Convert Kilograms and Newtons

Convert mass and weight

Sometimes it is said that something weighs so many kilograms. This has become part of everyday use, but it is not physically correct. A mass is given in grams (g) or kilograms (kg), a weight is weighed in newtons (N) or kilonewtons (kN). For weight it is important how strong the gravitational pull is, for mass it doesn't matter. Fortunately, the gravitational pull on Earth is fairly constant, so both statements can be used colloquially interchangeably. The average gravity (or gravitational acceleration) is 9.81 newtons per kilogram.

Acceleration of gravity : N/kg = m/s²

At the poles, gravity is about 9.83 Newtons per kilogram, at the equator about 9.78. So the difference is very small, often it is calculated with 10 if the result does not have to be very precise. On the moon, gravitational acceleration is only 1.62 N/kg. On Earth, a kilogram has an average weight of 9.81 newtons, and a newton weight is caused by about 102 grams.

Kilogram is the SI coherent unit of mass, newton is the SI coherent unit of force. Sometimes the specification daN is seen. This stands for decanewton, 1 daN = 10 N.

The kilogram was first defined in France in 1790 as the mass of one cubic decimeter (liter) of water. This definition is simple, but not very precise. Today's definition using Planck units is exact, but no longer easy to understand. A prototype kilogram made of platinum and iridium is located in Paris, and was the reference unit until 2019.
The newton was proposed as a unit of force in 1913 and has been official since 1948. It was of course named after Isaac Newton. One newton is the force needed to make one kilogram move one meter per second faster in one second.

Physics commonly uses SI units. Here is a calculator to convert units.