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As described in the Units, the specification of units SHOULD follow the International System of Units (SI, abbreviated from the French Système international (d'unités)).

The CMIXF-12 convention for encoding units is RECOMMENDED to achieve maximum portability and limited variability of representation. If a CMIXF-12 representation of a unit is not possible, the unit can be declared as custom units and defined in an accompanying JSON file, as described in the units section. Earlier versions of the BIDS standard listed the following Unicode symbols, and these are still included for backwards compatibility:

  1. U+03BC (μ) or U+00B5 (µ)
  2. U+03A9 (Ω) or U+2126 (Ω)
  3. U+00B0 (°)

Note that for the first two entries in this list, two characters are permissible for each, but the first character in each entry is preferred, per Unicode rules (see the section on "Duplicated Characters" on page 11 in the unicode report).

It is RECOMMENDED that units be CMIXF-12 compliant or among these five Unicode characters. Please note the appropriate upper- or lower- casing when using CMIXF-12.

For cases that are unspecified by this appendix or the units section, the CMIXF-12 convention applies.

You can use the cmixf Python package to check whether your formatting is compliant.

Examples for CMIXF-12 (including the five unicode symbols mentioned above):

  1. Different formatting of "micro Volts":

    1. RECOMMENDED: uV or µV
    2. NOT RECOMMENDED: microV, µvolt or 1e-6V
  2. Combinations of units:

    1. RECOMMENDED: V/us for the Slew rate
    2. NOT RECOMMENDED: volts per microsecond

Unit table

Unit name Unit symbol Quantity name
meter m length
kilogram kg mass
liter L volume
second s time
ampere A electric current
kelvin K thermodynamic temperature
mole mol amount of substance
candela cd luminous intensity
radian rad angle
steradian sr solid angle
hertz Hz frequency
newton N force, weight
pascal Pa pressure, stress
joule J energy, work, heat
watt W power, radiant flux
coulomb C electric charge or quantity of electricity
volt V voltage (electrical potential), emf
farad F capacitance
ohm Ohm resistance, impedance, reactance
siemens S electrical conductance
weber Wb magnetic flux
tesla T magnetic flux density
henry H inductance
degree Celsius oC temperature relative to 273.15 K
lumen lm luminous flux
lux lx illuminance
becquerel Bq radioactivity (decays per unit time)
gray Gy absorbed dose (of ionizing radiation)
sievert Sv equivalent dose (of ionizing radiation)
katal kat catalytic activity



Prefix name Prefix symbol Factor
deca da 101
hecto h 102
kilo k 103
mega M 106
giga G 109
tera T 1012
peta P 1015
exa E 1018
zetta Z 1021
yotta Y 1024


Prefix name Prefix symbol Factor
deci d 10-1
centi c 10-2
milli m 10-3
micro u 10-6
nano n 10-9
pico p 10-12
femto f 10-15
atto a 10-18
zepto z 10-21
yocto y 10-24