How Many? A Dictionary of Units of Measurement © Russ Rowlett and the University of North Carolina at Chapel Hill |
In addition to the seven base units, the General Conference on Weights and Measures has approved 22 names for SI units defined as combinations of the base units. Within the SI, these units are the named derived units.
Following are links to the definitions of these 22 units. The third column shows how each unit is derived from the preceding units, and the fourth column gives the formal equivalence of the unit in terms of the base units.
Derived Unit | Measures | Derivation | Formal Definition |
hertz (Hz) | frequency | /s | s^{-1} |
newton (N) | force | kg·(m/s^{2}) | kg·m·s^{-2} |
pascal (Pa) | pressure | N/m^{2} | kg·m^{-1}·s^{-2} |
joule (J) | energy or work | N·m | kg·m^{2}·s^{-2} |
watt (W) | power | J/s | kg·m^{2}·s^{-3} |
coulomb (C) | electric charge | A·s | A·s |
volt (V) | electric potential | W/A | kg·m^{2}·s^{-3}·A^{-1} |
farad (F) | electric capacitance | C/V | kg^{-1}·m^{-2}·s^{4}·A^{2} |
ohm (omega) | electric resistance | V/A | kg·m^{2}·s^{-3}·A^{-2} |
siemens (S) | electric conductance | A/V | kg^{-1}·m^{-2}·s^{3}·A^{2} |
weber (Wb) | magnetic flux | V·s | kg·m^{2}·s^{-2}·A^{-1} |
tesla (T) | magnetic flux density | Wb/m^{2} | kg·s^{-2}·A^{-1} |
henry (H) | inductance | Wb/A | kg·m^{2}·s^{-2}·A^{-2} |
degree Celsius (°C) | temperature | K - 273.15 | K |
radian (rad) | plane angle | m·m^{-1} | |
steradian (sr) | solid angle | m^{2}·m^{-2} | |
lumen (lm) | luminous flux | cd·sr | cd·sr |
lux (lx) | illuminance | lm/m^{2} | cd·sr·m^{-2} |
becquerel (Bq) | activity | /s | s^{-1} |
gray (Gy) | absorbed dose | J/kg | m^{2}·s^{-2} |
sievert (Sv) | dose equivalent | Gy·(multiplier) | m^{2}·s^{-2} |
katal (kat) | catalytic activity | mol/s | mol·s^{-1} |
The term derived unit covers any algebraic combination of the base units, but it is only the 22 combinations listed above that have approved special names. For example, the SI derived unit of momentum (mass times velocity) has no special name; momentum is stated in kilogram meters per second (kg·m/s) or in newton seconds (N·s). A few SI derived units do have special names that have been defined but not approved. Here are some examples:
Derived Unit | Measures | Derivation |
stere (st) | volume | m^{3} |
diopter (dpt) | refractive power | m^{-1} |
thermal ohm | thermal resistance | K/W |
poiseuille (Pl) | dynamic viscosity | Pa·s |
rayl | sound impedance | Pa·s/m |
acoustic ohm | sound resistance | Pa·s/m^{3} |
daraf | electric elastance | F^{-1} |
talbot | luminous energy | lm·s |
nit (nt) | luminance | cd/m^{2} |
molal | chemical concentration | mol/kg |
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You are welcome to email the author (rowlett@email.unc.edu) with comments and suggestions.
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Latest update July 14, 2004