Kilogram vs Kilogram-Force (kg vs kgf)

The kilogram (kg) is a unit of mass: how much matter an object contains. The kilogram-force (kgf) is a unit of force: the weight of a one-kilogram mass under standard gravity. By definition, 1 kgf = 9.80665 N exactly.

A bathroom scale measures the force your body presses on it, then shows the result in kilograms by assuming a value for gravity. On the Earth's surface a mass of 1 kg weighs close to 1 kgf, so the shortcut works for everyday use.

Older technical documents, tire charts and some tools still give forces or pressures in kgf or kgf/cm². When you meet these units, convert them to newtons or pascals before mixing them with SI values.

Gravitational acceleration ranges from about 9.78 m/s² at the equator to about 9.83 m/s² at the poles. The kilogram-force uses the fixed standard value of 9.80665 m/s², so it is a precise unit, even though the real weight of a 1 kg mass varies by about half a percent around the world.

To convert kilograms-force to newtons, multiply by 9.80665. Modern specifications usually use kilonewtons: 1 kN ≈ 101.97 kgf, and 1 kgf ≈ 2.20462 lbf.

If the quantity is an amount of matter (a sack of flour, a person), it is a mass and the unit is the kilogram. If it is a pull or push, such as the tension in a cable or the thrust of an engine, it is a force and the SI unit is the newton. Kilogram-force is an older engineering unit for force and is not part of the SI.

To change a kilogram-force figure into newtons, multiply by 9.80665, which gives for example 196.133 N for 20 kgf.

The kilogram-force is also called the kilopond (kp), mainly in older German and European documents. The two names mean exactly the same unit.

Heavy loads are sometimes given in metric tonnes-force, where 1 tf = 1000 kgf. That is 9.80665 kN. Lifting gear and crane charts that say "tonnes" usually mean the mass that can be lifted, not a force, so read the label carefully.