# Pharmaceutical Calculations

Filed under Basic Science

rate 1 star rate 2 star rate 3 star rate 4 star rate 5 star

CHAPTER 2 Pharmaceutical Calculations

# SYSTEMS OF MEASURE

Summary of conversion between metric, apothecaries’ and avoirdupois systems:

Note that in the apothecaries’ and avoirdupois systems there is only one common unit of measure, the grain. The other measurement units carry different values when comparing the systems. When converting between the two, the pharmacist should convert the value down to the grain amount in the one system, then convert to the other system.

Per the United States Pharmacopeia, 1 grain = 64.8 mg.

## METRIC SYSTEM

### Prefixes

 kilo- 103 1 thousand (1000) times the basic unit hecto- 102 1 hundred (100) times the basic unit deka- 10 1 ten (10) times the basic unit deci- 10−1 1 tenth (0.1) times the basic unit centi- 10−2 1 hundredth (0.01) times the basic unit milli- 10−3 1 thousandth (0.001) times the basic unit micro- 10−6 1 millionth times the basic unit nano- 10−9 1 billionth times the basic unit pico- 10−12 1 trillionth times the basic unit

## AVOIRDUPOIS SYSTEM

A system of masses based on a pound weighing 16 ounces mostly commonly used in the United States for commercial purposes.

# Example

If 5 tablets contain 1625 mg of acetaminophen, how many tablets should contain 2925 mg?

# Example 1:

How many grams of drug should be used to prepare 120 grams of a 2% w/w solution in water?

# Example 2

What is the percentage strength (w/v) of a solution of drug if 40 mL contain 5 grams?

# DILUTION, CONCENTRATION, AND ALLIGATION

## For females

If the individual is obese or not within 30% of their ideal body weight, other methods of calculating creatinine clearance should be used. Ideal body weight (IBW) or adjusted body weight (ideal body weight plus 40% of obese weight) instead of actual body weight in the Cockcroft-Gault equation will provide a better estimate of creatinine clearance.