In the SI system of units, Force is measured in Newtons (N).

When a force is applied to a mass, it will accelerate. The force F (in Newtons), the mass m (in kilograms) and the acceleration, a (in m/s²) are related according to Newton’s Second Law:

When a mass is acted upon by the acceleration of gravity (called g), we call the force “Weight.” So substituting weight, W for F in the above equation, and substituting g for the acceleration, we get the following equation:

The gravitational acceleration varies over the surface of the earth, but in North America, g is usually taken as 9.81 m/s².

Thus the weight of a 1 kg mass is 1 kg x 9.81 m/s² = 9.81 Newtons