The Syntax for Unit Strings

Conversion between units systems relies on the use of a specific syntax for the Unit attribute. If the value of the Unit attribute does not conform to this syntax, then an error will be reported if an attempt is made to use it to determine an inter-unit MappingMapping (this will never happen if the Unit attribute is passive).

The adopted syntax is that described in FITS-WCS paper I "Representation of World Coordinate in FITS" by Greisen & Calabretta. We distinguish here between “basic” units and “derived” units: derived units are defined in terms of other units (either derived or basic), whereas basic units have no such definitions. Derived units may be represented by their own symbol (e.g. “Jy”—the Jansky) or by a mathematical expression which combines other symbols and constants to form a definition of the unit (e.g. “km/s”—kilometres per second). Unit symbols may be prefixed by a string representing a standard multiple or sub-multiple.

In addition to the unit symbols listed in FITS-WCS Paper I, any other arbitrary unit symbol may be used, with the proviso that it will not be possible to convert between Frames using such units. The exception to this is if both Frames refer to the same unknown unit string. For instance, an axis with unknown unit symbol "flop" could be converted to an axis with unit "Mflop" (Mega-flop).

Unit symbols (optionally prefixed with a multiple or sub-multiple) can be combined together using a limited range of mathematical operators and functions, to produce new units. Such expressions may also contain parentheses and numerical constants (these may optionally use “scientific” notation including an “E” character to represent the power of 10).

The following tables list the symbols for the basic and derived units which may be included in a units string, the standard prefixes for multiples and sub-multiples, and the strings which may be used to represent mathematical operators and functions.


Basic units
Quantity Symbol FullFull Name
length m metre
mass g gram
time s second
plane angle rad radian
solid angle sr steradian
temperature K Kelvin
electric current A Ampere
amount of substance mol mole
luminous intensity cd candela


Derived units
Quantity Symbol Full Name Definition
area barn barn 1.0E-28 m**2
area pix pixel
area pixel pixel
electric capacitance F Farad C/V
electric charge C Coulomb A s
electric conductance S Siemens A/V
electric potential V Volt J/C
electric resistance Ohm Ohm V/A
energy J Joule N m
energy Ry Rydberg 13.605692 eV
energy eV electron-Volt 1.60217733E-19 J
energy erg erg 1.0E-7 J
events count count
events ct count
events ph photon
events photon photon
flux density Jy Jansky 1.0E-26 W /m**2 /Hz
flux density R Rayleigh 1.0E10/(4*PI) photon.m**-2 /s/sr
flux density mag magnitude
force N Newton kg m/s**2
frequency Hz Hertz 1/s
illuminance lx lux lm/m**2
inductance H Henry Wb/A
length AU astronomical unit 1.49598E11 m
length Angstrom Angstrom 1.0E-10 m
length lyr light year 9.460730E15 m
length pc parsec 3.0867E16 m
length solRad solar radius 6.9599E8 m
luminosity solLum solar luminosity 3.8268E26 W
luminous flux lm lumen cd sr
magnetic field G Gauss 1.0E-4 T
magnetic flux Wb Weber V s
mass solMass solar mass 1.9891E30 kg
mass u unified atomic mass unit 1.6605387E-27 kg
magnetic flux density T Tesla Wb/m**2
plane angle arcmin arc-minute 1/60 deg
plane angle arcsec arc-second 1/3600 deg
plane angle mas milli-arcsecond 1/3600000 deg
plane angle deg degree pi/180 rad
power W Watt J/s
pressure, stress Pa Pascal N/m**2
time a year 31557600 s
time d day 86400 s
time h hour 3600 s
time yr year 31557600 s
time min minute 60 s
D Debye 1.0E-29/3 C.m


Prefixes for multiples & sub-multiples
Sub-multiple Name Prefix Sub-multiple Name Prefix
$10^{-1}$ deci d $10$ deca da
$10^{-2}$ centi c $10^{2}$ hecto h
$10^{-3}$ milli m $10^{3}$ kilo k
$10^{-6}$ micro u $10^{6}$ mega M
$10^{-9}$ nano n $10^{9}$ giga G
$10^{-12}$ pico p $10^{12}$ tera T
$10^{-15}$ femto f $10^{15}$ peta P
$10^{-18}$ atto a $10^{18}$ exa E
$10^{-21}$ zepto z $10^{21}$ zetta Z
$10^{-24}$ yocto y $10^{24}$ yotta Y


Mathematical operators & functions
String Meaning
sym1 sym2 multiplication (a space)
sym1*sym2 multiplication (an asterisk)
sym1.sym2 multiplication (a dot)
sym1/sym2 division
sym1**y exponentiation ($y$ must be a numerical constant)
sym1^y exponentiation ($y$ must be a numerical constant)
log(sym1) common logarithm
ln(sym1) natural logarithm
exp(sym1) exponential
sqrt(sym1) square root