How to Use SI Units in Technical Writing

Definitions of SI units and examples of how to properly use them in scientific and technical writing. Jennifer Kirchhoff, PhD

PhD, Physics
Florida State University

See more articles by Jennifer Kirchhoff, PhD

Le Système international d’unités (SI), or International System of Units, is used in the metric system and as the accepted convention in technical and scientific writing. There are seven base units and twenty-two defined derived units (thus named because they are derived from the base units), but a number of other units can be formed using this system. The standards of SI units (used to define the base units) are based on the meter-kilogram-second (mks) system rather than on the centimeter-gram-second (cgs) system.

In writing, the names of SI units are always written in lowercase. However, the symbols of units named after a person are capitalized (e.g., ampere and A). These symbols are not abbreviations, so periods are not required. Additionally, a space should always be included between a number and the SI unit, except for the degree symbol. Italics are usually not used with SI units.

Base units

The base units are the building blocks of the SI system. There are seven base units and symbols for the seven base quantities, which are assumed to be independent. Although not its official SI symbol, second is sometimes abbreviated as sec in technical writing.

Base Quantity Base Unit Symbol Definition
length meter m distance traveled by light in vacuum in 1/299,792,456 seconds
mass kilogram kg mass of the international prototype kilogram
time second s duration of 9,192,631,770 periods of the standard Cs-133 transition
electric current ampere A constant current in two infinitely long parallel conductors of negligible cross-section placed 1 m apart that would produce a force of 2x10-7 newtons per meter length
thermodynamic temperature kelvin K 1/273.16 of the thermodynamic temperature of the triple point of water
amount of substance mole mol amount that contains as many elementary entities as there are atoms in 0.012 kg of carbon-12
luminous intensity candela cd luminous intensity of a source with a frequency of 540x1012 hertz and a radiant intensity of 1/683 watts per steradian

Derived units

There are 22 derived units that have been given special names and symbols for the sake of convenience. All derived units are combinations of base units (products, powers, etc.), although the radian and steradian are actually unitless.

Quantity Name Symbol Base Unit Representation
solid angle steradian sr m2/m2 (unitless)
frequency hertz Hz 1/s
force/weight newton N kg*m/s2
pressure/stress pascal Pa kg/(m*s2) or N/m2
energy/work/heat joule J kg*m2/s2 or N/m
power/radiant flux watt W kg*m2/s3 or J/s
electric charge coulomb C s*A
voltage volt V kg*m2/(s3*A) or W/A
electric capacitance farad F s4*A2/(kg*m2) or C/V
electric resistance ohm Ω kg*m2/(s3*A2) or V/A
electric conductance siemens S s3*A2/(kg*m2) or A/V
magnetic flux weber Wb kg*m2/(s2*A) or V*s
magnetic field strength tesla T kg/(s2*A) = Wb/m2
inductance henry H kg*m2/(s2*A2) = Wb/A
temperature degree Celsius °C K-273.15
luminous flux lumen lm cd*sr
illuminance lux lx cd*sr/m2 or lm/m2
absorbed dose gray Gy m2/s2 or J/kg
equivalent dose sievert Sv m2/s2 or J/kg
catalytic activity katal kat mol/s

Other units

While there are 22 defined derived units, there are many more combinations that can be made with the seven base units. Some common derived quantities that have not been given special names and symbols include area (m2), volume (m3), velocity (m/s), acceleration (m/s2, including the unit g, used to indicate multiples of the gravity acceleration), wave number (1/m) and density (kg/m3).

Additionally, there are units that are often used in papers with SI units that are not SI units themselves. The common non-SI units accepted for use with SI units include units of time (minute = 60 s, min; hour = 3,600 s, h or hr; and day = 86,400 s, d), volume (liter = 10-3 m3, whose symbol is an exception to the capitalization rule: L not l), and pressure (bar = 105 Pa, bar).

We hope that today’s tip provides clarification about how to use the correct abbreviations for SI units in your writing. If you have questions, please write to us at [email protected]. Thanks for reading!

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