Conversions among Measurement Systems

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Chapter 5

Conversions among Measurement Systems

image Tech Alert

When making conversions between two measurement systems, the answers will usually not be exact, and the accepted difference between the measurement systems is in the range of 10% either above or below the actual calculated amount. Other factors in calculating approximate measurements do exist, such as the viscosity of the medication and the size of the utensils that will be used to provide the medication. In this text, please be sure that your conversion is as close to an exact amount as possible, so use the conversion factor that will give you an answer that can be administered with the utensil available. For example, in some cases a teaspoon may be 4 milliliters, but in others, when calculating a teaspoon, the conversion may be 5 milliliters. In these cases, you will need to complete the problem using both conversions to find the answer that is measurable. You always want to find the measurable amount for the ease of administration. For example, if your answer from the conversion indicates 4.4 milliliters is to be administered and this medication will be given with a utensil holding either 4 or 5 milliliters, such as a dosespoon, you would indicate that 4 milliliters would be given; however, if this is being given with a teaspoon that holds 5 milliliters, measuring 4.4 milliliters would not be possible, so you would state to administer a teaspoon of medication. Finally, if the dosespoon or other measurement utensil is found in 0.01-milliliter increments, the exact amount may be administered. This concept is difficult to understand, but as you go through this chapter, please use the conversion factor that supplies the approximate dose that is measurable. Also remember that some of the conversions used in the mathematical calculations in this chapter are based on the materials presented in Chapter 4. This text builds on previously learned materials, and materials in previous chapters will be important for use starting with this chapter on conversions among measurement systems.

Pretest

Be sure to use the correct numerical system when solving the problems in the Pretest. Show your calculations and round to the nearest hundredth. Remember that in this chapter you are converting among measurement systems and not within the measurement system. In converting drops to milliliters, you will have two answers based on the commonly used conversions of drops to milliliters. The two lines are provided for the problems needing two answers.

1. 20 mL = __________ gtts __________ gtts

2. 4 tsp = image __________

3. 15 tbsp = __________ oz

4. 2 oz = __________ tsp

5. 6″ = __________ cm

6. 16 oz = __________ pt

7. 8 c = __________ qt

8. image xxxii = __________ mL

9. image vi = __________ gtts

10. image = __________ mg

11. 88# = __________ kg

12. 120 mg = __________ gr

13. image viii = __________ mL

14. 8 kg = __________ mg

15. 15 gtts = image __________

16. 6 tbsp = __________ oz

17. 15 tsp = image __________

18. gr xv = __________ mg

19. gr v = __________ mg

20. 60 mL = image __________

21. 4 c = image __________

22. 25 cm =__________ ′

23. 3 tbsp = image __________

24. 250 mg = gr __________

25. 0.125 mg = __________ mcg

26. image = __________ mg

27. gr xxx = __________ mg

28. gr image = __________ mg

29. 4.56 g = __________ mcg

30. 60″ = __________′

31. 2′ = __________ cm

32. 5 kg = __________ #

33. 44 # = __________ kg

34. image = __________ mg

35. image vi = __________ tsp

36. image xv = __________ tbsp

37. gr image= __________ mg

38. 156 # = __________ kg

39. image vi = __________ tbsp

40. image image = __________ tsp

Introduction

In the previous chapter you learned the basic measurements found in the household, metric, and apothecary systems of measurements. Each of these is used today in prescribing and administering medications. Another measurement system, avoirdupois, is also available, but this system is not normally found in calculations or conversions. In this system, 16 ounces = one pound (7000 grains = one pound). Two other measures are used in dosage calculations—milliequivalents and units—that will be discussed in Chapter 6.

The household measures are those that use common measuring tools found in most homes. Therefore for the person taking medication to be able to use utensils available, you must convert any orders written in apothecary or metric systems to the household measurements for ease of drug administration. If a physician writes an order in the apothecary system and the medication is available in a metric dose, you must convert the apothecary dose to an approximate metric equivalent to ensure the patient receives the correct amount of medication. As the metric system becomes more widely accepted, the need for conversion will decrease, although the need to calculate a dose in the household measure will remain as long as the United States is measuring in household equivalents.

