Metric and Imperial Conversion Guide
The UK uses a mix of metric and imperial units that can be confusing — road distances in miles, weights in kilograms at the supermarket but stones and pounds for body weight, temperatures in Celsius for weather but Fahrenheit in older recipes. This guide covers the most common conversions across length, weight, volume and temperature, with the key figures you need and an explanation of how to apply them correctly.
Length and distance conversions
The metric unit of length is the metre (m). The imperial units in common UK use are inches, feet, yards and miles.
Key conversions: 1 inch = 2.54 cm exactly. 1 foot = 30.48 cm. 1 yard = 0.9144 m. 1 mile = 1.60934 km. 1 metre = 3.28084 feet. 1 km = 0.621371 miles.
Quick mental approximations: a kilometre is roughly 0.6 miles; a mile is roughly 1.6 km. For heights, 6 feet is about 183 cm; 5 feet is about 152 cm.
Weight conversions
The metric unit is the kilogram (kg). Imperial units in UK use include ounces, pounds, stones and (for large quantities) tons.
Key conversions: 1 ounce = 28.35 g. 1 pound = 453.592 g (0.4536 kg). 1 stone = 14 pounds = 6.35 kg. 1 kg = 2.20462 pounds. 1 tonne (metric) = 1,000 kg = approximately 0.984 imperial tons.
Note that a metric tonne (1,000 kg) and a UK imperial ton (2,240 lbs ≈ 1,016 kg) are different quantities. The US short ton (2,000 lbs ≈ 907 kg) is different again. Check which is meant when tons appear in specifications or freight documents.
Quick mental approximations: 1 kg is roughly 2.2 lbs. 10 stone is roughly 63.5 kg. 70 kg is roughly 11 stone.
Volume conversions
The metric unit is the litre (l) and millilitre (ml). Imperial units in UK use include fluid ounces, pints, quarts and gallons.
Key conversions: 1 UK fluid ounce = 28.41 ml. 1 UK pint = 568 ml. 1 UK quart = 1.136 litres. 1 UK gallon = 4.546 litres. 1 litre = 1.76 UK pints.
Important: UK (imperial) and US volume units are different. 1 US pint = 473 ml (versus 568 ml for a UK pint). 1 US gallon = 3.785 litres (versus 4.546 litres for a UK gallon). This matters for fuel economy (MPG), recipes from American sources and any US specification involving liquid volumes.
Temperature conversions
Unlike most unit conversions, temperature cannot be converted with a simple multiplier because Celsius and Fahrenheit have different zero points.
Celsius to Fahrenheit: °F = (°C × 9/5) + 32
Fahrenheit to Celsius: °C = (°F − 32) × 5/9
Key reference points: 0°C = 32°F (water freezes). 100°C = 212°F (water boils). 37°C = 98.6°F (normal body temperature). 180°C = 356°F (common oven temperature). −40°C = −40°F (the scales coincide).
Quick mental approximations: double the Celsius figure and add 30 to get a rough Fahrenheit equivalent (e.g. 20°C → 40 + 30 = 70°F; actual is 68°F). Subtract 30 from Fahrenheit and halve to get rough Celsius.
Area conversions
Area is measured in square units. 1 square metre (m²) = 10.764 square feet. 1 square foot = 0.0929 m². 1 hectare = 10,000 m² = 2.471 acres. 1 acre = 4,047 m² = 0.405 hectares.
For decorating, note that paint coverage figures are given in m² per litre — this is the metric area you can cover with one litre. If your room dimensions are in feet, convert to metres before calculating coverage.
Speed conversions
UK road speeds are in miles per hour (mph). Metric speed is kilometres per hour (km/h). 1 mph = 1.60934 km/h. 1 km/h = 0.621371 mph.
Common reference points: 30 mph = 48.3 km/h. 60 mph = 96.6 km/h. 70 mph = 112.7 km/h. 100 km/h = 62.1 mph (the typical motorway limit in Europe).
Why precision matters in conversions
Every unit conversion introduces a small rounding error when the conversion factor is not exact. For most everyday purposes this is negligible, but rounding early in a multi-step calculation can accumulate errors that matter for engineering, construction or scientific work. The general rule is to carry extra decimal places through intermediate steps and round only the final result.
Be careful with "near enough" approximations in contexts where precision matters. Using 1 kg ≈ 2 lbs (instead of 2.205) introduces a 0.5% error — acceptable for estimating luggage weight, less so for pharmaceutical or chemical applications.
Common questions about metric and imperial conversion
Why does the UK still use both systems?
The UK officially adopted the metric system for trade and official purposes but retained imperial units in several areas — road distances and speeds by law, pints for draught beer and milk, and informal everyday use for body weight and height. This hybrid situation developed gradually over decades rather than through a single switch.
Are UK and US gallons the same?
No. A UK (imperial) gallon is 4.546 litres. A US gallon is 3.785 litres. The difference is about 20%, which is significant when comparing fuel economy or liquid quantities from American sources.
How do I convert a recipe from cups to metric?
US cup measurements are volume-based, not weight-based. 1 US cup = 240 ml. However, the weight of a cup varies by ingredient — 1 cup of flour is about 120-130 g, while 1 cup of sugar is about 200 g. For baking accuracy, weigh ingredients rather than converting cups to millilitres and then measuring by volume.
Is a metric tonne the same as a ton?
No. A metric tonne is exactly 1,000 kg. A UK imperial ton is 2,240 lbs (approximately 1,016 kg). A US short ton is 2,000 lbs (approximately 907 kg). All three are different.
Related tools and guides
Use Unit Switch to convert any length, weight, volume, temperature, area or speed instantly. For journey fuel cost calculations using miles and litres, see Mile Minder. For scaling recipe quantities between metric and imperial, see How to Scale a Recipe. Browse all LibraireStudios guides.
LibraireStudios tools are designed for everyday planning and general information. For engineering, scientific, pharmaceutical or legal applications, use authoritative reference sources and verify conversion factors.