Guide

Telling mass, weight and force apart

Kilograms and newtons are often treated as the same thing in everyday life. Physically they are different quantities, and the conversion factor between them is gravitational acceleration.

Mass is location-independent, weight is not

The mass of a body is the same everywhere. Its weight, by contrast, depends on how strongly gravity pulls at its location. On the Moon the same mass weighs about one sixth, while its mass is unchanged. Force is mass times acceleration, here F = m × g.

The factor between kilogram and newton

Standard gravity is defined exactly as 9.80665 m/s². A mass of 1 kg therefore produces a weight of 9.80665 N. For rough estimates a factor of 10 is enough: 80 kg corresponds to about 785 N, roughly 800 N.

Kilopond, kilogram-force and pound-force

Because that factor is inconvenient, dedicated units exist: the kilopond (kp, also kilogram-force, kgf) is exactly 9.80665 N, the pound-force (lbf) exactly 4.4482216152605 N. Both are tied by definition to standard gravity and are therefore common for engineering figures such as tensile strength or rope loads. A kilopond is not “one kilogram” but the force with which one kilogram pulls downwards under standard gravity.

Which unit to use when

For masses — shopping, recipes, body weight, freight — use the kilogram. For forces — spring force, load, thrust, tension — use the newton. If a figure is given in kilograms and a force is needed, the factor must be applied, and in the right direction: 1 kp is 9.80665 N, not 0.102 N.

Matching conversions

Matching tables

Unit family: Force

Sources