Three quantities, one principle
School measures three big families of quantities: length (a distance), mass (what something weighs) and capacity (what a container can hold). Each has its base unit — the metre, the gram, the litre — and the same prefixes to show multiples or fractions: kilo (× 1000), hecto (× 100), deca (× 10), then deci (÷ 10), centi (÷ 100), milli (÷ 1000).
Units of length
From biggest to smallest: km (kilometre), hm, dam, m (metre), dm, cm (centimetre), mm (millimetre). In practice at school, km, m, cm and mm are the ones used most.
Example: 2.5 m = 250 cm (multiply by 100, since there are 100 cm in 1 m); 1,300 m = 1.3 km (divide by 1000).
Units of mass
From biggest to smallest: kg (kilogram), hg, dag, g (gram), dg, cg, mg (milligram). In practice, kg and g are used most, plus sometimes the tonne (1,000 kg) for very large masses.
Example: 3 kg = 3,000 g; 750 g is a bit less than 1 kg (1,000 g).
Units of capacity
From biggest to smallest: hL, daL, L (litre), dL, cL (centilitre), mL (millilitre). In practice: L, cL and mL are the most common (a bottle of juice, a glass, a dose of medicine).
Example: 1.5 L = 150 cL; a small 330 mL bottle holds less than a third of a litre.
Using the conversion table
The safest method to never make a mistake: the conversion table. Write one column per unit, place a single digit per column (just like a place-value table), then read the result in the column you want.
| m | dm | cm | mm |
|---|---|---|---|
| 2 | 5 | 0 |
2.50 m = 250 cm — read from the "cm" column: 2, 5, 0
- Place the digit for the units of the starting unit in its column (here, the 2 in "2.50 m" goes in the "m" column).
- Fill in the following columns with the remaining digits, in order.
- Read the number starting from the "target unit" column — fill with zeros if a column before the decimal point is empty.
Mistakes to avoid
- Going the wrong way: multiplying instead of dividing (or the reverse) when changing units.
- Skipping a column in between (for example missing the dam when converting m → km).
- Not adding the missing zeros when a column is empty.
- Mixing up mass and capacity (a litre of water weighs 1 kg, but they are two different quantities!).
Worked exercises
Convert 4 m into cm
Convert 2,500 g into kg
How many mL in 3 L?
Now it's your turn!
On EasyMaths, your child practises converting lengths, masses and capacities with kind marking, for free.
Start the exercises →Frequently asked questions
How do you convert units of length easily?
Use a conversion table with one column per unit (km, m, cm, mm). Place one digit per column, then read the number in the unit you want, adding zeros where needed.
What is the difference between mass and weight?
In primary school we use the word mass for what you weigh on scales (in grams, kilograms), even though everyday language often says weight instead. Weight is actually a different concept in physics (the force of gravity).
What age do children learn unit conversions?
Everyday units (m, cm, kg, g, L) are used from around age 7-8. The full conversion table and converting between non-adjacent units are learned around age 9-10 and consolidated at age 10-11.