Conversions among the systems are only necessary if the two factors needed for calculation are not found within the same measurement system. If both factors are already in the metric system, then moving within the system is all that is necessary, as seen in Chapter 4. This is likewise true for the apothecary and household measurements. However, if one factor is in one system, such as the metric system, and the other factor is in another system, such as household, proportional equations are used to find the unknown approximation.

Either ratio and proportion or an advanced system of ratio and proportion, called dimensional analysis, may be used.

Both systems are presented in this chapter so you can decide which method is easier for your personal use. Find the system with which you feel comfortable and use it regularly to perfect its use. Only through practice will this become a skill that is easy for you to use for making pharmaceutical calculations.

One further hint is necessary before beginning the process of converting measurements: Because the measurement systems are not identical, any conversions among them are approximate in the conversion. You cannot find an exact equivalent in most conversions; rather the number will be an approximate amount. An example is that a dram is often used as a symbol for a teaspoon when a dram is actually 4 mL and a teaspoon may be either 4 or 5 mL depending on the utensil. This conversion therefore becomes an approximate amount.

Using Ratio and Proportion for Conversion of Units

Ratio is the relationship of one quantity to another, whereas proportion is the relationship between two equal ratios. In ratio and proportion, an unknown is solved using “x” as the unknown. This provides a logical and systematic means of finding the equivalent unknown when knowns are used for calculations. Two equal ratio sets (or the relationship between two equivalents, such as a [extreme] : b [mean] :: c [mean] : d [extreme]) is indicated for calculating proportional equivalents. When one of the equivalents is unknown, an “x” is used to indicate the unknown. (See Chapter 2 if you need to review this material.)

When the ratios have been properly aligned into proportion(s), multiply the two inside numbers (means, or “insies”) and the two outside numbers (extremes, or “outsies”) and then solve for “x.” Remember that what is done to numbers on one side of the equation must be done to numbers on the other side of the equation. If you need more review for ratio and proportion, refer to Chapter 2.

Using Dimensional Analysis for Converting Among Units

Dimensional analysis is actually ratio and proportion in multiple sections expressed as fractional forms written across one fractional equation.

To use dimensional analysis, the system uses the multiplication of a series of fractions in which the numerator and denominator contain related conversion factors. Each factor has a number and unit of measurement. By using dimensional analysis, remembering multiple formulas to solve drug calculations is not necessary for drug dosages to be accurate and safe.

Using dimensional analysis involves using a series of ratios or factors that are arranged as a fractional equation. Each factor is written as a fraction, and the factors must be related to each other and to the problem that is being solved. As with all fractions, each factor must have a numerator and a denominator. Each problem requires the use of only one equation to determine the answer. If the units are not in the same measurement system, the conversion to the system becomes part of the equation. If both quantities are in the same measurement system, the conversion among systems is not indicated.

Conversion factors are the equivalents between two measurements whether in the same system or not. Each conversion factor includes a value (numerical value) and a label (units of measurement). For example, 12 in = 1 ft. This is a conversion between feet and inches. It can be used to convert measurements for length within the U.S. system but not for direct analysis if the measurement is given in meters or centimeters found in the metric system. In that case, the factor for the metric system must be placed within the equation for the answer to be correct.

To perform dimensional analysis, always start with what you are looking for, or the unknown (x). Place the x to the left of the equation so that you will not forget what you are solving for. As you work the problem, all matching symbols in the equation must be removed by identifying and canceling those identifying symbols/abbreviations that are alike in the numerators and denominators as the equation evolves.

There are six distinct steps in setting up a fractional analysis, or the fractional equation used in dimensional analysis. To illustrate these steps, the following example used the familiar household measurement system. A recipe calls for image cup of an ingredient for a cake. At the time of baking, the only available measurement device is a tablespoon. How can you help the person baking the cake know the proper amount of the ingredient to be used when only a tablespoon is available? (image c = __________ tbsp)

1. Find the known, or given, quantity—image c.

2. What is the desired, or wanted, amount?—x tablespoons.

3. What are the conversion factors that are needed to make the necessary calculation? In this case, we need to know that 2 tablespoons equals 1 oz and 8 oz equals 1 c.

