Unit Converter
Recipes, road signs, science homework and travel all mix metric and imperial units. This converter changes a value between units in the same category: length, mass, temperature, volume, area or speed. It shows the exact conversion factor and a few other units so you can see the scale.
How to use the unit converter
- Pick a category first; the “From” and “To” lists update to units in that category.
- Enter the value and choose the units you are converting from and to.
- Read the result and the conversion factor. For temperature there is no single factor because the scales have different zero points.
Formula
For most units the tool converts to a base unit and then to the target: result = value × (from-factor ÷ to-factor). Base units are metres, kilograms, litres, square metres and metres per second. Temperature uses formulas: °F = °C × 9/5 + 32 and K = °C + 273.15.
Worked examples
Distance
5 miles × 1.609344 = 8.04672 kilometres. A 5 km run is about 3.1 miles.
Body temperature
98.6 °F − 32 = 66.6, × 5/9 = 37 °C, which is 310.15 K.
Quick reference for common conversions
| Category | Handy equivalents |
|---|---|
| Length | 1 in = 2.54 cm · 1 ft = 30.48 cm · 1 mi = 1.609 km |
| Mass | 1 lb = 0.4536 kg · 1 oz = 28.35 g · 1 kg = 2.205 lb |
| Volume | 1 US gal = 3.785 L · 1 cup = 236.6 mL |
| Speed | 60 mph ≈ 96.6 km/h · 1 knot ≈ 1.852 km/h |
| Temperature | 0 °C = 32 °F · 100 °C = 212 °F |
Metric versus imperial
The metric system is based on powers of ten, so converting between kilometres, metres and millimetres only means moving a decimal point. Imperial and US customary units have irregular factors (12 inches in a foot, 5,280 feet in a mile), which is why a converter is so useful. Nearly every country uses metric officially, while the United States mixes both in daily life.
Getting precise results
Conversions here use exact or internationally agreed factors, such as the inch being defined as exactly 25.4 mm. Rounding is applied only when displaying, so chained conversions do not accumulate error. For cooking, small differences between US and UK measures matter, so check which cup or pint your recipe uses.
Estimating in your head
To convert miles to kilometres roughly, multiply by 1.6 (or add half and a tenth). Kilograms to pounds: double and add 10%. Celsius to Fahrenheit: double, subtract 10%, add 32. These shortcuts are good for travel; use the tool when accuracy counts.
Practice problems
Try these yourself first, then check your answer. The answers come from the calculator above.
- How many kilometres is a 26.2-mile marathon?
Show answer
26.2 mi in km: 42.1648128
- Convert 72 °F to Celsius.
Show answer
72 °F in °C: 22.22222222
- How many litres are in 5 US gallons?
Show answer
5 gal in L: 18.92705892
Common mistakes to avoid
- Confusing US and UK gallons. This tool uses the US gallon (3.785 L); the imperial gallon is about 4.546 L.
- Mixing mass and weight. Pounds and kilograms measure mass here, which is what everyday scales report.
- Converting temperature differences with the temperature formula. A 10 °C rise is a 18 °F rise, not 50 °F.
Frequently asked questions
How many kilometres are in a mile?
One mile is exactly 1.609344 kilometres.
How do I convert Celsius to Fahrenheit?
Multiply by 9/5 and add 32. So 20 °C is 20 × 1.8 + 32 = 68 °F.
Is a US cup the same as a metric cup?
No. A US cup is about 236.6 mL; a metric cup is 250 mL. This tool uses the US cup.
Why do I get an error for temperature below −273.15?
Absolute zero, −273.15 °C (0 K), is the lowest possible temperature.
Reviewed September 26, 2026 by the SolveCalcPro editorial team. Found a mistake? Tell us; see our editorial policy.