It is often necessary in the practice of pharmacy to convert from one system to another in order to dispense the substances that have been ordered in their proper amounts. Although the denominations of the metric system are not commensurate with those of the common systems, the Bureau of International Standards has established conversion standards that will satisfy the degree of accuracy required in almost any practical situation. Ordinary pharmaceutical procedures generally require something between two-and three-figure accuracy, and the following tables of conversion are more than sufficient for practical use. Naturally, if potent agents are involved, you must use a more precise conversion factor for purposes of calculation. Conversion Table for Weights and Liquid Measures

1 grain = 0.065 gram or 65 miligrams

1 gram = 15.432 grains

1 mililiter = 16.23 minims

1 fluid ounce = 29.57 mililiters

1. gr to g - gr/15.432 = g

2. ml to fl oz - ml/29.57

3. minims to ml - minims/16.23 = ml

4. mg to gr - mg/65 = gr

5. g to gr - g x 15.432 = gr

6. fl oz to ml - fl ox x 29.57

7. ml to minims - ml x 16.23 = minims

8. gr to mg - gr x 65 = mg

In compounding, you will often find it necessary to reduce or enlarge the original recipe or formula. Most of the formulas in the USP-NF are given in quantities of 100g or 1000g of weight or milliliters of volume total.

There are many ways of reducing and enlarging formulas. The methods most commonly used are:

Example: Reduce the following formula for potassium arsenite solution to make 120 ml.

Arsenic trioxide ..................... 10g

Potassium bicarbonate ............... 7.6 g

Alcohol .............................30ml

Distilled water, a sufficient quantity to make, ............... 1000ml

By using ratio and proportion, the amount of arsenic trioxide to be used:

1000 : 120 :: 10 : X

1000 X = 1200

X = 1.2g of arsenic trioxide needed.

For potassium bicarbonate:

000 : 120 :: 7.6 : X

1000 X = 912

X = 0.912g of potassium bicarbonate needed.

1000 : 120 :: 30 : X

1000 X = 3600

x = 3.6 ml of alcohol needed.

The new formula is written as follows:

Arsenic trioxide ..................... 1.2 g

Potassium bicarbonate ............. 0.912 g

Alcohol ............................3.6 ml

Distilled water, q.s. ................120.0 ml

The numerator will be the amount of the new formula, and the denominator will be the amount of the original formula. Example: Reduce the formula for the potassium arsenite solution in the preceding item to make 120 ml.

120/1000 x 10 = 1.2g

120/1000 x 7.6 = 0.912g

120/1000 x 30 = 3.6ml

q.s. to 120 ml