Imperial and Metric Units

Two families of units

Around the world, people measure with two different systems.

• The metric system uses metres, grams and litres, all scaling by tens. You met these in Converting Metric Units.
• The imperial system uses older units like inches, feet, miles, pounds and pints. These scale by irregular numbers — 12 inches make a foot, 16 ounces make a pound, 1760 yards make a mile.

Metric is the standard in science everywhere. Imperial is still common in everyday life in some countries, especially the United States, and partly in the United Kingdom for roads (miles) and people's height (feet and inches).

Why metric is easier to calculate with

The big advantage of metric is that everything scales by 10, 100 and 1000, so converting just means moving digits. Imperial uses awkward numbers that you have to memorise:

System	Length steps
Metric	10 mm = 1 cm, 100 cm = 1 m, 1000 m = 1 km
Imperial	12 in = 1 ft, 3 ft = 1 yard, 1760 yd = 1 mile

This is exactly why scientists and most of the world chose metric: tidy tens beat 12s and 1760s every time.

Key conversions between the systems

To switch between metric and imperial, you multiply or divide by a conversion factor. Learn these handy rough values:

Quantity	Rough link
Length (small)	1 inch ≈ 2.5 cm
Length (big)	1 mile ≈ 1.6 km
Mass	1 kg ≈ 2.2 lb
Capacity	1 litre ≈ 1.75 pints

The ≈ sign means "approximately equal to". Most of these are rounded — 1 inch is exactly 2.54 cm — but the rough values are fine for quick work.

Worked example 1: inches to centimetres

A book is 8 inches tall. About how many centimetres is that?

1. Use the factor: 1 inch ≈ 2.5 cm.
2. An inch is bigger than a centimetre, so multiply: 8 × 2.5 = 20 cm.

The book is about 20 cm tall. Sensible — it should be more centimetres than inches, because each inch packs in 2.5 cm.

Worked example 2: miles to kilometres

A road sign says the town is 15 miles away. How far is that in kilometres?

1. Factor: 1 mile ≈ 1.6 km.
2. A mile is longer than a km, so multiply: 15 × 1.6 = 24 km.

The town is about 24 km away.

Worked example 3: going the other way

A parcel weighs 5 kg. The courier in the US needs it in pounds. How many pounds?

1. Factor: 1 kg ≈ 2.2 lb.
2. A kilogram is heavier than a pound, so multiply: 5 × 2.2 = 11 lb.

The parcel is about 11 lb.

Checking your answer makes sense

After converting, always ask: is the number a sensible size? Use the idea that a bigger unit gives a smaller number of those units.

• An inch is bigger than a cm → there should be more cm than inches. (8 in → 20 cm. ✓)
• A kilogram is bigger than a pound → there should be more pounds than kg. (5 kg → 11 lb. ✓)

If you ever get fewer cm than inches, you have divided when you should have multiplied. This sense-checking is the same skill used in Converting Metric Units.

Why this matters

We live in a mixed world. A recipe from an American website lists ounces and pints; a road trip abroad shows kilometres; a running app might report miles. Being able to convert between systems means none of that confuses you. It also shows clearly why metric won the scientific argument: when every step is a power of ten, the maths is effortless, and there is far less room for costly mistakes — a real concern in engineering and medicine.

Try it yourself

Use the rough conversion factors above:

1. Your height in feet and inches — convert it to centimetres. (Remember 12 inches = 1 foot.)
2. A 26-mile marathon — about how many kilometres is that?
3. A 2-litre bottle of drink — roughly how many pints?
4. A 3 kg bag of flour — about how many pounds?

Check each answer is a sensible size for the unit you converted to.

Great work!

You now know the difference between metric and imperial units, the key conversion factors, how to convert between the systems, and how to check your answers make sense. Strengthen your metric skills in Converting Metric Units or revise the basics in Units of Length, Mass and Capacity.