4. Set up the problem with factors that are available for conversion factors. You want to cancel out like factors by placing them in a numerator of the first fraction followed by the denominator in the next fraction as follows:

xtbsp=1/4c1×8oz1c×2tbsp1oz

image

5. Cross out the unwanted units. Just as with any other mathematical equation, if units appear in both the numerator and the denominator, you may cancel them to find the unit that is desired. In this case, the remaining unit is a tablespoon, which is the unit you are seeking.

image

6. Multiply the numerators, multiply the denominators, and divide the product of the numerators by the product of the denominators. This will give you the desired quantity that was originally your unknown factor.

xtbsp=1/4×8×2tbsp1×1×1or16/4tbsp1or4tbsp1orx=4tbsp

image

So, after completing the problem:

1/4c=4tbsp of the ingredient needed in the cake

image

Now let’s look at other examples found within the same measurement systems.

Conversions Between Household and Apothecary Systems

The apothecary system has only a few significant measurements that, although not commonly used, do convert into household measurements. Of importance are minims to drops, drams to teaspoons, and ounces to tablespoons (Figure 5-1). Cups, pints, quarts, and gallons are used by both systems. Length is not found in the apothecary system, so the conversions are not included in Table 5-1. As the chapter unfolds, the metric system will be added to combined tables for conversions among measurement systems.

TABLE 5.1

Conversions between Apothecary and Household Systems of Measurement

PARAMETER APOTHECARY UNITS HOUSEHOLD UNITS METRIC UNITS
Volume image i (1 minim) 1 gtt (1 drop)
image i (1 dram, image 60) 1 tsp (1 teaspoon)
image image (image ounce) 1 tbsp (1 tablespoon, 3 tsp)
image i (1 oz) 2 tbsp (2 tbsp, 6 tsp)
image viii 1 c (8 oz, 1 cup, image pint)
1 pt (1 pint, 2 c, image xvi) 1 pt (1 pint, 2 c, 16 oz)
1 qt (1 quart, 2 pt, 4 c, image 32) 1 qt (1 quart, 2 pt, 4 c, 32 oz)
1 gal (4 qt, 8 pt, 16 c, image 128) 1 gal (4 qt, 8 pt, 16 c, 128 oz)
Mass/Weight image i (1 oz, 60 gr)* 1 oz
1# (1 lb, image xii) 1# or 1 lb (16 oz)*
Length N/A N/A

image

*Note that an apothecary pound contains only 12 oz, while a household pound contains 16 oz.

Example 5-6

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Ratio and Proportion Dimensional Analysis
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Practice Problems A

Calculate the following problems using either ratio and proportion or dimensional analysis. Show all of your calculations. Be sure to use the correct numeral indicator for the measurement system of the unknown. Remember these answers may be approximates, NOT EXACT ANSWERS.

1. 6 tsp = image __________

2. 2 tsp = image __________

3. 16 gtts = image __________

4. 4 tbsp = image __________

5. image image= __________ tsp

6. 24 gtts = image __________

7. image v = __________ tsp

8. 75 gtts = __________ tsp

9. 3 tbsp = image __________

10. 10 tsp = image __________

11. 2 tsp = __________ gtts

12. 25 gtts = image __________

13. image vi = image __________

14. 8 c = image __________

15. image xxiv = __________ c

16. 75 gtts = image __________

17. 4 tsp = image __________

18. 30 tsp = image__________

19. 16 tbsp = image __________

20. image xxx = __________ tbsp

21. image image = image __________

22. image xxiv = __________ tsp

23. image vi = image __________

24. 15 tsp = __________ tbsp

25. 45 gtts = image __________ = image __________

26. imageA physician writes a prescription for image ii of Robitussin cough medication q4h.

27. imageThe label on the bottle of Maalox reads that the medication may be taken image image every 3 to 4 hours as needed for indigestion.

28. imageA child is to take Tri-Vi-Sol image viii daily.

29. imageA physician writes a prescription for MiraLax one capful in H2O image viii.

30. imageA physician instructs the parent to give a child 30 gtts of Benadryl for an allergy.

