Three things we measure

Think about a trip to the shops: you weigh a bag of apples on the scales, pour a litre of milk into the basket, and check that a new shelf is short enough to fit your wall. Measuring is something we do every single day — and almost all of it comes down to just three questions.

Almost everything we measure in everyday life is one of three things: how long it is, how heavy it is, or how much it holds. We have special names for these — length, mass and capacity — and each one comes with a small unit for little things and a large unit for big things. The clever part is choosing the unit that fits.

• Length — how long, tall or far something is. We measure it in millimetres (mm), centimetres (cm), metres (m) and kilometres (km). A fingernail is a few millimetres, a pencil is best in centimetres, a person's height is in metres, and the distance along a road is in kilometres.
• Mass — how heavy something is. We measure it in grams (g) and kilograms (kg). A grape is a few grams, an apple is about a hundred grams, and a bag of potatoes is a few kilograms.
• Capacity — how much a container can hold. We measure it in millilitres (ml) and litres (l). A teaspoon holds a few millilitres, a cup holds a couple of hundred millilitres, and a big bottle of water holds about one litre.

Always pick a unit that suits the size of the thing. You would not measure a long road in millimetres — the number would be enormous! — and you would not weigh a feather in kilograms, because it would barely move the scales at all.

The ladder of tens

Each large unit is built out of a tidy number of small ones — and that number is always a bundle of tens, just like place value. Climb up the ladder by gathering small units into big ones; climb down by splitting big units back into small ones.

To go from a big unit to a small one you multiply (each big unit becomes many small ones). To go the other way you divide. Here are three worked examples:

• A ribbon is 2\text{ m} long. How many centimetres? One metre is 100\text{ cm}, so two metres is 2 \times 100 = 200\text{ cm}.
• A melon has a mass of 3\text{ kg}. How many grams? One kilogram is 1000\text{ g}, so 3 \times 1000 = 3000\text{ g}.
• A carton holds half a litre. How many millilitres? A whole litre is 1000\text{ ml}, and half of that is 1000 \div 2 = 500\text{ ml}.

Reading a ruler

To measure a length, line one end of the object up with the 0 on the ruler, then read off the number where the other end stops. Step through the figure: first a centimetre ruler numbered from 0 to 10, then an object resting on it.

Weighing works the same way: you put the thing on a scale and read where the pointer (or the number) settles. Capacity works the same way too: you pour a liquid into a marked jug and read where the surface reaches.

Read the measuring jug

Here is a measuring jug marked every 100\text{ ml} up to 500\text{ ml}. Read the level by looking at the line the water reaches. Press Refresh for a new pour and read it again.