AMA Manual of Style - Stacy L. Christiansen, Cheryl Iverson 2020
SI Units
Units of Measure
The English system of measurements appears to be the fever dream of a crazy person.
Samuel Arbesman^{1}
The presentation of quantitative scientific information is an integral component of biomedical publication. Accurate communication of scientific knowledge and presentation of numerical data require a scientifically informative system for reporting units of measure.
17.1 SI Units.
The International System of Units (le Système International d’Unités or SI) represents a modified version of the metric system that has been established by international agreement and currently is the official measurement system of most nations of the world.^{2}^{,}^{3} The SI promotes uniformity of quantities and units, minimizes the number of units and multiples used in other measurement systems, and can express virtually any measurement in science, medicine, industry, and commerce.
In 1977, the World Health Organization recommended the adoption of the SI by the international scientific community. Since then, many biomedical publications throughout the world have adopted SI units as their preferred and primary method for reporting scientific measurements. Although SI units have long been the dominant measurement system used in science,^{3} in the United States most physicians and other health care professionals use conventional units for many common clinical measurements (eg, blood pressure), and many clinical laboratories report most laboratory values by means of conventional units. Accordingly, some biomedical publications, including JAMA Network journals, have adopted an approach for reporting units of measure that includes a combination of conventional units and SI units (see 17.5, Conventional Units and SI Units in JAMA Network Journals). Authors, scientists, clinicians, editors, and others involved in preparing and processing manuscripts for biomedical publication should be familiar with appropriate use of units of measure and should ensure that the presentation and reporting of scientific information are clear and accurate, including any necessary conversion from conventional units to SI units or vice versa.
17.1.1 Base Units.
The SI is based on 7 fundamental units (base units) that refer to 7 basic quantities of measurement (Table 17.1-1). These units form the structure from which other measurement quantities are composed.
Table 17.1-1. The International System of Units (SI) Base Units and Symbols
Quantity |
Base unit name |
SI unit symbol |
Length |
meter |
m |
Mass |
kilogram |
kg |
Time |
second |
s |
Electric current |
ampere |
A |
Thermodynamic temperature |
kelvin |
K |
Luminous intensity |
candela |
Cd |
Amount of substance |
mole |
mol |
Although not included among the 7 base units, the liter is widely used in the SI as a fundamental measure of capacity or volume. The liter is the recommended unit for measurement of volume for liquids and gases, whereas the cubic meter is the SI unit of volume for solids. Although the kelvin is the SI unit for thermodynamic temperature, Celsius is used with the SI for temperature measurement in biomedical settings.
17.1.2 Derived Units.
Other SI measurement quantities are referred to as derived units and are expressed as products or quotients of the 7 base units. Certain derived SI units have special names and symbols and may be used in algebraic relationships to express other derived units (Table 17.1-2).
Table 17.1-2. The International System of Units (SI) Derived Units and Symbols
Quantity |
Name |
SI symbol |
Derivation from base unit |
Area |
square meter |
m^{2} |
m^{2} |
Volume |
cubic meter |
m^{3} |
m^{3} |
Speed, velocity |
meter per second |
m/s |
m/s |
Density, mass density |
kilogram per cubic meter |
kg/m^{3} |
kg/m^{3} |
Specific volume |
cubic meter per kilogram |
m^{3}/kg |
m^{3}/kg |
Concentration |
mole per cubic meter |
mol/m^{3} |
mol/m^{3} |
Frequency |
hertz |
Hz |
s^{−1} |
Force |
newton |
N |
kg · m · s^{−2} |
Pressure, stress |
pascal |
Pa |
kg · m^{−1} · s^{−2} (N/m^{2}) |
Work, energy |
joule |
J |
kg · m^{2} · s^{−2} (N · m) |
Luminous flux |
lumen |
lm |
m^{2} · m^{−2} · cd = cd |
Power, radiant flux |
watt |
W |
m^{2} · kg · s^{−3} (J/s) |
Electric potential |
volt |
V |
m^{2} · kg · s^{−3} · A^{−1} |
Electric charge |
coulomb |
C |
A · s |
Electric resistance |
ohm |
Ω |
m^{2} · kg · s^{−3} · A^{−2} (V/A) |
Capacitance |
farad |
F |
m^{−2 ·} kg^{−1} · s^{4} · A^{2} (C/V) |
Magnetic flux |
weber |
Wb |
m^{2} · kg ·s^{−2} · A^{−1} (V · s) |
Magnetic flux density |
tesla |
T |
kg · s^{−2} · A^{−1} (Wb/m^{2}) |
Inductance |
henry |
H |
m^{2} · kg ·s^{−2} · A^{−2} |
17.1.3 Prefixes
Prefixes are combined with base units and derived units to form multiples of SI units. The factors designated by prefixes are powers of 10, and most prefixes involve exponents that are simple multiples of 3, thereby facilitating conversion procedures using successive multiplications by 10^{3} or 10^{−3} (Table 17.1-3).
Table 17.1-3. The International System of Units Prefixes
Factor |
Prefix |
Symbol |
10^{24} |
yotta |
Y |
10^{21} |
zetta |
Z |
10^{18} |
exa |
E |
10^{15} |
peta |
P |
10^{12} |
tera |
T |
10^{9} |
giga |
G |
10^{6} |
mega |
M |
10^{3} |
kilo |
k |
10^{2} |
hecto |
h |
10^{1} |
deka (deca) |
da |
10^{−1} |
deci |
d |
10^{−2} |
centi |
c |
10^{−3} |
milli |
m |
10^{−6} |
micro |
μ |
10^{−9} |
nano |
n |
10^{−12} |
pico |
p |
10^{−15} |
femto |
f |
10^{−18} |
atto |
a |
10^{−21} |
zepto |
z |
10^{−24} |
yocto |
y |
Compound prefixes formed by the combination of 2 or more SI prefixes generally are not used. It is preferable to use an expression with a single prefix.
Avoid: |
mμm (millimicrometer) |
Better: |
nm (nanometer) |
The kilogram is the only SI base unit with a prefix as part of its name and symbol (kg). However, because compound prefixes are not recommended, prefixes relating to mass are combined with gram (g) rather than kilogram (kg).
Avoid: |
μkg (microkilogram) |
Better: |
mg (milligram) |
Note: For the abbreviation μ (micro), some use the abbreviation “mcg” instead, specifically when communicating medical information, owing to the possibility that the prefix μ (micro) might be misread as the prefix m (milli), resulting in a thousand-fold overdose. JAMA Network journals use the abbreviation μ.