Conversions Between Household and Metric Units

Now that the household system has been compared with the apothecary system, it is time to compare the household system with the metric system, which is more frequently used in drug measurements (Table 5-2). Recall that the metric system is formed on the base 10 for ease of conversion. Most conversions from metric to household, such as milliliters to teaspoons, or milliliters to ounces, are volume conversions, although length from inches to centimeters is used in the medical field. Also notice that conversions between household and metric measurements are most commonly found in volume, but length and weight are found minimally when converting between these two systems for pharmaceutical purposes (Figure 5-2).

TABLE 5.2

Conversions between Household and Metric Systems of Measurement

PARAMETER APOTHECARY UNITS HOUSEHOLD UNITS METRIC UNITS
Volume 1 gtt (1 drop) N/A
15 or 16 gtts* 1 mL
1 tsp (1 teaspoon) 5 mL or cc
1 tbsp (1 tablespoon, 3 tsp) 15 mL
2 tbsp (2 tablespoons) 30 mL
1 oz (2 tbsp) 30 mL
1 c (8 oz, 1 cup, image pint) 240 mL
1 pt (1 pint, 2 c, 16 oz) 480 mL
1 qt (1 quart, 2 pt, 4 c, 32 oz) 960 mL
1 gal (4 qt, 8 pt, 16 c, 128 oz) 3840 mL
Mass/Weight 2.2# 1 kg
Length 1″ 2.54 cm

image

*Remember that the equivalents are approximate and not exact when doing conversions.

image

image

Practice Problems B

Calculate the following problems. Remember that the only time a conversion is necessary is when the units are not in the same measurement system. Also remember that all calculations in conversions are approximates and should be rounded to the nearest fractional number or hundredth as appropriate. Show all of your calculations.

1. 8 tsp = __________ tbsp

2. 16 oz = __________ mL

3. 8 kg = __________ #

4. 3 tsp = __________ mL

5. 2 pt = __________ mL

6. 5″ = __________ cm

7. 3 c = __________ mL

8. 90 mL = __________ oz

9. 6 tsp = __________ tbsp

10. image iv = __________ tsp

11. image ii = __________ tbsp

12. image x = __________ gtts

13. 10 cm = __________ ″

14. 30 cm = __________ ft

15. 75 mL = __________ oz

16. 10 kg = __________ #

17. 720 mL = __________ pt

18. 720 mL = __________ qt

19. 10 tsp = __________ mL

20. 2 pt = __________ c

21. 500 mL = __________ qt

22. 6 tsp = __________ mL

23. 1.5 L = __________ pt

24. 3120 mL = __________ qt

25. 10 c = __________ L

26. 5 ft = __________ cm

27. imageA physician writes an order for an antihelmintic for a child weighing 35#. The dosage must be calculated in mg/kg body weight.

28. imageA physician writes a prescription for Rondec DM 15 gtts.

29. imageA physician writes a prescription for 5 mL of azithromycin liquid for a child.

30. imageA parent has an infant with colic. A bottle of Mylicon drops is available for relief of the pain. The label reads to give 15 drops for a child who is the age of the infant.

Conversions Between Metric and Apothecary Systems

Remember that the metric system uses grams, liters, and meters, whereas the apothecary system frequently uses grains, minims, drams, and ounces. Also remember that in converting between two systems of measure, the equivalents are approximate. This is important when converting between apothecary and metric systems. If the answer is a portion of a minim, you should round to the closest full amount because minims cannot be divided (e.g., if the calculation is for image minims, the amount should be rounded to 5 minims; if the calculation is image minims, the amount should be rounded to 4 minims). Portions of grains, milligrams, grams, and other measures can be expressed with fractional units. Again, with these conversions, ratio and proportion or dimensional analysis may be used, depending on the method that is most comfortable for you. Table 5-3 shows the equivalents between the apothecary system and the metric system.

TABLE 5.3

Conversions between Apothecary and Metric Systems of Measurement

PARAMETER APOTHECARY UNITS HOUSEHOLD UNITS METRIC UNITS
Volume image xv (15) (or xvi [16])* 1 mL
  image i 4 or 5 mL *
  image image 15 mL
  image i 30 mL
  image viii 240 mL
  image xvi or 1 pt 480 mL
  image 32 or 1 qt 960 mL
  image 128 or 1 gal 3840 mL
Mass/Weight gr i 60 mg
gr xv 1 g

image

*Remember that all equivalents are approximates when converting between measurement systems.

