Maths
What is the difference between mass and weight?
Quick answer
Mass is how much stuff is in an object, measured in grams and kilograms. Weight is the pull of gravity on that mass. In everyday life people say weight for both.
In one sentence
Mass is how much matter something contains, and weight is how hard gravity pulls on it, although most people use "weight" for both.
Explained
Hold an apple in one hand and a melon in the other. The melon feels heavier because it has more mass: more stuff packed into it. In maths lessons, mass is measured in:
| Unit | Short | Example |
|---|---|---|
| gram | g | a paper clip is about 1 g |
| kilogram | kg | 1000 g, a bag of sugar |
| tonne | t | 1000 kg, a small car |
apple
elephant
Converting works in thousands. To change kg to g, multiply by 1000: 2.5 kg = 2500 g. To change g to kg, divide by 1000: 750 g = 0.75 kg. These are decimals at work.
Imperial units are common in the US and on UK recipes and bathroom scales:
- 16 ounces (oz) = 1 pound (lb)
- 14 pounds = 1 stone (used in the UK and Ireland for body weight)
- 1 kg is about 2.2 lb
Mass or weight? Scientists draw a careful line. Mass stays the same wherever you go. Weight is a force, the pull of gravity on that mass, and scientists measure it in newtons (N). On Earth, a 1 kg bag of flour weighs about 10 N. On the Moon, gravity is about one sixth as strong, so the same bag weighs about 1.6 N, but its mass is still 1 kg. In shops and kitchens, "weight" in kilograms really means mass, and that is fine for everyday maths.
Reading scales. Kitchen scales often have a dial with marks between the numbers. Work out what each small mark is worth first. If the gap from 0 to 100 g has 5 spaces, each space is 20 g.
Examples
- 4 kg = 4000 g
- 600 g + 900 g = 1500 g = 1.5 kg
- A recipe needs 250 g of flour for 10 cookies, so 20 cookies need 500 g.
- A suitcase limit of 23 kg is about 50 lb.
- 3 tonnes = 3000 kg
Common mistakes
- Thinking bigger means heavier. A balloon is bigger than a stone but much lighter.
- Converting by 100 instead of 1000. There are 1000 g in a kilogram.
- Misreading scales by assuming each small mark is 1. Fix: count the spaces between labelled numbers.
Quick quiz
Mass and weight
Check what you know with 4 quick questions.
Question 1 of 4
How many grams are in 1 kilogram?
Practise it
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Browse maths worksheets for measuring and scales practice.
For grown-ups
Related topics
Length and measuring
Length is how long, tall or far something is. It is measured with a ruler or tape in units such as millimetres, centimetres, metres and kilometres (or inches, feet and miles).
Capacity
Capacity is how much a container can hold, usually measured in millilitres (ml) and litres (l). A large bottle of water often holds 2 litres, which is 2000 ml.
Decimals
Decimals are numbers with a decimal point. Digits after the point show tenths, hundredths and so on, so 2.5 means two and five tenths.
Estimation
Estimating means making a sensible guess at an answer, usually by rounding the numbers first. It tells you roughly what to expect and helps you spot mistakes.
Gravity
Gravity is the force that pulls objects towards each other. Earth's gravity pulls everything towards its centre, which is why things fall down and why the Moon orbits Earth.
2D shapes
2D shapes are flat shapes with length and width but no thickness, like circles, triangles, squares and hexagons. You can draw them on paper.