Weight is mass times gravity, so kilograms alone are not a force
A kilogram is a measure of mass; a newton is a measure of force. To turn a mass into the weight force it feels, multiply by the local gravity. At Earth's standard 9.80665 m/s², 1 kg weighs 9.81 N. For a quick estimate, multiply the kilograms by 10: 70 kg → ~700 N, about 2% over the exact 686.5 N.
Quick reference (Earth, g = 9.80665)
| Mass | Weight (N) | Kilogram-force (kgf) |
|---|---|---|
| 1 kg | 9.81 N | 1 kgf |
| 5 kg | 49.03 N | 5 kgf |
| 10 kg | 98.07 N | 10 kgf |
| 25 kg | 245.2 N | 25 kgf |
| 50 kg | 490.3 N | 50 kgf |
| 75 kg | 735.5 N | 75 kgf |
| 100 kg | 980.7 N | 100 kgf |
Kilogram-force is a legacy unit: 1 kgf is defined as the weight of 1 kg under standard gravity, so 1 kgf = 9.80665 N and the mass number and the kgf number always match.
The same mass, a different weight on each world
Move a 100 kg object off Earth and its mass stays 100 kg, but its weight follows local gravity. This is why an astronaut can leap on the Moon and would be pinned on Jupiter.
Same 100 kg of matter, weighed under each world's gravity. The Moon pulls at about one sixth of Earth; Jupiter about two and a half times.
Getting it right
- Reverse it by dividing. To go from newtons back to mass, divide by gravity: 960 N ÷ 9.80665 = 97.9 kg.
- Use the right g off Earth. For the Moon use 1.62, not 9.81 — using Earth gravity overstates lunar weight sixfold.
- Always state the unit. "2 kg = 20" is meaningless; it is 20 N. Mass in kg, weight in N.
Common questions
What is the difference between a kilogram and a newton?
A kilogram measures mass, the amount of matter in an object, and it never changes. A newton measures force, including the weight that gravity gives that mass. On Earth 1 kg weighs about 9.81 N; the same 1 kg on the Moon weighs only about 1.62 N.
How many newtons is 75 kg on Earth?
75 kg times 9.80665 is 735.5 newtons. A rough answer is 75 times 10, or 750 N.
What is standard gravity, 9.80665?
It is the agreed nominal value for gravity at the Earth surface, set internationally in 1901 for a latitude of 45 degrees at sea level. Real gravity ranges from about 9.764 at the equator to 9.832 at the poles.
