Weight Calculator (Mass to Force in Newtons)
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 9/5/2026
Weight (a force, in newtons) is calculated by multiplying mass (in kg) by the acceleration of gravity, about 9.81 m/s² on Earth. A mass of 70 kg therefore weighs about 686.7 N.
Explanation
In physics, weight and mass are two different quantities, often mixed up in everyday language. Mass (in kilograms) is an intrinsic property of matter, the same everywhere in the universe. Weight (in newtons) is a force, the one gravity exerts on that mass: it therefore depends on where you are. On Earth, standard gravitational acceleration is about 9.81 m/s² (the exact standardized value is 9.80665 m/s², see the FAQ), giving the formula weight = mass × 9.81. On the Moon, where gravity is about 6 times weaker (about 1.62 m/s²), the same 70 kg mass would weigh only about 113 N. This calculator applies standard Earth gravity; for the energy a moving object can develop rather than its weight at rest, see our kinetic energy calculator.
Example: a mass of 70 kg
Inputs
Mass: 70 kg.
Calculation
Weight = mass × g = 70 × 9.81 = 686.7.
Result
A mass of 70 kg weighs about 686.7 N on Earth.
Frequently asked questions
What's the difference between mass and weight?
Mass (in kg) measures the amount of matter in an object and never changes, wherever you are. Weight (in newtons) is the force gravity exerts on that mass: it varies by location (weaker on the Moon, stronger on Jupiter). In everyday language, the two are often mixed up when saying "I weigh 70 kg," whereas in the strict physical sense, it's the mass that equals 70 kg and the weight that equals about 686.7 N.
Why use 9.81 and not the exact value 9.80665?
9.81 m/s² is the commonly used rounded value in teaching and practical calculations; the exact international standardized value of standard gravity is 9.80665 m/s². The difference between the two (less than 0.001%) is negligible for everyday use, but explains a tiny gap if you compare against a calculation done with the exact value.
Is weight the same everywhere on Earth?
Almost, but not exactly: actual gravity varies slightly with latitude and altitude (about 9.78 m/s² at the equator versus 9.83 m/s² at the poles). The value 9.81 m/s² is a standard average, accurate enough for the vast majority of uses.