The important conversions to remember are as follows and these are approximations as stated early in the chapter:

MASS VOLUME
gr i = 60 mg (or 65 mg) 1 mL = image xv or xvi (use the figure that will give you as close to a whole number as possible)
1 g = gr xv image i = 4 or 5 mL
  image i = 30 mL

image

A quick way to remember conversions between grains and milligrams is to use a clock as the basis for conversions. There are 60 minutes in an hour just as there are 60 mg to a gr. Figure 5-3 shows how this can be used.

If you use the clock as a basis for conversions, gr image or image hour is 15 milligrams or minutes; gr image is 30 milligrams, while image hour is 30 minutes; gr image or image hour equals 45 milligrams or hours; and finally gr i equals 60 milligrams, while 1 hour equals 60 minutes. This same clock can be used for 1/6, 1/3, and 2/3 grains or hours just as easily. See Figure 5-3, A and B.

The clock can also be used to convert drams to minims as shown in Figure 5-3, C. As with the other clock examples, drams can be converted to minims by keeping the minim on the outside of the clock and the drams on the inside.

As with previous conversions, ratio and proportion or dimensional analysis may be used for conversions as desired.

image

image

Before beginning practice problems, remember the conversions are approximates and may not be exact equivalents. Also remember that a minim is the smallest unit that can be measured in the apothecary liquid measure, so round the amount as appropriate. Finally, remember that if you are using equivalents within the same measurement system, you do not need the conversions between measurement systems.

Practice Problems C

Calculate the following practice problems. Round as appropriate, being sure the answer is measurable; for instance, image drop would be 2 drops because a drop cannot be divided for administration. Show all of your calculations.

1. gr ii = __________ mg

2. 5 mg = __________ mcg

3. image xvi = __________ mL

4. 6 g = gr __________

5. image iv = __________ mL

6. 45 mL = image __________

7. 15 mg = gr __________

8. 60 mL = image __________

9. 5 mL = image __________

10. 1.5 mL = image __________

11. gr image = __________ g

12. 750 mL = image __________

13. image iv = image __________

14. gr image = __________ mg

15. gr image= __________ mg

16. 0.6 mg = gr __________

17. 15 cc = image __________

18. 12 image = __________ mL

19. 2 L = image __________

20. 0.1 mg = gr __________

21. 16 mL = image __________

22. 55 mL = __________ cc

23. gr image = __________ mcg

24. gr image = __________ mg

25. 4 mL = image __________

26. image 48 = image __________

27. imageA physician orders a parent to give a young child 2 mL of amoxicillin suspension.

28. imageA patient reads a prescription for Maalox 16 mL.

29. imageA patient has a prescription for Seconal gr image. The available medication is Seconal 100 mg/capsule.

30. A prescription is written for phenobarbital gr image. The medication stock bottle reads phenobarbital 30 mg per unscored tablet.

Putting It All Together—Metric, Household, and Apothecary Measurements and Their Conversions

Throughout this chapter you have converted between two measurement systems at a time. Now you need to put all of the conversions together. Table 5-4 is a combination of all tables previously shown in this chapter. Please remember that all conversions are approximations and could be changed depending on the conversion used for calculation, such as the number of mL in a dram. The conversion should always be made on the basis of a measurable dose.

TABLE 5.4

Conversions among Metric, Apothecary, and Household Systems of Measurement

PARAMETER APOTHECARY UNITS HOUSEHOLD UNITS METRIC UNITS
Volume image i 1 gtt N/A
image xv (or xvi)* 15 or 16 gtts 1 mL
image i 1 tsp 4 or 5 mL
image image 1 tbsp or 3 tsp 15 mL
image i 2 tbsp or 6 tsp 30 mL
image viii 1 c (8 oz, image pt) 240 mL
image xvi or 1 pt 1 pt (2 c, 16 oz) 480 mL
image 32 or 1 qt 1 qt (2 pt, 4 c, 32 oz) 960 mL
image 128 or 1 gal 1 gal (4 qt, 8 pt, 16 c, 128 oz) 3840 mL
Mass/Weight gr i N/A 60 mg
gr xv N/A 1 g
image i (60 gr) 1 oz 4 g
N/A 2.2# 1 kg
Length N/A 1″ 2.54 cm

image

*Remember that all equivalents are approximates when converting among measurements.

