International System of Units

Systems of Measurements used in Pharmacy

In Pharmacy, three notable systems of measurement exist:

1. Metric or International System of Units
2. Apothecaries’ System
3. Avoirdupois System

In practice, the Metric or International System of Units is the only acceptable system of measurement in pharmacy and medicine; though knowledge of the other two major systems ( Apothecaries’ and Avoirdupois ) is essential because of their heavy use in commerce and daily life.

Metric or International Systems of Units

This system is the internationally recognized decimal system of weights and measurements.

This system is advantageous for its use of a definitive or primary unit for measurement; variations can be made through the use of appropriate prefixes.

This system is the system of choice for pharmacy and medicine; since it is a decimal system, derived units can be generated as a 10th power multiple of the base unit.

See also: Units and Conversions

The Correct SI Metric Symbols for Pharmacy

Symbol	For
$\text{m}$	meter
$\text{cm}$	centimeter
$\text{mm}$	millimeter
$\text{L}$ or $\text{l}$	liter
$\text{mL}$ or $\text{ml}$	milliliter
$\text{kg}$	kilogram
$\text{g}$	gram
$\text{mg}$	milligram
$\mu \text{g}$ ( $\text{mcg}$ for drug orders )	microgram
$\text{h}$	hour
$\text{s}$	second
${\text{cm}}^3$ ( $\text{mL}$ for drug orders)	cubic centimeter
$°\text{C}$	degrees Celsius
$\text{K}$	kelvin

Prescription Writing Style Using the SI Units

Apothecary System

This system was commonly used in the past by pharmacists and physicians and the standard system of weights and measures for dispensing medications.

Despite being largely replaced by SI Units, it is still commonly used in a variety of products.

Writing Apothecary Units

In the Apothecary System of Units, quantities are written with the units first, followed by the quantity written in roman numerals.

Example: $\text{3 fluid dram}=\text{flʒ}\text{ iii}$

Apothecaries’ Measures of Volume and Weight

Measures of Volume

Unit	Equivalent Unit
60 minims ($\text{min lx}$ )	1 fluid dram ( $\text{flʒ i}$ )
8 fluid drams ( $\text{flʒ viii}$ )	1 fluid ounce ( $\text{fl℥ i}$ or $1\text{fl oz}$)
16 fluid ounces ( $\text{fl℥ xvi}$ or $16\text{fl oz}$)	1 pint ( $\text{pt i}$ or $\text{O i}$ )
2 pints ( $\text{pt ii}$ or $\text{O ii}$ )	1 quart ( $\text{qt i}$ )
4 quarts ( $\text{qt iv}$ )	1 gallon ( $1\text{gal }$ or $\text{C i}$ )

Measures of Weight

Unit	Equivalent Unit
20 grains ( $\text{gr xx}$ )	1 scruple ( $\text{℈ i}$ )
3 scruples ( $\text{℈ iii}$ )	1 dram ( $\text{ʒ i}$ )
8 drams ( $\text{ʒ viii}$ )	1 ounce ( $\text{℥ i}$ )
12 ounces ( $\text{℥ xii}$ )	1 pound ( $\text{℔ i}$ )

Avoirdupois System

This is another system of measurement like the Metric and Apothecaries’ System, but this system is just a system of weight measurements.

Just like the Apothecaries’ system, the base unit is the grain ( $gr$ ).

The Avoirdupois ounce and pound differ in weight and symbols from those of the Apothecaries’ system.

The Avoirdupois pound is the pound we are all accustomed to in our daily lives.

The Avoirdupois pound is the weight measure in which bulk chemicals and over the counter pharmaceuticals are bought and sold; whereas the Apothecaries’ ounce is only used in prescriptions or medication orders.

Avoirdupois Measurements of Weight

Unit	Equivalent Unit
437.5 grains ( $437.5\text{gr}$ )	1 ounce ( $1\text{oz}$ )
16 ounces ( $16oz$ )	1 pound ( $1\text{lb}$ )

Other Systems of Measurements

Measurments of Volume

Unit	Equivalent Unit
3 teaspoonfuls ( $3\text{t}$ or $3\text{tsp}$ )	1 tablespoonful ( $1\text{T}$ or $1\text{tbsp}$ )
2 tablespoonfuls ( $2\text{T}$ or $2\text{tbsp}$ )	1 fluid ounce ( $1\text{fl oz}$ )
8 fluid ounces ( $8\text{fl oz}$ )	1 cup ( $1\text{Cu}$ )

Measurements of Weight

Unit	Equivalent Unit
16 ounces ( $16\text{oz}$ )	1 pound ( $1\text{lb}$ )

Measures of Length

Unit	Equivalent Unit
12 inches ( $12\text{in}$ or $12^{\prime \prime }$ )	1 foot ( $1\text{ft}$ or $1^{\prime }$ )
3 feet ( $1\text{ft}$ or $1^{\prime }$ )	1 yard ( $1\text{yd}$ )

Conversion Equivalents Commonly Used in Pharmacy and Medicine

Conversion Equivalents of Volume

Unit	Equivalent Unit
1 drop ( $\text{gtt}$ )	0.05 milliliters
1 milliliter	20 drops
1 teaspoonful	5 milliliter
1 tablespoonful	15 milliliter
1 milliliter	16.23 minims
1 minim	0.06 milliliters
1 fluid dram	3.69 milliliters
1 fluid ounce	29.57 milliliters
1 pint	473 milliliters
1 quart	946 milliliters
1 gallon	3785 milliliters

Conversion Equivalents of Weight

Unit	Equivalent Unit
1 gram	15.432 grains
1 kg	2.20 pounds ( av )
1 grain	64.8 milligrams
1 grain ( av )	1 grain ( ap)
1 ounce ( av )	28.35 grams
1 pound ( av )	454 grams
1 pound ( av )	7000 grains
1 ounce ( ap )	31.1 grams
1 pound ( ap )	373 grams
1 pound ( ap )	5760 grains

Conversion Equivalents of Weight

Unit	Equivalent Unit
1 inch	2.54 centimeter
1 m	39.37 inches

Key Definitions

• International System of Units
  • also known as the Metric System of Units;
  • the only acceptable system of measurements in pharmacy and medicine;
• Apothecaries’ System
  • a system of weights and measures used by pharmacists in the past;
  • largely replaced but is still used;
• Avoirdupois System
  • a system of weight measurements;
  • borrows the pound and ounce unit from the Apothecaries’ System but with different values