Recall the key approximate conversion facts between metric and imperial units
Convert from metric to imperial and from imperial to metric for length, mass and capacity
Decide whether to multiply or divide when converting between systems
Compare quantities given in different systems
Estimate and check answers in real-world contexts
Real-World Applications
Travel: UK road signs show distances in miles while many European countries use kilometres — converting between the two helps drivers estimate journey times and fuel consumption accurately.
Cooking: UK recipes often use both systems — older recipes list ingredients in ounces and pints while modern ones use grams and millilitres, so cooks frequently need to convert between the two.
Healthcare: Patients often know their height in feet and inches and weight in stones and pounds, but healthcare professionals record these in centimetres and kilograms, requiring accurate conversion.
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You are not expected to memorise all conversion facts — some will be given in the exam. However, the ones below appear frequently enough that knowing them saves time and builds confidence. Learn them in the format shown: metric ↔ imperial.
📍 Distance — key fact
8 km ≈ 5 miles
Think: both are even numbers, easy ratio
📏 Length
2.54 cm = 1 inch
This one is exact — memorise it
⚖ Mass
1 kg ≈ 2.2 lb
A bag of sugar ≈ 1 kg ≈ just over 2 lbs
🍺 Capacity
1 litre ≈ 1.75 pints
A 2-litre bottle ≈ 3.5 pints
🍶 Capacity (large)
4.5 litres ≈ 1 gallon
8 pints = 1 gallon
⚖ Mass (small)
25 g ≈ 1 ounce
A £1 coin weighs about 8.75 g
💡 Memory Tips:
• Miles to km: 8 km = 5 miles. Think of it as a ratio 8:5. Double it: 16 km = 10 miles. Triple it: 24 km = 15 miles.
• Kilograms to pounds: "A kilo is a little more than 2 lbs." 1 kg ≈ 2.2 lbs.
• Litres to pints: "A litre and three-quarters of a pint." 1 litre ≈ 1¾ pints.
• Centimetres to inches: 2.54 is exact. 30 cm ≈ 1 foot (actually 30.48 cm) — close enough for estimates.
To convert between metric and imperial, you need to: (1) identify the correct conversion fact, (2) decide whether to multiply or divide, and (3) calculate.
Choosing the operation: if your answer should be a BIGGER number than you started with → multiply. If it should be SMALLER → divide.
💡 Quick sense-check: A mile is longer than a kilometre, so any distance in miles should be a smaller number than the same distance in km. If you're converting 10 miles to km, your km answer must be bigger than 10. If you get a smaller number, you divided when you should have multiplied!
Estimating before you calculate: convert 197 miles to km
Step 1 — Round first for a quick estimate
Round 197 to a nearby easy number: 200. Round the conversion factor 1.6 to a nearby easy number too, if it helps: keep it as 1.6 here since it's already simple.
Step 2 — Estimate
\(200 \times 1.6 = 320\text{ km}\) — so the precise answer should land close to 320.
Step 3 — Now calculate precisely, and check against the estimate
\(197 \times 1.6 = 315.2\text{ km}\). This is close to the 320 estimate, so the answer is reasonable — if you'd got something like 31.52 or 3152, the estimate would immediately flag a decimal-point slip.
📝 Worked Example A: Convert 10 miles to kilometres
Step 1 — Conversion fact
8 km ≈ 5 miles. So 1 mile ≈ 8 ÷ 5 = 1.6 km.
Step 2 — Direction check
Miles → km: km are smaller units, so we need MORE of them. Multiply.
Step 3 — Calculate
10 × 1.6 = 16 km
10 miles ≈ 16 km
📝 Worked Example B: Convert 24 km to miles
Step 1 — Conversion fact
8 km ≈ 5 miles.
Step 2 — Direction check
km → miles: miles are longer, so we need FEWER of them. Divide.
Step 3 — Calculate using ratio
24 ÷ 8 × 5 = 3 × 5 = 15 miles
24 km ≈ 15 miles
📝 Worked Example C: Convert 5 kg to pounds
Step 1 — Conversion fact
1 kg ≈ 2.2 pounds
Step 2 — Direction check
kg → pounds: pounds are a smaller unit, so we need MORE. Multiply.
Step 3 — Calculate
5 × 2.2 = 11 pounds
5 kg ≈ 11 pounds
📝 Worked Example D: Convert 3 litres to pints
Step 1 — Conversion fact
1 litre ≈ 1.75 pints
Step 2 — Direction check
litres → pints: pints are smaller, so we need MORE. Multiply.
