About This Unit Converter
This tool converts between the most commonly needed units of length, weight, volume, temperature, area and speed: the measures people need most often in everyday life, cooking, DIY, travel, and international communication. Every conversion runs instantly in your browser using precise conversion factors.
Length Conversions
Length units covered include millimetres, centimetres, metres, kilometres, inches, feet, yards, miles, nautical miles and fathoms. The core relationship to remember: 1 inch = 2.54 cm exactly (this is the internationally defined exact conversion), 1 foot = 30.48 cm, 1 mile = 1.60934 km. These conversions matter constantly for anyone working between metric-using countries (most of the world, including Finland and Sweden) and imperial-using countries (primarily the United States, with the UK using a mix of both systems in everyday life).
Weight Conversions
Weight units covered include grams, kilograms, ounces, pounds, and stones. 1 kilogram = 2.20462 pounds, 1 pound = 453.592 grams, 1 stone (used mainly in the UK and Ireland for body weight) = 14 pounds = 6.35 kg. Cooking recipes from different countries frequently use different weight units, making quick conversion essential for following international recipes accurately.
Temperature Conversions
Temperature conversion works differently from length and weight because it isn't a simple multiplication — it involves an offset. Celsius to Fahrenheit: °F = (°C × 9/5) + 32. Fahrenheit to Celsius: °C = (°F − 32) × 5/9. Celsius to Kelvin: K = °C + 273.15. The United States uses Fahrenheit for everyday temperature; nearly every other country, including Finland and Sweden, uses Celsius. Kelvin is used primarily in scientific contexts since it starts at absolute zero.
Volume Conversions
One litre is 1,000 millilitres, or 10 decilitres, the unit many Nordic recipes use. The US and the UK use gallons and pints of different sizes, which is a common source of mistakes: a US gallon is 3.785 litres, while a UK (imperial) gallon is 4.546 litres, about 20% more. A US pint is 473 ml and a UK pint 568 ml. In cooking, one US cup is 236.6 ml, one tablespoon 14.8 ml and one teaspoon 4.9 ml.
Fuel and water tanks, aquariums and swimming pools are often measured in cubic metres: one cubic metre holds exactly 1,000 litres.
Area Conversions
Area grows with the square of length, so the factors are larger than you might expect: one square metre is 10.764 square feet, not 3.28. A hectare is 10,000 square metres (100 m × 100 m), and an acre is 4,046.9 square metres, about 0.405 hectares. Flat and house sizes are usually given in square metres in Europe and in square feet in the US and the UK.
Speed Conversions
At sea and in the air, speed is measured in knots: one knot is one nautical mile per hour, exactly 1.852 km/h or about 1.151 mph. To convert km/h to m/s, divide by 3.6, so 36 km/h is 10 m/s.
Quick Reference Table
| From | To | Multiply by |
|---|---|---|
| Inches | Centimetres | 2.54 |
| Feet | Metres | 0.3048 |
| Miles | Kilometres | 1.609344 |
| Kilometres | Miles | 0.621371 |
| Pounds | Kilograms | 0.45359237 |
| Kilograms | Pounds | 2.20462 |
| Ounces | Grams | 28.3495 |
| Stones | Kilograms | 6.35029 |
Useful Temperatures to Remember
Water freezes at 0 °C (32 °F) and boils at 100 °C (212 °F) at sea level. Normal body temperature is about 37 °C (98.6 °F). A warm summer day of 25 °C is 77 °F, and −20 °C, a cold winter day in northern Finland or Sweden, is −4 °F. A quick mental estimate from Celsius to Fahrenheit is "double it and add 30": 20 °C gives about 70 °F (the exact answer is 68 °F).
Units at Sea and in the Air
Ships and aircraft use their own units. One nautical mile is exactly 1,852 metres, based on one minute of latitude, and one knot is one nautical mile per hour, about 1.852 km/h. A ferry cruising at 20 knots travels at about 37 km/h.
Frequently Asked Questions
Why is 1 inch defined as exactly 2.54 cm?
This exact relationship was formally established by international agreement in 1959 (the International Yard and Pound Agreement), standardising the previously slightly inconsistent imperial measurements used across different English-speaking countries into an exact metric equivalent.
What's the difference between a UK stone and other weight units?
The stone is a unit historically used across the British Isles, still commonly used in the UK and Ireland for expressing body weight (e.g., "12 stone 4" rather than kilograms or pounds alone), though it has largely fallen out of use elsewhere.
Why does temperature conversion need an offset, unlike length or weight?
Celsius and Fahrenheit scales have different zero points — 0°C is the freezing point of water, while 0°F was originally based on a different reference point (a brine solution). Because the scales don't share a common zero, conversion requires both a multiplication (for the different degree sizes) and an addition/subtraction (to align the zero points).
