UNITS AND MEASUREMENTS

Pharmaceutical Calculations and Techniques: Units and Measurements

1. International System of Units (SI)

Formerly called the metric system; abbreviation SI comes from the French name Le Système International d'Unités.

Internationally recognized decimal system of weights and measures.

Pharmaceutically relevant measurements: Length, Weight, Volume.

Prefixes and Relative Values of the SI System

Prefix

Meaning

Exponential Value

Subdivisions

atto-

one quintillionth

10^{-18}

femto-

one quadrillionth

10^{-15}

pico-

one trillionth

10^{-12}

nano-

one billionth

10^{-9}

micro-

one millionth

10^{-6}

milli-

one thousandth

10^{-3}

centi-

one hundredth

10^{-2}

deci-

one tenth

10^{-1}

Multiples

deka-

10 times

10^1

hecto-

100 times

10^2

kilo-

1,000 times

10^3

myria-

10,000 times

10^4

mega-

1 million times

10^6

giga-

1 billion times

10^9

tera-

1 trillion times

10^{12}

peta-

1 quadrillion times

10^{15}

exa-

1 quintillion times

10^{18}

SI Dosing & Practice Guidelines

Capitalization: Unit names and symbols generally are not capitalized EXCEPT when used at the beginning of a sentence or in headings.

Exception: The symbol for liter (L) may be capitalized or lowercase (4\text{ L}\ or\ 4\text{ l},4\text{ mm},4\text{ g}).

Decimal Points:

In the U.S., decimal points are placed on the line with the value (4.5\text{ mL}).

In non-U.S. countries, a comma or raised dot may be used (4,5\text{ mL}or4\cdot5\text{ mL}).

Symbols vs. Spelled-out terms: Do not combine symbols with spelled-out terms (3\text{ mg/mL}\ or\ 3\text{ mg/milliliter}).

Microgram Abbreviation: Two symbols exist: \mu\text{g} (SI) and \text{mcg} (widely preferred in pharmacy practice, package labeling, and prescription writing to avoid errors).

Note: The term gamma (\gamma) is customarily used for microgram in biochemical literature.

Cubic Centimeters: 1\text{ cm}^3 is considered equivalent to 1\text{ mL}. While used in practice, the abbreviation "\text{cc}" is not an official SI symbol.

Leading & Trailing Zeros:

Always use a leading zero before a decimal point to prevent errors: 0.18\text{ g}(Correct)\ vs..18\text{ g}\ (Incorrect).

Never use a trailing zero following a whole number: 3\text{ mg}\ (Correct)\ vs.\ 3.0\text{ mg }(Incorrect).

SI Tables of Measure

Length (Primary Unit: Meter):

1\text{ kilometer (km)}=1,000\text{ meters}

1\text{ hectometer (hm)}=100\text{ meters}

1\text{ dekameter (dam)}=10\text{ meters}

1\text{ decimeter (dm)}=0.1\text{ meter}

1\text{ centimeter (cm)}=0.01\text{ meter}

1\text{ millimeter (mm)}=0.001\text{ meter}

1\text{ micrometer }(\mu\text{m})=0.000001\text{ meter}

1\text{ nanometer (nm)}=0.000000001\text{ meter}

Weight / Mass (Primary Unit: Kilogram / Gram in Pharmacy):

1\text{ kilogram (kg)}=1,000\text{ grams}

1\text{ hectogram (hg)}=100\text{ grams}

1\text{ dekagram (dag)}=10\text{ grams}

1\text{ decigram (dg)}=0.1\text{ gram}

1\text{ centigram (cg)}=0.01\text{ gram}

1\text{ milligram (mg)}=0.001\text{ gram}

1\text{ microgram }(\mu\text{g/mcg})=0.000001\text{ gram}

1\text{ nanogram (ng)}=0.000000001\text{ gram}

Volume (Primary Unit: Liter):

1\text{ kiloliter (kL)}=1,000\text{ liters}