SI Unit of Force
The International System of Units (SI) is widely used for trade, science, and engineering. The SI unit of force is the newton, symbol N. The base units relevant to force are:
- the metre, unit of length, symbol m,
- the kilogram, unit of mass, symbol kg,
- the second, unit of time, symbol s.
Force is defined as the rate of change of momentum. For an unchanging mass, this is equivalent to mass x acceleration. Thus 1 N = 1 kg·m·s-2.
Historically there have been a variety of units of force, and conversion factors to some of these are given in the table below. Exact conversions are shown in bold, others are quoted to seven significant figures.
Some force conversion factors from non-SI units:
| Unit | Symbol | Equivalent SI Value |
| dyne | dyn | 10.0 µN |
| grain-force | grf | 635.460 2 µN |
| gram-force | gf | 9.806 65 mN |
| poundal | pdl | 138.255 0 mN |
| ounce-force (avdp) | ozf | 278.013 9 mN |
| pound-force | lbf | 4.448 222 N |
| kilogram-force | kgf | 9.806 65 N |
| kilopond | kp | 9.806 65 N |
| sthène | sthène | 1.0 kN |
| kip (= 1 000 lbf) | kip | 4.448 222 kN |
| US ton-force (= 2 000 lbf) (short) | tonf (US) | 8.896 443 kN |
| tonne-force (= 1 000 kgf) (metric) | tf | 9.806 65 kN |
| UK ton-force (= 2 240 lbf) (long) | tonf (UK) | 9.964 016 kN |
The use of abbreviated forms for large and small numbers is encouraged by the SI system. SI prefixes represent multiples of 103 or 10‑3. A list of further SI units and SI prefixes can be found in the SI Units area.