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 | Fluid | 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 | Metric | Imperial |
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
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