The Pharmaceutics and Compounding Laboratory
Accuracy in Measurements

Measurement Systems

Two of the most crucial steps in compounding any pharmaceutical product are the accurate calculation and measurement of the component ingredients of the formulation. In order to carry out these critical functions, the pharmacist must have a working knowledge of three systems of measurement: the Metric system, the Apothecary system, and the Avoirdupois system.

The Metric System

The metric system is the preferred and most frequently used system of measurement in pharmacy. Since it is a decimal system, other denominations of measure in the system are easily and quickly generated as a 10nth multiple at the basic unit. To convert from larger to smaller units, one need only move the decimal the appropriate number of places to the right. The decimal is moved to the left to convert from smaller to larger units.

The Apothecary System

The Apothecary system was commonly used in the past by pharmacists and physicians as the system of weights and measures for prescribing and dispensing medications. Although it has largely been replaced by the less cumbersome metric system, the pharmacist still encounters these symbols in his/her routine practice. Indeed, the apothecary system of fluid measure is still commonly used in a variety of products, both pharmaceutical and nonpharmaceutical, and everyone should be familiar with the fluid ounce, pint, quart, and gallon.

Since quantities of ingredients in the Apothecary system are commonly written in Roman rather than in Arabic numerals, you should recall the following:

ss or ss = 1/2   L or l = 50 - I or i = 1       C or c = 100  - V or v = 5       D or d = 500 - X or x = 10      M or m = 1000

The Avoirdupois System

The Avoirdupois system is a system of weight measurement only. Its basic unit, the grain, is the same as in the Apothecary system. The Avoirdupois ounce and pound differ in weight and symbols from those in the Apothecary system. The Avoirdupois pound is the pound to which we are all accustomed in our daily lives. It is also the weight measure in which bulk chemicals and over the counter pharmaceuticals are bought and sold. It is important to make this distinction from weights in the Apothecary system, which are used only in the prescription or medication order.