Deca (International spelling as used by the International Bureau of Weights and Measures;[1] symbol: da) or deka (American spelling[2]) is a decimal unit prefix in the metric system denoting a factor of ten. The term is derived from the Greek déka (δέκα) meaning ten.

The prefix was a part of the original metric system in 1795. It is not in very common usage, although the decapascal is occasionally used by audiologists. The decanewton is also encountered occasionally, probably because it is an SI approximation of the kilogram-force. Its use is more common in Central Europe. In German, Polish, Czech, Slovak, and Hungarian, deka (or deko) is common, and used in self-standing form, always meaning decagram. A runway number typically indicates its magnetic azimuth in decadegrees.

Before the symbol as an SI prefix was standardized as da with the introduction of the International System of Units in 1960, various other symbols were more common, such as dk (e.g., UK and Austria), D (e.g., Germany, Eastern Europe), and Da. For syntactical reasons, the HP 48, 49, 50 series, as well as the HP 39gII and Prime calculators use the unit prefix D.[3][4][5]


  • The blue whale is approximately 30 metres or 3 decametres in length.[6]
PrefixBase 10 Decimal Adoption
[nb 1]
quettaQ1030 10000000000000000000000000000002022[7]
ronnaR1027 1000000000000000000000000000
yottaY1024 10000000000000000000000001991
zettaZ1021 1000000000000000000000
exaE1018 10000000000000000001975
petaP1015 1000000000000000
teraT1012 10000000000001960
gigaG109 1000000000
megaM106 10000001873
kilok103 10001795
decid10−1 0.11795
centic10−2 0.01
millim10−3 0.001
microμ10−6 0.0000011873
nanon10−9 0.0000000011960
picop10−12 0.000000000001
femtof10−15 0.0000000000000011964
attoa10−18 0.000000000000000001
zeptoz10−21 0.0000000000000000000011991
yoctoy10−24 0.000000000000000000000001
rontor10−27 0.0000000000000000000000000012022
quectoq10−30 0.000000000000000000000000000001
  1. Prefixes adopted before 1960 already existed before SI. The introduction of the CGS system was in 1873.

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