Units of Measurement

What you will learn

• Say what a unit is and why a measurement needs both a number and a unit.
• Name the units for length, mass, capacity, time and temperature.
• Choose a sensible unit for what you are measuring.

Before you start: you should be able to count and compare numbers. Comparing Length, Mass and Capacity is a gentle first step, and Measuring Length with a Ruler comes next.

The idea

If someone tells you a pencil is “15 long”, you can’t tell whether they mean 15 centimetres, 15 inches or 15 paper clips. A unit is an agreed amount that everyone measures against. Every measurement has two parts: a number and a unit.

Measuring with everyday things

Young children often start by measuring with hand spans, footsteps or cubes. This works, but your hand span is different from mine, so our answers don’t match. Standard units fix that problem: a metre is the same length everywhere in the world.

The main things we measure

The metric system and its prefixes

The metric system is built on tens. A few prefixes do all the work:

• kilo- means a thousand: $1$ kilometre $= 1000$ metres; $1$ kilogram $= 1000$ grams.
• centi- means a hundredth: $100$ centimetres make $1$ metre.
• milli- means a thousandth: $1000$ millilitres make $1$ litre.

Many countries also use imperial units such as inches, feet, miles, pounds and pints. You will meet both, and our conversions lesson shows how to switch between them.

Choosing a sensible unit

Pick a unit that keeps the number easy to picture. Measure an ant in millimetres, a room in metres and a journey in kilometres. A bag of sugar is about $1$ kg, a teaspoon holds about $5$ ml, and a door is about $2$ m tall. These benchmarks help you estimate.

Worked examples

Example 1: Which unit?

Choose the best unit for: the length of a football pitch, the mass of an apple, and the water in a bath.

1. A pitch is about a hundred metres long, so use metres.
2. An apple is light, so use grams (about $150$ g).
3. A bath holds a lot of water, so use litres (about $150$ L).

Answer: metres, grams and litres.

Example 2: Comparing measurements

Which is longer: $1.2$ m or $95$ cm?

1. Write both in the same unit. $1.2$ m $= 1.2 \times 100 = 120$ cm.
2. Compare: $120$ cm $> 95$ cm.

Answer: $1.2$ m is longer, by $25$ cm.

Example 3: Adding measurements

A recipe needs $750$ g of flour and $1.5$ kg of potatoes. What is the total mass in kilograms?

1. Convert grams to kilograms: $750 \div 1000 = 0.75$ kg.
2. Add: $0.75 + 1.5 = 2.25$ kg.

Answer: $2.25$ kg

Mass and weight are different in science: mass is how much matter something contains (kg); weight is the force of gravity on it (newtons). In everyday speech, people say “weight” for both.

Why it matters

Units keep the world working together. Builders, doctors, cooks and engineers all need to agree exactly what a measurement means. In 1999 a NASA spacecraft was lost at Mars partly because one team used imperial units while another expected metric. At home, measuring medicine in millilitres, following a recipe or checking whether a sofa will fit through the door all depend on getting units right.