National Physical Laboratory

Non-SI Units

There are certain units, which are accepted for use with the SI. It includes units which are in continuous everyday use, in particular the traditional units of time and of angle, together with a few other units which have assumed increasing technical importance.

There are also units which are currently accepted for use with the SI to satisfy the needs of commercial, legal and specialist scientific interests or are important for the interpretation of older texts.

Non-SI units accepted for use with the International System
Name Symbol Value in SI Units
minute min 1 min = 60 s
hour (1) h 1 h = 60 min = 3600 s
day d 1 d = 24 h = 86 400 s
degree of arc (2) ° 1° = (π/180) rad
minute of arc ' 1' = (1/60)° = (π/10 800) rad
second of arc " 1" = (1/60)' = (π/648 000) rad
litre (3) l, L 1 l = 1 dm3 = 10-3 m3
tonne (4, 5) t 1 t = 103 kg
neper (6, 8) Np 1 Np = 1
bel (7, 8) B 1 B = (1/2) ln 10 (Np)(9)


Notes:

  1. The symbol of this unit is included in Resolution 7 of the 9th CGPM (1948; CR, 70).
  2. ISO 31 recommends that the degree be subdivided decimally rather than using the minute and second.
  3. This unit and the symbol l were adopted by CIPM in 1879 (PV, 1879, 41). The alternative symbol, L, was adopted by the 16th CGPM (1979, Resolution 6; CR, 101 and Metrologia, 1980, 16, 56-57) in order to avoid the risk of confusion between the letter l and the number 1. The present definition of the litre is given in Resolution 6 of the 12th CGPM (1964; CR, 93).
  4. This unit and its symbol were adopted by the CIPM in 1879 (PV, 1879, 41).
  5. In some English-speaking countries this is called 'metric ton'.
  6. The neper is used to express values of such logarithmic quantities as field level, power level, sound pressure level, and logarithmic decrement. Natural logarithms are used to obtain the numerical values of quantities expressed in nepers. The neper is coherent with the SI, but not yet adopted by the CGPM as an SI unit. For further information see International Standard ISO 31.
  7. The bel is used to express values of such logarithmic quantities as field level, power level, sound pressure level, and attenuation. Logarithms to base ten are used to obtain the numerical values of quantities expressed in bels. The submultiple decibel, dB, is commonly used. For further information see International Standard ISO 31.
  8. In using these units it is particularly important that the quantity be specified. The unit must not be used to imply the quantity.
  9. Np is enclosed in parentheses because, although the neper is coherent with the SI, it has not yet been adopted by the CGPM.