Use the fractional method as above.

Example: Calculate the amounts of ingredients for a gallon of the following liquid:

Glycerin............................3 fl dr

Liq. phenol........................ .1fldr

Water q.s.......................... .4 fl dr

There are 128 fl oz in a gallon. The fraction would then become 128/4 or 32. Multiply each ingredient by 32.

3 fl dr x 32= 96 fl dr or 12 fl oz of glycerin.

1 fl dr x 32 = 32 fl dr or 4 fl oz of liq phenol.

Then add sufficient quantity of water to make the total volume measure 128 fl oz (one gallon).

Use ratio and proportion.

Example: We have 1/2 gr tablets on hand, and we want to give the patient a 1/8 gr dose. Use 24 minims of water as the solvent.

1/2gr.................24 minims of solvent

1/8gr................. X minims of solution

1/2: 1/8::24:X

1/2X=3

X=6

Give the patient 6 minims of the solution.

Use ratio and proportion.

Example: A tablet contains 1/2 gr of phenobarbital, and we wish to give the patient 3/4 gr.

Dissolve 2 tablets in 4 ml of water:

1 gr......................4 ml of solvent

3/4 gr.................... X ml of solution

1: 3/4 :: 4 : X

X = 3

Give the patient 3 ml of the solution.

A review of basic mathematics will help you understand the important phases of pharmaceutical calculations.

The decimal point represents a power of 10. Every time the decimal is moved one digit to the right, the number is multiplied by 10 and conversely, every time it is moved one digit to the left, the number is divided by 10.

Example:

3.0—if we move the decimal one digit to the right: 30.0, we have multiplied the number 3 x 10 = 30. If we move it another digit to the right, we have again multiplied by 10:

30.0 x 10 = 300.0.

3. 0—moving the decimal one digit to the left, we will have divided the number 3 by 10: 3 ÷ 10 = 0.3.

If we move the decimal another digit to the left, we will have divided by 10 again: 0.3 ÷ 10 = 0.03, and so forth.

The number, or numbers, left of a decimal point are whole numbers or units; the numbers to the right of the decimal point are fractional parts of the same unit. If you compare the decimal with our monetary system, this is readily understood.

Example: 3.85 3 gram 3.85 3 dollars

grams = 8 decigrams dollars = 8 dimes

5 centigrams 5 cents

Addition of Decimals

When adding, keep the decimal points in a vertical line to avoid confusing fractional numbers with whole numbers.

Examples:

1.30

.50

11.2

.015

= 13.015

.065

1.435

23.015

456.65

= 481.165