## 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 Although it has largely been replaced by the less cumbersome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:

### 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.