Imperial units such as miles, pounds, and pints are still in everyday use in the UK alongside metric units. At KS3, you need to know the key conversion facts between the two systems and apply them confidently in both directions — converting metric to imperial and imperial back to metric.

Why do we still use imperial units in the UK?

The UK officially adopted the metric system, but imperial units persist in everyday life. Road signs show distances in miles, people's heights are often given in feet and inches, body weight in stone and pounds, and beer is sold in pints. KS3 pupils are expected to convert fluently between the two systems using the standard approximations.

What are the key conversion facts to memorise?

You are not expected to recall every conversion — but these core facts appear regularly in KS3 and GCSE exams. Exam questions will either state the conversion or expect you to know the ones below.

Imperial unit Approximate metric equivalent
1 mile ≈ 1.6 km (equivalently: 5 miles ≈ 8 km)
1 foot ≈ 30 cm
1 inch ≈ 2.5 cm
1 pound (lb) ≈ 450 g (more precisely 454 g)
1 stone = 14 pounds
1 gallon ≈ 4.5 litres
1 pint ≈ 568 ml (roughly 0.57 litres)
1 kg ≈ 2.2 pounds

The most frequently tested at KS3 are: miles ↔ km, kg ↔ pounds, and litres ↔ gallons/pints.

How do you convert from imperial to metric?

Method: Multiply by the conversion factor.

Example 1 — miles to kilometres:

Using 1 mile ≈ 1.6 km:

26 miles × 1.6 = 41.6 km

A marathon is approximately 41.6 km.

Example 2 — pounds to kilograms:

Using 1 kg ≈ 2.2 lb, so 1 lb ≈ 1 ÷ 2.2 ≈ 0.454 kg:

10 pounds × 0.454 ≈ 4.54 kg

Alternatively: 10 ÷ 2.2 ≈ 4.5 kg

Example 3 — gallons to litres:

Using 1 gallon ≈ 4.5 litres:

6 gallons × 4.5 = 27 litres

How do you convert from metric to imperial?

Method: Divide by the conversion factor (or multiply by its reciprocal).

Example 1 — kilometres to miles:

Using 1 mile ≈ 1.6 km:

120 km ÷ 1.6 = 75 miles

Or use the 5 miles ≈ 8 km ratio: 120 ÷ 8 × 5 = 75 miles ✓

Example 2 — kilograms to pounds:

Using 1 kg ≈ 2.2 lb:

65 kg × 2.2 = 143 pounds

Example 3 — litres to pints:

Using 1 pint ≈ 0.568 litres, so 1 litre ≈ 1 ÷ 0.568 ≈ 1.76 pints:

2 litres × 1.76 ≈ 3.5 pints

How do you choose whether to multiply or divide?

Use this simple check:

  • Going from a smaller unit to a larger unit? → Divide (you end up with fewer of the larger unit).
  • Going from a larger unit to a smaller unit? → Multiply (you end up with more of the smaller unit).

For example, converting 5 miles to km: a kilometre is smaller than a mile, so you will get more of them → multiply: 5 × 1.6 = 8 km. ✓

Alternatively, always set up a ratio and cross-multiply:

5 miles      x km
─────────  = ──────
1 mile       1.6 km

x = 5 × 1.6 = 8 km

What are common mistakes?

  • Dividing when you should multiply (or vice versa). Always ask: am I converting to a bigger or smaller unit? Bigger unit → divide; smaller unit → multiply.
  • Using an inaccurate conversion factor. Stick to the values given in the question. If the question says "use 1 kg = 2.2 lb", use exactly 2.2 — do not use 2.205 or you may introduce a rounding error.
  • Forgetting that 1 stone = 14 pounds. Height and weight conversions sometimes require a two-step process: stone to pounds, then pounds to kg.
  • Mixing up miles and km on speed problems. A speed of 60 mph converted to km/h is 60 × 1.6 = 96 km/h — not divided by 1.6.

Frequently asked questions

How many km is 10 miles?

Using 1 mile ≈ 1.6 km: 10 × 1.6 = 16 km. Using the ratio 5 miles = 8 km: 10 miles = 2 × 8 = 16 km. ✓ Both methods confirm the answer is 16 km.

What is 5 feet 6 inches in centimetres?

Convert feet and inches to inches first: 5 feet = 5 × 12 = 60 inches; total = 60 + 6 = 66 inches. Then convert to cm: 66 × 2.54 ≈ 167.6 cm. In most KS3 contexts the approximation 1 inch ≈ 2.5 cm gives 66 × 2.5 = 165 cm.

Is the conversion between litres and pints exact?

No — the UK pint is exactly 568.261 ml, but the approximation of 1 pint ≈ 568 ml (or 0.568 litres) is used in exams. The US pint is different (≈ 473 ml), but UK exam questions use the British Imperial pint.

Why does the UK still use miles on road signs?

It is a legal requirement under the Road Traffic Regulation Act 1984. The UK Government has not set a date to convert road signs to kilometres, partly due to cost and the need to replace millions of signs. As a result, driving speeds and distances remain in mph and miles in everyday UK life.

For step-by-step measurement coaching that connects imperial and metric to real-life contexts, visit aitutors.me.