Metrology is the science of measurements and it is the key to achieving accuracy. In pharmacy, it provides the accurate and reliable measurements for safety and efficacy of drugs.
Metric system
1. Basic units
- Mass = g or gram
- Length = m or meter
- Volume = L or liter
Note: 1 cc (cubic centimeter) of water is approximately equal to 1 mL and weighs 1 g.
2. Prefixes
- kilo = 10^3, or 1000 times the basic unit
- hekto = 10^2, or 100 times the basic unit
- deka = 10^1, or 10 times the basic unit
- deci = 10^-1, or 0.1 times the basic unit
- centi = 10^-2, or 0.01 times the basic unit
- milli = 10^-3, or 0.001 times the basic unit
- micro = 10^-6, or one-millionth of the basic unit
- nano = 10^-9, or one-billionth of the basic unit
- pico = 10^-12, or one-trillionth of the basic unit
Note: Examples of these prefixes include milligram (mg), which equals one-thousandth of a gram, and deciliter (dL), which equals 100 mL, or 0.1 L.
Apothecary system
1. Volume (fluids or liquid)
- 60 minims (♍) = 1 fluidrachm or fluidram (fÊ’)or (Ê’)
- 8 fluidrachms (480 minims) = 1 fluid ounce (f ℥ or ℥)
- 16 fluid ounces = 1 pint (pt or 0)
- 2 pints (32 fluid ounces) = 1 quart (qt)
- 4 quarts (8 pints) = 1 gallon (gal or C)
2. Mass (weight)
- 20 grains (gr) = 1 scruple (℈)
- 3 scruples (60 grains) = 1 drachm or dram (Ê’)
- 8 drachms (480 grains) = 1 ounce (℥)
- 12 ounces (5760 grains) = 1 pound (lb)
Avoirdupois system
1. Volume
- 1 fluidrachm = 60 min.
- 1 fluid ounce = 8 fl. dr. = 480 min.
- 1 pint = 16 fl. oz. = 7680 min.
- 1 quart = 2 pt. = 32 fl. oz.
- 1 gallon = 4 qt. = 128 fl. oz.
2. Mass (weight)
The grain is common to both the apothecary and the avoirdupois systems.
- 437.5 grains (gr) = 1 ounce (oz)
- 16 ounces (7000 grains) = 1 pound (lb)
Exact equivalents
Exact equivalents are used for the conversion of specific quantities in pharmaceutical formulas and prescription compounding.
1. Length
- 1 meter (m) = 39.37 in.
- 1 inch (in) = 2.54 cm.
2. Volume
- 1 ml = 16.23 minims (♍)
- 1 ♍ = 0.06 mL
- 1 fÊ’ = 3.69 mL
- 1 f ℥ = 29.57 mL
- 1 pt = 473 mL
- 1 gal (U.S.) = 3785 mL
3. Mass
- 1 g = 15.432 gr
- 1 kg = 2.20 lb (avoir.)
- 1 gr = 0.065 g or 65 mg
- 1 oz (avoir.) = 28.35 g
- 1℥ (apoth.) = 31.1 g
- 1 lb (avoir.) = 454 g
- 1 lb (apoth.) = 373.2 g
4. Other equivalents
- 1 oz (avoir.) = 437.5 gr
- 1℥ (apoth.) = 480 gr
- 1 gal (U.S.) = 128 fl℥
- 1 fl℥ (water) = 455 gr
- 1 gr (apoth.) = 1 gr (avoir.)
Approximate equivalents
Physicians may use approximate equivalents to prescribe the dose quantities using the metric and apothecary systems of weights and measures, respectively. Household units are often used to inform the patient of the size of the dose. In view of the almost universal practice of using an ordinary house -hold teaspoon to administer medication, a teaspoon may be considered 5 mL. However, when accurate measurement of a liquid dose is required, the USP recommends the use of a calibrated oral syringe or dropper.
- 1 fluid dram = 1 teaspoonful = 5 mL
- 4 fluid ounces = 120 mL
- 8 fluid ounces = 1 cup = 240 mL
- 1 grain = 65 mg
- 1 kg = 2.2 pounds (lb)
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