Capacity and Volume in Litres

Capacity and volume: two related ideas

These two words are easy to mix up, so let's be precise.

• Capacity is how much a container can hold — the maximum amount of liquid that fits inside.
• Volume is how much space a substance actually takes up.

A 2-litre bottle has a capacity of 2 litres. If you pour in 1.5 litres of juice, the juice has a volume of 1.5 litres, and there is still room for 0.5 litres more. Both are measured in the same units.

The units: litres and millilitres

We measure capacity and liquid volume in:

• millilitres (ml) — small amounts, like a spoonful of medicine (about 5 ml).
• litres (l) — larger amounts, like a carton of milk.

The link is the familiar metric step:

1 litre = 1000 millilitres

So 0.5 litres = 500 ml, and 2.5 litres = 2500 ml. Converting is just multiplying or dividing by 1000, exactly as in Converting Metric Units.

The link to cubic centimetres

Here is a fact that connects liquids to solid shapes:

1 litre = 1000 cubic centimetres (cm³), and 1 ml = 1 cm³

This is no accident. The litre was defined as the space inside a cube measuring 10 cm on every edge. That cube's volume is 10 × 10 × 10 = 1000 cm³, which holds exactly 1 litre. This means you can work out how much liquid a box-shaped container holds by finding its volume in cm³, a skill from Surface Area and Volume.

Reading a measuring jug

A measuring jug is a scale for capacity. Two rules keep readings accurate:

• Stand the jug on a flat surface so the liquid is level.
• Read at eye level, where the surface of the liquid meets the scale. Looking from above or below makes the level appear at the wrong mark.

As with any scale, first find what one mark is worth: subtract two labelled marks to get the gap, count the spaces, and divide.

Worked example 1: reading a jug

A jug is marked 0, 500, 1000 ml, with 5 spaces between each pair.

1. Gap: 500 − 0 = 500.
2. Spaces: 5.
3. One mark: 500 ÷ 5 = 100 ml.

If the water reaches the 2nd mark above 500, that is 500 + 200 = 700 ml.

Worked example 2: a capacity problem

A jug holds 2 litres. You pour in 650 ml, then another 800 ml. How much more will fit?

1. Add what is already in, in millilitres: 650 + 800 = 1450 ml.
2. The capacity in ml: 2 litres = 2000 ml.
3. Space left: 2000 − 1450 = 550 ml.

So 550 ml more will fit. Always work in one unit before adding or subtracting.

Worked example 3: volume of a tank

A fish tank is 30 cm long, 20 cm wide and 25 cm deep. How many litres of water does it hold when full?

1. Volume = length × width × height = 30 × 20 × 25 = 15 000 cm³.
2. Convert to litres: 15 000 ÷ 1000 = 15 litres.

The tank holds 15 litres. This connects volume of a cuboid straight to a real capacity.

Choosing a sensible unit

Container	Sensible unit
A teaspoon	ml
A drinks can	ml
A water bottle	litres
A bucket	litres
A bathtub	litres
A medicine cup	ml

Pick the unit that keeps the number tidy. A bath is far easier to describe as 80 litres than 80,000 ml.

Why this matters

Capacity comes up every day — filling a kettle, mixing a drink, fuelling a car, or following a recipe. The link between litres and cubic centimetres is especially powerful: it lets engineers and scientists work out how much liquid any container holds just from its measurements, without ever filling it. Once you see that 1 ml is 1 cm³, the worlds of liquids and solid shapes join up into one idea.

Try it yourself

Find a measuring jug and a few containers — a mug, a glass and a small bottle.

1. Estimate how many millilitres each one holds.
2. Fill each with water, then pour it into the jug and read the real value at eye level.
3. How close were your estimates?
4. Add the three capacities together. Is the total more or less than 1 litre?

Great work!

You now know the difference between capacity and volume, that 1 litre = 1000 ml = 1000 cm³, how to read a measuring jug accurately, and how to solve capacity problems. Take the volume idea further in Surface Area and Volume, or revise the units in Converting Metric Units.