Now add one last conversion to the clock (see Figure 5-3, D). Remember that you are now adding a liquid measure that should not be confused with the weight or solid measure.

Using the clock, the calculation of minims to drams can easily be accomplished by using 60 minims equals 1 fl dram. Again, remember to place all symbols or abbreviations ending in “r” in the center of the clock and all beginning with “m” on the outside of the clock. The fractional parts as seen with the grains and hours will again apply to fluid drams, such as 10 minims is 1/6 fluid dram.

Review

In this chapter the concepts of conversion among the metric, apothecary, and household systems have been shown. The conversions may be accomplished using either ratio and proportion, dimensional analysis, or in some cases the use of the clock. As a student, you should find one method with which you are the most comfortable and use it regularly. Although conversions are made, the fact that the conversion is not exact but is an approximation is an important concept to remember. Conversions are necessary only when more than one measurement system is presented in the problem. Remember that the more you use conversions and approximate equivalencies, more comfortable you will be with this concept.

Posttest

Before taking the Posttest, retake the Pretest to check your understanding of the materials presented in this chapter. Be sure to use the correct numeral indications for each system. Also be sure the approximate calculation could be administered and makes a sensible dose.

Use either ratio and proportion or dimensional analysis to solve these problems. If two lines appear for an answer line, two conversions are possible for that answer based on the difference in the conversion factors. Round all answers to hundredths.

1. 3 tsp = image __________

2. 6 tsp = __________mL

3. gr image = __________ mg

4. image v = __________mL __________ mL

5. 88 # = __________ kg

6. 0.6 mg = __________ gr

7. 1250 mL = __________ pt

8. 2.5 L = __________ mL

9. 2.5 mL = __________ gtts

10. 8 tbsp = __________ oz

11. 4 qt = __________ L

12. 5050 mL = __________ L

13. 45 mg = gr __________

14. image 24 = __________ c

15. 15 tsp = __________ tbsp

16. 16 tbsp = image __________

17. 16 tbsp = image __________

18. 16″ = __________ cm

19. 46 # = __________ kg

20. 0.1 mg = gr __________

21. 2500 g = __________ #

22. 12 mL = __________ gtts

23. 45 mL = image __________

24. 45 mL = __________ tsp

25. gr image= __________ mg

26. 430 mg = __________ g

27. 5 tbsp = image __________

28. 4 tsp = image __________

29. 10 kg = __________ #

30. 324 cm = __________ ″

31. 2000 mL = __________ pt

32. 12 mg = gr __________

33. 11 mL = image __________

34. 28 mL = image __________

35. 2 yd = __________ cm

36. 2784 mL = __________ L

37. 2784 mL = __________ cc

38. 35 mL = __________ tsp

39. gr 45 = __________ g

40. 0.6 kg = gr __________

41. gr xx = __________ mg

42. 2500 mL = __________ pt

image 43 A physician orders acetylsalicylic acid 325 mg. Available are gr v tablets.

image 44 A young child is to receive Benadryl elixir 2 mL.

image 45 An older sibling of the child mentioned in question 44 is to receive Benadryl elixir 7.5 mL. The parent needs to give this with a teaspoon.

image 46 A physician orders codeine gr image stat for pain.

47. A chemotherapeutic medication will be ordered by weight in kilograms. The patient weighs 166 #.

48. A child is measured in a teaching hospital in centimeters. The height is 104 cm.

image 49 A physician orders atropine sulfate 0.4 mg as a preoperative order.

How many grains will the patient receive if the medication label is in grains? ____________________

image 50 A prescription for amoxicillin must be reconstituted with image iii of water.

Review of Rules

Rules of Conversion among Measurement Systems Using Ratio and Proportion