Methods of Dosage Calculation

Desired dosage (D) is the dosage ordered

Available dosage(H) equals the dosage on hand (e.g., the number of milligrams or the number of milliequivalents) or available volume (H) is the amount of solution or number of tablets containing the drug (e.g., milliliter, minims, tablets) on hand

Vehicle (V) is the drug form (number of tablets or amount of solution containing the available dosage)

Amount (A) is the volume/amount of the drug to be administered (e.g., number of milliliters, minims, tablets)

Method 1: Basic Equation

D/H × V = A

Example

The doctor's order (D) is 400 mg. The available dosage (H) is 200 mg available in the vehicle (V) of one tablet.

400/200 × 1 tablet = A

2 × 1 tablet = A

A = 2 tablets

Method 2: Ratio and Proportion

H : V :: D : A

Example

The doctor's order (D) is 400 mg. The available dosage (H) is 200 mg available in the vehicle (V) of one tablet.

200 : 1 :: 400 : A

200/1 = 400/A

Cross multiply to find A:

200 × A = 400 × 1

A = 400/200

A = 2 tablets