How the unit converter works
This converter handles three of the categories people need most: length, weight (mass) and temperature. Choose a category, type a value, and pick the unit you are converting from and to. The main result appears immediately, and a table below it shows the same value in all the other units of that category, which saves repeating the conversion.
Length and weight conversions use a base unit. Every unit is first converted to the base (metres for length, kilograms for weight) using an exact defined factor, and then converted from the base into the target unit. This is how the tool can support any pair of units with only one factor per unit. The factors come from the international definitions: an inch is exactly 2.54 centimetres, a foot is 0.3048 metres, a mile is 1,609.344 metres, and a pound is exactly 0.45359237 kilograms.
Temperature is different because scales have different zero points as well as different step sizes. Celsius, Fahrenheit and Kelvin cannot be converted by multiplication alone. The tool converts through Celsius with the proper offsets and rejects values below absolute zero.
Key formulas
Length: value × (factor of from-unit in metres) / (factor of to-unit in metres)
°F = °C × 9/5 + 32
°C = (°F − 32) × 5/9
K = °C + 273.15
Useful quick facts: 1 inch = 2.54 cm, 1 mile = 1.609 km, 1 kg = 2.2046 lb, 1 lb = 16 oz, 1 stone = 14 lb.
Worked examples
Miles to kilometres. A 5 mile run is 5 × 1609.344 = 8,046.72 metres, or 8.04672 km.
Pounds to kilograms. 150 lb × 0.45359237 = 68.04 kg.
Fahrenheit to Celsius. A room at 72°F: subtract 32 to get 40, multiply by 5/9 to get 22.2°C. Interestingly, −40 is the same on both scales, which is a handy check for any temperature formula.
Tips for accurate conversions
- Watch the unit names. Ounces here are weight (avoirdupois) ounces, not fluid ounces, which measure volume.
- Round last. Keep full precision until the final answer, especially when converting in several steps.
- Use sensible precision. A measurement of 5 miles does not justify six decimal places in practice, but the extra digits are there if you need them.
- Cooking and baking. For recipes, weight is far more reliable than volume.
- Check the scale in science and medicine, where Kelvin and exact dosing units are essential, and confirm with official references.
Frequently asked questions
How many kilometres are in a mile?
One mile is exactly 1.609344 kilometres. To convert miles to kilometres, multiply by 1.609344. To go the other way, divide by 1.609344 or multiply by about 0.6214.
How do I convert Celsius to Fahrenheit quickly?
Multiply by 9/5 (or 1.8) and add 32. As a rough mental shortcut, double the Celsius value and add 30. It is close enough for everyday weather temperatures.
What is the difference between mass and weight?
Mass is the amount of matter in an object and is measured in kilograms. Weight is the force gravity exerts on that mass. In everyday use, "weight" in kg or lb really means mass, and that is what this converter handles.
Why does the calculator reject some temperatures?
Nothing can be colder than absolute zero, which is 0 K, −273.15°C or −459.67°F. Values below that are impossible and are reported as an error.
Are the conversion factors exact?
Yes, for the units offered here the factors are defined exactly by international agreement (for example 1 inch = 2.54 cm and 1 lb = 0.45359237 kg). Any rounding comes only from how many digits are displayed.