Weights and measures - Pharmaceutics - I B. Pharma 1st Semester

Weights and measures

Objective

At the end of this lecture, student will be able to

• List the various systems of weights and measures

• Convert units within the various systems

• Compare the different systems

• Relate common household measurements to the imperial and metric measures

Weights and measures 

A thorough knowledge of weights and measures – to understand various types of calculations used in dispensing.

The two systems of weights and measures

• The Imperial system

• The Metric system

The imperial system

An old system of weights and measures

The Imperial system type:

• Apothecaries system

• Avoirdupois system

1. Avoirdupois weight measures

– ‘Pound’ is the standard unit for weighing

– All measures of mass are derived from the Imperial standard pound (lb)

Avoirdupois measure of weight

Pound	Ounce	Grain
1	16	7000
	1	437.5

2. Apothecaries weight measures

– Also known as ‘Troy’ system

– ‘Grain’ is the standard unit

Apothecaries’ measures of weight

• 20 grains = 1 scruple (℈)

• 3 scruples = 1 drachm/ dram (ʒ)

• 8 drachms = 1 ounce (oz )

• 12 ounces  = 1 pound (lb)

A typical set of Apothecary weight will contain the following units:

• 2 fluid ounce

• 1 fluid ounce

• 2 fluid dram

• 1 fluid dram

• ½ dram

• 2 scruple

• 1 scruple

• ½ scruple

• 5, 4, 3, 2, 1, ½ grain

Pound	Ounce	Drachm	Scruple	Grain
1	12	96	228	5760
	1	8	24	480
		1	3	60
			1	20

Conversion table

I Lb	16 oz (avoir)
1 Lb	7000 grains
1oz	7000/16 = 473.5 grains
20 grains	1 scruple
60 grains	1 drachm
480 grains	1 ounce
12 ounce	1 pound
5760 grains	1 pound

Measurement of capacity in Imperial system

• The standard unit for capacity is ‘gallon’ in both apothecaries and avoirdupois system.

Gallon	Quart	Pint	Fluid ounce	Fluid drachm	Minim
1	4	8	128	1024	61440
	1	2	32	256	15360
		1	16	128	7680
			1	8	480
				1	60

Imperial measures for volume

60 minims (♏) = 1 fluid drachm (fʒ)

8 fluid drachm = 1 fluid ounce (f℥)

16 fluid ounces = 1 pint (pt or O)

2 pints = 1 quart ( qt)

4 quarts = 1 gallon ( gal or C )

The Metric System

• Most commonly used measure of weights and measures

• The three basic units of metric system – meter, gram and litre

• Other units are formed by adding prefix to one of the basic units

• Litre and gram – most commonly used basic units in pharmacy

PREFIX	VALUE
Micro	1/10,00,000
Milli	1/1,000
Centi	1/100
Deci	1/10
Deca	10
Hecto	100
Kilo	1,000

Conversions – Imperial to Metric

IMPERIAL	METRIC
15.43 grains	1 gram
1 grain	65 mg
2.2 pounds	1 kg
16.23 minims	1 ml
1 minim	0.06 ml
1 fluid drachm	3.69 ml
1 fluid ounce	29.57 ml
1 pint	473 ml
1 gallon	3785 ml

Household / Domestic measures

Domestic measures	Metric system	Imperial system
1 drop	0.06 ml	1 minim
1 teaspoonful	4 ml/5 ml	1 fluid drachm
1 desert spoonful	8 ml/10 ml	2 fluid drachms
1 tablespoonful	15 ml	4 fluid drachms
1 wineglassful	60 ml	2 fluid ounces
1 teacupful	120 ml (4 fluid ounce)	4 fluid ounces
1 tumblerful	240 ml (8 fluid ounce)	8 fluid ounce

Summary

• Two basic systems of weights and measures – Imperial and metric

• The two imperial systems of measures – Avoirdupois and Apothecaries

• Apothecary measure of weight – Grain, scruple, drachm ounce pound

• Avoirdupois measure of weight - Grain ounce pound

• Imperial measure of volume – minim, scruple, drachm, fluid ounce pint quart

• Common household measurements