Maths
What are metric units?
Quick answer
Metric units measure length, mass and capacity in steps of 10, 100 and 1,000. Millimetres, metres, grams, kilograms and litres are all metric units.
In one sentence
The metric system is a way of measuring in which units are linked by multiples of 10, so changing between them is a matter of moving the decimal point.
Explained
We use metric units to measure length (how long), mass (how heavy) and capacity (how much a container holds). Almost every country uses them. The United States still uses its own units, such as inches and pounds, in daily life, but science everywhere is metric.
ruler
bucket
cup
The system is easy because it uses prefixes that always mean the same:
| Prefix | Meaning | Example |
|---|---|---|
| milli- | one thousandth | 1 millimetre = 1/1,000 of a metre |
| centi- | one hundredth | 1 centimetre = 1/100 of a metre |
| kilo- | one thousand | 1 kilometre = 1,000 metres |
The main units are:
- Length: millimetre (mm), centimetre (cm), metre (m), kilometre (km).
- Mass: milligram (mg), gram (g), kilogram (kg), tonne (t).
- Capacity: millilitre (ml), litre (l).
Useful conversions: 10 mm = 1 cm, 100 cm = 1 m, 1,000 m = 1 km, 1,000 g = 1 kg, 1,000 kg = 1 tonne, 1,000 ml = 1 litre.
To change units, think about whether the new unit is smaller or bigger. Going to a smaller unit means more of them, so you multiply: 2.5 km × 1,000 = 2,500 m. Going to a bigger unit means fewer, so you divide: 350 cm ÷ 100 = 3.5 m. Multiplying and dividing by 10, 100 and 1,000 just shifts the digits, so place value helps. You can read more about length and measuring, mass and weight and capacity.
Examples
| From | To | Answer |
|---|---|---|
| 2.5 km | m | 2,500 m |
| 350 cm | m | 3.5 m |
| 4 l | ml | 4,000 ml |
| 1,500 g | kg | 1.5 kg |
| 60 mm | cm | 6 cm |
Common mistakes
- Multiplying when you should divide. Going from small units to big ones needs a divide.
- Mixing units in a sum. Convert first: 1 m + 20 cm is 120 cm.
- Using the wrong prefix. Milli is small, kilo is big.
Quick quiz
Metric units
Check what you know with 4 quick questions.
Question 1 of 4
How many centimetres are in 1 metre?
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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).
Mass and weight
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.
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.
The 24-hour clock
The 24-hour clock counts the hours from 00:00 at midnight to 23:59, so there is no am or pm. For afternoon times, add 12: 3 pm is 15:00.
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.
3D shapes
3D shapes are solid shapes with length, width and height, like cubes, spheres, cylinders and pyramids. They have faces, edges and vertices.