About this calculator
Choose force (F = ma) or weight (W = mg). For force, enter a mass and an acceleration. For weight, enter a mass and see what it weighs on four different worlds.
Answers come in newtons, with kilograms-force and pounds-force for comparison.
Worked examples
Real numbers, worked out by the same calculator. Press “Use these numbers” to try one above.
10 kg accelerating at 2 m/s²
- Force
- 20 N
- In kilograms-force
- 2.039432 kgf
- In pounds-force
- 4.496179 lbf
A force of 20 N is needed to give 10 kg an acceleration of 2 m/s².
Show the working
- Newton's second law: force = mass × acceleration.
- = 10 × 2 = 20 N.
Weight of a 70 kg person
- Weight on Earth
- 686.4655 N
- Weight on the Moon
- 113.4 N
- Weight on Mars
- 259.7 N
- Weight on Jupiter
- 1,735.3 N
- In kilograms-force (Earth)
- 70 kgf
- In pounds-force (Earth)
- 154.323584 lbf
A mass of 70 kg weighs 686.4655 N on Earth (113.4 N on the Moon).
Show the working
- Weight = mass × gravity (W = mg).
- On Earth, g = 9.80665 m/s², so 70 × 9.80665 = 686.4655 N.
- Mass stays the same everywhere; weight changes with gravity.
1,200 kg car accelerating at 3 m/s²
- Force
- 3,600 N
- In kilograms-force
- 367.097837 kgf
- In pounds-force
- 809.312195 lbf
A force of 3,600 N is needed to give 1,200 kg an acceleration of 3 m/s².
Show the working
- Newton's second law: force = mass × acceleration.
- = 1,200 × 3 = 3,600 N.
Newton's second law
Force equals mass times acceleration: F = m × a. A force of one newton gives a one-kilogram mass an acceleration of one metre per second squared.
Mass and weight are different
Mass is how much matter something contains (kilograms) and never changes. Weight is the force of gravity on that mass: W = m × g. A 70 kg person weighs about 686 N on Earth but only about 113 N on the Moon, where gravity is weaker. Their mass is 70 kg in both places.
Kilograms-force and pounds-force
Everyday “weight in kilograms” is really a mass. The kilogram-force is the force gravity exerts on one kilogram at Earth's surface (9.80665 N), and the pound-force is 4.4482 N. The calculator shows both so you can compare.
Frequently asked questions
What is the formula for force?
Force = mass × acceleration (F = ma).
What is the difference between mass and weight?
Mass is the amount of matter and stays constant. Weight is the gravitational force on it and changes with gravity.
How many newtons is 1 kg?
On Earth, 1 kg weighs about 9.81 N.
What is a newton?
The force needed to accelerate 1 kg by 1 m/s².
Formulas tested against hand-worked answers. Last reviewed 29 September 2026. These calculators do arithmetic only; they are not financial, tax or legal advice.