Mass vs weight: what's the difference?
By Engr. Mehmood Kakar · Updated
Quick answer
Mass is how much matter an object has, measured in kilograms. Weight is the force of gravity on that mass, measured in newtons. Mass is the same everywhere; weight depends on the gravitational field strength, g. They are linked by W = m × g, so on Earth (g = 9.8 N/kg) a 70 kg person weighs 686 N.
The difference at a glance
| Mass | Weight | |
|---|---|---|
| What it is | The amount of matter in an object | The force of gravity on the object |
| Unit | kilogram (kg) | newton (N) |
| Scalar or vector? | Scalar | Vector (it acts downwards) |
| Changes on the Moon? | No | Yes, it gets smaller |
| Measured with | A balance | A newtonmeter |
Linking them: W = mg
Weight depends on mass and on the gravitational field strength, g, where the object is:
weight (N) = mass (kg) × gravitational field strength (N/kg)
Calculate the weight of a 70 kg person on Earth. g = 9.8 N/kg. (2 marks)
- W = m × g
- W = 70 × 9.8
- W = 686 N
The weight calculator also works backwards, to find mass from weight or g from both.
Why weight changes on the Moon
The Moon has much less mass than Earth, so its gravitational field is weaker: about 1.6 N/kg. Take the same 70 kg person there and nothing about their body changes, so their mass is still 70 kg. Their weight is 70 × 1.6 = 112 N, about a sixth of what it was on Earth.
That is why astronauts can bounce around on the Moon. The pull on them is smaller, but there is just as much of them to move.
How each is measured
Weight is measured with a newtonmeter, a calibrated spring balance. The spring stretches more when the pull of gravity is bigger, so a newtonmeter would give a smaller reading on the Moon.
Mass is measured with a balance, which compares an object with known masses. Because both sides feel the same gravity, a balance gives the same mass anywhere.
"I weigh 60 kg"
In everyday speech, people say "weight" when they mean mass, and bathroom scales show kilograms. In a physics answer, keep them separate: kilograms are for mass, newtons are for weight. Writing a weight in kilograms loses the unit mark.
Practice
1. Calculate the weight of a 0.50 kg apple on Earth. g = 9.8 N/kg.
W = 0.50 × 9.8 → W = 4.9 N
2. An object has a weight of 196 N on Earth. g = 9.8 N/kg. Calculate its mass.
m = W ÷ g = 196 ÷ 9.8 → m = 20 kg
3. An astronaut weighs 128 N on the Moon, where g = 1.6 N/kg. What is her mass, and what would she weigh on Earth (g = 9.8 N/kg)?
m = 128 ÷ 1.6 = 80 kg → on Earth W = 80 × 9.8 = 784 N. Her mass is 80 kg in both places.
Frequently asked questions
What is the difference between mass and weight?
Mass is the amount of matter in an object, in kilograms, and it does not change from place to place. Weight is the force of gravity on the object, in newtons, and it depends on the gravitational field strength.
Is weight measured in kg or N?
In physics, weight is a force, so it is measured in newtons (N). Kilograms are the unit of mass.
Does your mass change on the Moon?
No. Your mass stays the same. Your weight is smaller because the Moon’s gravitational field strength is about 1.6 N/kg, compared with about 9.8 N/kg on Earth.
Are mass and weight proportional?
Yes. In the same gravitational field, weight is directly proportional to mass: double the mass and the weight doubles.
Written by Engr. Mehmood Kakar
Physics educator and engineer (MSc Physics, B.Ed, M.Ed) with 8 years of hands-on teaching, from college students to teacher training. About the author · LinkedIn (opens in a new tab)
Checked against the AQA GCSE Physics (8463) specification on 14 November 2026.