It is often necessary for the pharmacist to compound solutions of a desired percentage strength. Percentage in that respect means parts of active ingredient per 100 parts of total preparation.

Following are the three basic rules to remember in solving percentage problems:

1. To find the amount of the active ingredient when the percentage strength and the total quantity are known, multiply the total weight or volume by the percent (expressed as a decimal fraction).

Table 6-2.-Table of Metric Doses with Approximate Equivalents

Table 6-3.-Conversion Table for Weights and Liquid Measures

Table 6-4.-Examples of Weight and Liquid Conversions

2. To find the total quantity of a mixture when the percentage strength and the amount of the active ingredient are known, divide the weight or volume of the active ingredient by the percent (expressed as a decimal fraction).

3. To find the percentage strength when the amount of the active ingredient and the total quantity of the mixture are known, divide the weight or volume of the active ingredient by the total weight or volume of the mixture. Then multiply the resulting answer by 100 to convert the decimal fraction to percent.