Different Systems of Weights and Measures and Relationships Relevant to a Pharmacist

A brief account of different systems of weights and measures and relationships relevant to a pharmacist are given below.

(a) Measures of Mass

The difference between weight and mass is that the weight changes with the altitude, as it is dependent on the gravitational force, whereas mass remains unchanged.

The unit of weight is the Gram, which is defined as 1/1000 of the mass of the International Prototype Kilogram, e.g.,

(b) Measures of Length

The unit of length is the Meter (M), which is defined as the length of the International Prototype Meter bar

(c) Measures of Capacity

The unit of capacity is the litre or liter (1. or L.) that is defined as the volume of 1 kg of water at 4°C. There is difference between millilitre and cc. 1 ml is the volume occupied by 1 g of water at 4°C, whereas 1 cc is the volume of a cube each side of which is 1 cm length. 1 cc = 0.99984 ml or 1 litre = 1000.028 cc. However, for all practical purposes ml and cc are taken as the same.

(d) Imperial System

Imperial system of weights and measures, commonly known as Avoirdupois system was recognized in the British Pharmacopoeia, though at present UK also has switched over to the metric system.

(i) Avoirdupois Weights and measures

Measures of Capacity (Volume)

Relationship of capacity to mass

1. 1 Gallon = the volume of 10 pounds or 70,000 grains of distilled water at 62°F

2.1 Fluid ounce or 480 minims = the volume at 62°F of 1 oz. or 437.5 grains of distilled water

3. 109.71 minims (taken as 110 minims) = the volume at 62°F of 100 grains of distilled water.

(ii) Apothecaries’ Weights and Measures

Measures of Mass

Grain is the same in both Avoidupois and Apothecaries systems.

The measures of volume are the same in both the systems. In Avoirdupois system the abbreviations are fl.dr and fl.oz while in Apothecarees system they are and for dram and ounce, respectively.

United States of America : Weights and Measures

1 Fluid ounce (480 minims) = 454.6 grains at 25°C (USA standard) = 437.5 grains at 16.7°C (Imperial Standard)

1 pint =16 fl.oz. (USA) = 20 fl oz. (Imperial)

1 Gallon = 128 fl. Oz. (USA) = 3.7853 liters

1 Gallon = 160 fl.oz. (Imperial) = 4.5436 litres

1 hundredweight = 100 pounds (USA) =112 pounds (Imperial)

1 ton = 2000 pounds (USA) = 240 pounds (Imperial)

Household Measures

The accepted approximate dose equivalents for the household measures are given below-

The drops of various liquids vary in size and it is therefore necessary to calibrate the dropper to get the correct dose of a potent liquid preparation.