Step 3 — Calculate
3 × 1.75 = 5.25 pints
3 litres ≈ 5.25 pints
Convert
Conversion fact to use
Operation
Example
miles → km
1 mile = 1.6 km (or 8 km = 5 miles)
× 1.6 (or ÷5 ×8)
3 miles = 3 × 1.6 = 4.8 km
km → miles
8 km = 5 miles
÷ 1.6 (or ÷8 ×5)
40 km = 40 ÷ 8 × 5 = 25 miles
kg → pounds
1 kg = 2.2 lbs
× 2.2
3 kg = 3 × 2.2 = 6.6 lbs
pounds → kg
1 kg = 2.2 lbs
÷ 2.2
11 lbs = 11 ÷ 2.2 = 5 kg
litres → pints
1 litre = 1.75 pints
× 1.75
4 litres = 4 × 1.75 = 7 pints
pints → litres
1 litre = 1.75 pints
÷ 1.75
3.5 pints = 3.5 ÷ 1.75 = 2 litres
inches → cm
1 inch = 2.54 cm
× 2.54
6 inches = 6 × 2.54 = 15.24 cm
cm → inches
1 inch = 2.54 cm
÷ 2.54
50.8 cm = 50.8 ÷ 2.54 = 20 inches
Try It: Metric-Imperial Converter
Enter a value, choose the conversion, and see the result instantly.
✓ Quick Check 2 — Converting Between Metric and Imperial
Question 1 of 10
Comparing and Estimating
A common exam question type asks: "Which is bigger?" or "Which is better value?" — where amounts are given in different systems. The key is to convert both to the same unit before comparing.
📝 Example: Which is further — 100 miles or 100 km?
Convert 100 miles to km
100 ÷ 5 × 8 = 20 × 8 = 160 km
Compare
160 km vs 100 km. Clearly 100 miles is longer.
100 miles ≈ 160 km, so 100 miles is further than 100 km.
📝 Example: A person is 5 feet 8 inches tall. What is this in cm?
Here are some everyday comparisons that are useful to know:
Everyday item / situation
Imperial
Metric equivalent
Standard 2-litre soft drink bottle
≈ 3.5 pints
2 litres
UK motorway speed limit
70 mph
≈ 112 km/h
Marathon race distance
26.2 miles
≈ 42.2 km
Average adult male height
5 ft 10 in (70 in)
≈ 178 cm
A bag of flour
≈ 3.5 lbs
1.5 kg
A pint of milk
1 pint
≈ 568 ml
A 5-gallon petrol can
5 gallons
≈ 22.7 litres
Distance from London to Birmingham
≈ 110 miles
≈ 177 km
🎖 US Cups: Some recipes (especially American ones) use "cups." 1 US cup = 240 ml. So 2 cups ≈ 480 ml ≈ just under half a litre. There is no direct imperial equivalent — treat cups as a metric-ish measure at 240 ml each.
✓ Quick Check 3 — Comparing and Real-Life Contexts
Question 1 of 10
Practice: Metric and Imperial Conversions
Enter your answers as numbers, using the approximations above.
#
Question
Answer
Result
1
Convert 5 miles to km (1 mile ≈ 1.6 km)
2
Convert 10 kg to pounds (1 kg ≈ 2.2 lb)
3
Convert 4 litres to pints (1 litre ≈ 1.75 pints)
4
Convert 6 feet to cm (1 foot ≈ 30 cm)
5
Convert 200 g to ounces (25 g ≈ 1 oz)
6
Convert 10 miles to km (1 mile ≈ 1.6 km)
7
Convert 5 kg to pounds (1 kg ≈ 2.2 lb)
8
Convert 8 litres to pints (1 litre ≈ 1.75 pints)
9
Convert 3 feet to cm (1 foot ≈ 30 cm)
10
Convert 100 g to ounces (25 g ≈ 1 oz)
⚠ Things to Watch Out For
Always check whether you need to multiply or divide — converting to a larger unit means dividing, to a smaller unit means multiplying.
Metric–imperial conversions are approximate — only use the exact value if the question specifies it.
Don't use the wrong conversion factor (e.g. confusing miles↔km with miles↔metres).
For multi-step conversions, complete each step in order and carry the full unrounded value through to the end.
Always state the units in your final answer — the number alone is not enough.