Apart from the SI units we have discussed so far, there are some other units in common usage. These are so well-known thatthey are often used in place of the appropriate SI unit. Sometimes they are simply more practical (for example, we don't measure temperature in kelvins, since using degrees celsius leads to smaller numbers), or they may simply be the historical units in a specialised field.
|Units||Abbr.||Used to measure ...||Notes|
|mass / weight||1 tonne = 1000 kg|
Not to be confused with the ton which is 2240 pounds
1 lb = 2.2 kg
millimetre of mercury
|pressure||1 atm = 10^5 Pa (* see note below)|
1 atm = 76 mm Hg
1 mb = 100 Pa (a meteorological unit)
|volume (capacity)||1 L = 10^(-3) m^3|
1 cup = 250 mL
1 tspn = 5 mL
|degree Celsius||° C||temperature||0 °C = 273.16 K|
A change of one degree Celsius is equal to a change of one kelvin.
|Nautical mile||n mile||distance||1 n mile = 1.852 km|
|kilometres per hour|
|velocity||More common than m/s|
1 kn = 1 n mile / h
|hectare||ha||area||1 ha = 10 000 m^2|
|kilowatt hour||kW||energy||1 kW h = 3.60 MJ (* see note below)|
|time||These are useful multiples of the second. Our system of time is based on multiples of 60 for historical reasons only.|
1 N = 1 kg m/s^2, is the force required to accelerate a mass of one kilogram at a rate of one metre per second every second.