Other Units In Common Use

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.

Some examples of common (non-SI) units

Units	Abbr.	Used to measure ...	Notes
tonne stone pound ounce	t st lb oz	mass / weight	1 tonne = 1000 kg Not to be confused with the ton which is 2240 pounds 1 lb = 2.2 kg
atmosphere millimetre of mercury millibar	atm mm Hg mb	pressure	1 atm = 10^5 Pa (* see note below) 1 atm = 76 mm Hg 1 mb = 100 Pa (a meteorological unit)
litre cup teaspoon	L tspn	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 the knot	k/h (kph) kn	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)
minute hour day year	min h	time	These are useful multiples of the second. Our system of time is based on multiples of 60 for historical reasons only.

Notes

• Pa is the abbreviation for Pascal, which is the SI unit for pressure. One Pascal is one newton per square metre (1 Pa = 1 N/m^2). The newton (N) is itself a shorthand notation for the SI unit of force.

  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.

• J is the abbreviation for Joule, a unit of energy or work. One joule is the amount of work required to apply a force of one newton over a distance of one metre. In other words, 1 J = 1 Nm = kg m^2/s^2. The watt (W) is an older measure of energy and does not convert so nicely to SI units (i.e. there is a conversion factor involved).

• Some of these units (such as minutes) are part of the metric system, however they are not SI units, as they are not derived from base units multiplied by powers of ten.