The short version
Mass is how much matter something contains. It's the same anywhere in the universe, and the SI unit is the kilogram. Weight is the force gravity exerts on that mass. It depends on where you are, and the SI unit is the newton.
In daily life the two words get swapped all the time. When you step on a bathroom scale and it says 70 kg, that's your mass. In physics class, your weight would be about 686 newtons.
The formula connecting them
Weight equals mass times the acceleration due to gravity, W = m x g. NIST lists standard gravity as 9.80665 m/s², the value used to define units like the kilogram-force. Real gravity varies slightly from place to place on Earth.
So a 70 kg person weighs 70 x 9.80665 = about 686.5 N on Earth. Flip the formula to find mass from weight: m = W / g. A force of 980.665 N corresponds to a mass of 100 kg under standard gravity.
Formula notes
Weight (N) = mass (kg) x g (m/s²)Mass (kg) = weight (N) / g (m/s²)Standard g = 9.80665 m/s²
Same mass, different weight: Earth and the Moon
NASA lists the Moon's surface gravity as 1.62 m/s², about 0.165 of Earth's. Take that 70 kg person to the Moon and their mass is still 70 kg. Their weight drops to 70 x 1.62 = about 113 N.
Anything they carry keeps its mass too. A heavy pack is just as hard to start or stop moving on the Moon as on Earth. It just presses down far less.
- Mass on Earth: 70 kg. Weight: about 686 N
- Mass on the Moon: 70 kg. Weight: about 113 N
- Mass in orbit: 70 kg. Feels weightless, but mass is unchanged
Why scales show kilograms and pounds
A typical bathroom or kitchen scale measures the force you press down with, then divides by Earth's gravity to show mass. That works because almost everyone uses it on Earth. Take it to the Moon and it would read about one-sixth of your real mass.
Pounds add their own twist. The pound (lb) is a unit of mass, exactly 0.45359237 kg. The pound-force (lbf) is a unit of force, about 4.448 N. Engineering work in U.S. units keeps them apart. Everyday labels just say pounds.
A worked case in U.S. units
Say a scale reads 150 lb. That's a mass of 150 x 0.45359237 = 68.04 kg. Under standard gravity, the weight is 68.04 x 9.80665, or about 667 N.
In U.S. engineering units the same weight is 150 lbf, because the pound-force is defined using standard gravity. So on Earth the number of pounds and pounds-force match, which is why people treat them as one thing. On the Moon the mass would still be 150 lb, but the weight would be only about 25 lbf.
Common mix-ups
Calling a kilogram reading "weight" is fine at the grocery store. In a physics problem, it'll cost you marks. If a question asks for weight, the answer should be in newtons.
The other mix-up is volume. A liter of water and a liter of oil take up the same space but have different masses, so you can't turn liters into kilograms without the density. And a kilogram-force (kgf) isn't a kilogram either: it's 9.80665 N. To convert between mass units themselves, our mass calculator handles kilograms, pounds, ounces, stones and more.
Frequently asked questions
Do you weigh less on the Moon?
Yes. Your weight there is about 0.165 of your Earth weight, because the Moon's surface gravity is 1.62 m/s². Your mass doesn't change.
Is weight measured in kg or newtons?
In physics, newtons. Kilograms measure mass. Everyday scales show kilograms because they assume Earth's gravity.
Does mass change with altitude?
No. Mass stays the same. Weight changes very slightly, because gravity gets a little weaker as you move farther from Earth's center.
Is a pound mass or weight?
The pound (lb) is a unit of mass, 0.45359237 kg. The pound-force (lbf) is the matching unit of force, about 4.448 newtons.
References
These sources support the method or guidance used for Mass Calculator. Verify time-sensitive rules at the source.
Try the calculator
Open Mass Calculator, enter your scenario, and compare its supporting rows with this guide's method and checks.