There are many thousands of measurement units outside the International System of Units (SI). A number of these non-SI units have been given the special status of being accepted for use with the SI.
Some of these units, such as the litre, have been given this status because of their compatibility with the SI and their usefulness in practical everyday applications. A few, such as the astronomical unit, have been included because they are still widely used in certain fields of science and are expected to continue to be used for many years. Three units used to express logarithmic ratios are also included.
SI prefixes can be used with several of these units, but not, for example, with the non-SI units of time. Units derived from coherent SI units and units accepted for use with the SI, such as km/h, are also accepted for use with the SI.
Non-SI units accepted for use with the SI
Over a period of many years, the number of non-SI units accepted for use with the SI has been reduced. Currently there are fifteen – the full list is given below.
| Unit name | Symbol | Definition and what is measured |
| minute | min | Time = 60 s |
| hour | h | Time = 60 min = 3 600 s |
| day | d | Time = 24 h = 86 400 s |
| hectare | ha | Area = 1 hm2 (square hectometre) = 10 000 m2 Used for land areas. |
| litre | l or L | Volume = 1 dm3 (cubic decimetre) = 1000 cm3 = 0.001 m3 The familiar litre in which we buy petrol, etc. Note that (for all practical everyday purposes) 1 litre of pure liquid water has a mass of 1 kg. |
| tonne | t | Mass = 1 Mg (megagram) = 1000 kg Used in transport, vehicle masses and weight limits, etc. Also the approximate mass of one cubic metre of water. (see litre above) |
| degree | ° | Angle = (π⁄180) rad |
| minute | ′ | Angle = (π⁄10 800) rad |
| second | ″ | Angle = (π⁄648 000) rad |
Used in certain fields of science
| Unit name | Symbol | Definition and what is measured |
| astronomical unit | au | Length = 149 597 870 700 m = 149.597 870 7 Gm Used in astronomy. |
| electronvolt | eV | Energy = 1.602 176 634 × 10-19 J Used in particle physics. |
| dalton | Da | Mass = 1.660 539 068 92(52) × 10-27 kg Used in physics and chemistry. The dalton (Da) and the unified atomic mass unit (u) are alternative names (and symbols) for the same unit, equal to 1⁄12 of the mass of a free carbon-12 atom, at rest and in its ground state. |
Logarithmic ratios
| Unit name | Symbol | Definition and what is measured |
| neper | Np | Logarithmic ratio Used to express the values of quantities whose numerical values are based on the use of the neperian (or natural) logarithm, ln = loge. |
| bel | B | Logarithmic ratio Used to express the values of logarithmic ratio quantities whose numerical values are based on the decadic logarithm, log10. |
| decibel | dB | Logarithmic ratio Used to express the values of logarithmic ratio quantities whose numerical values are based on the decadic logarithm, log10. Typically used to express the loudness of sound. |
Other non-SI units
The table below lists some of the non-SI units that have previously been accepted for use with the SI, but are now no longer accepted for use with the SI.
| Unit name | Symbol | Definition and what is measured |
| bar | bar | Pressure = 100 kPa = 1000 hPa In weather maps, the use of the millibar (mbar) has been superceded by the hectopascal (hPa) which is numerically equal to the millibar. |
| nautical mile | nmi NM |
Length = 1852 m Used in marine and aerial navigation.. |
| knot | kn | Speed = (1852⁄3600) m/s Used in marine and aerial navigation. |
| ångström | Å | Length = 0.1 nm = 10-10 m |
UK Metric Association 