How to use the unit converter
Step 1 — Pick what you are converting
Choose a category first — length, weight, temperature, cooking, data and twelve more. Everything after that is filtered to units that actually belong together, so it is impossible to ask for kilograms in miles and be told off for it.
Step 2 — Type the value
Enter any number, including decimals and negatives. Pasted numbers with thousands separators work too, in either the 1,234.5 or the 1 234,5 style. The result updates as you type.
Step 3 — Choose the two units
Pick what you are converting from and to, or press the swap button to reverse them. The large figure is your answer; underneath it the full sentence, and under that the factor or formula being used.
Step 4 — See every other unit too
The table below shows the same value in every unit of that category at once — so one look tells you what 2 metres is in feet, inches, yards and millimetres together. Any row copies with one click.
Step 5 — Tune the numbers
Set how many significant digits you want (4 to 12), choose decimal or scientific notation, and turn thousands separators on or off. Your choices are remembered for next time, on your device only.
Step 6 — Take it away
Copy the result on its own, copy the whole sentence for a message, or download the full table as a CSV. Your last ten conversions stay in the list underneath, each with a button to run it again.
About the unit converter
This tool converts between 139 units across 17 categories — length, weight, temperature, area, volume, cooking measures, speed, time, data, data rate, energy, power, pressure, force, angle, fuel economy and frequency. Enter a value, pick two units, and you get the answer plus the same value in every other unit of that category. It is free, needs no account, and runs entirely in your browser.
Why the factors here are exact
Most conversion factors people quote are rounded, and the rounding is usually invisible until it is not. A mile is not 1,609.34 metres, it is exactly 1,609.344. A pound is exactly 0.45359237 kilograms, not 0.453592. An acre is exactly 4,046.8564224 square metres. These are not approximations of a physical measurement — they are definitions, fixed by the international yard and pound agreement of 1959, and every factor in this tool uses the full definition rather than a shortened one.
That matters as soon as you multiply. Convert a marathon at the rounded figure and you are about 10 centimetres out; convert a shipping manifest in pounds and the error compounds with every line. It costs nothing to be exact, so the tool is.
Temperature is a formula, not a factor
Length, weight and most other units share a zero: nothing is zero metres long in one unit and something in another, so a single multiplication converts them. Temperature scales do not share a zero, so they need a formula, and the tool shows you the one it used:
- °F = °C × 9/5 + 32 — and the reverse, °C = (°F − 32) × 5/9.
- K = °C + 273.15. Kelvin has no degree sign and no negative values — 0 K is absolute zero, and the tool says so if you try.
- −40 is the one temperature where Celsius and Fahrenheit agree, which is a quick way to check any converter.
The unit pairs that are not the same unit
Several everyday units share a name and not a size. Getting these wrong is the most common cause of a conversion that looks right and is not, so they are listed separately here rather than merged into one entry:
- US and imperial gallons. A US gallon is 3.785411784 L; an imperial gallon is 4.54609 L — 20% larger. Pints and fluid ounces differ the same way.
- Kilobytes and kibibytes. A kilobyte is 1,000 bytes; the 1,024-byte unit is a kibibyte. Drive manufacturers count in kilobytes, Windows displays kibibytes and labels them KB, which is exactly why a “1 TB” drive shows as 931 GB.
- Horsepower. Mechanical horsepower is 745.69987 W; metric horsepower — PS, CV or pk — is 735.49875 W. European car figures normally use the second.
- Tons. A tonne is 1,000 kg, a US short ton is 907.18474 kg, and a UK long ton is 1,016.0469 kg.
- Calories. The calorie on a food label is a kilocalorie — 1,000 of the calories used in physics.
- Tablespoons. 15 mL in most of the world, 14.79 mL in the US, and 20 mL in Australia.
Fuel economy runs backwards
Litres per 100 km and miles per gallon do not just use different units, they run in opposite directions: a lower L/100km is better, a higher mpg is better. That makes the conversion a division rather than a multiplication, and it is why the difference between 5 and 6 L/100km matters far more than the difference between 40 and 41 mpg. The tool handles the inversion properly and tells you it is doing it.
Miles per gallon also comes in two sizes, because of the gallon: 30 mpg (US) is 36 mpg (UK) for exactly the same car.
Significant digits, not decimal places
Asking for “four decimal places” is the wrong question for a converter. Four decimal places turns 1 km into 1000000.0000 mm, which is noise, and turns 1 mm into 0.0000 km, which is wrong. This tool works in significant digits instead, so both come out readable: 1,000,000 and 0.000001. Very large and very small results switch to scientific notation automatically, or you can force either style.
Where this fits with the other tools
For money rather than measurements, Currency Converter uses live rates. Date Difference Calculator and Time Zone Converter handle the parts of time that are not simple multiplication, and Timestamp Converter deals with Unix time. For body measurements there is BMI Calculator, and for data work, CSV to JSON and Number to Words.
Frequently asked questions
Is the unit converter free?
Completely — no account, no email, no limit. Every conversion is arithmetic done in your browser, so there is nothing for us to meter.
How accurate is it?
The factors are the exact defined values, not rounded ones: a mile is 1,609.344 m, a pound is 0.45359237 kg, an acre is 4,046.8564224 m². Results are then shown to as many significant digits as you ask for, from four to twelve.
Why can I not convert kilograms to miles?
Because there is no answer. You choose a category first and the unit lists only contain units of that quantity, which removes the whole class of mistake — the previous version let you select the pair and then refused to convert it.
Why is 1 TB not 1,000 GB on my computer?
It is, in the sense the manufacturer means. Drives are sold in kilobytes of 1,000 bytes, while Windows measures in units of 1,024 and labels them KB, MB and GB anyway. One terabyte is 931.32257 gibibytes, which is the “missing” space. Both units are in the Data category here, correctly named, so you can see the difference rather than argue with it.
Which gallon does the tool use?
Whichever you choose. US and imperial gallons, pints and fluid ounces are separate units in the Volume list, because they are separate units — an imperial gallon holds 20% more than a US one.
How do I convert Celsius to Fahrenheit by hand?
Multiply by 9, divide by 5, add 32. To go back, subtract 32, multiply by 5, divide by 9. For a rough figure in your head, double the Celsius and add 30 — that is within a couple of degrees over normal weather temperatures.
Can I see one value in several units at once?
That is the table under the converter: your value in every unit of the chosen category, updating as you type, with a copy button on each row and a CSV download for the lot.
Does it remember my settings?
Significant digits, notation, separators, the category you were in and your last ten conversions are stored in your own browser, on your device. Nothing is sent anywhere and clearing your browser data removes them.
What is a metric horsepower?
735.49875 watts, written PS in Germany, CV in France and pk in the Netherlands. Mechanical horsepower — the figure used in the UK and US — is 745.69987 watts, about 1.4% more. Both are in the Power list.
Why does the fuel economy conversion behave differently?
Because litres per 100 km is an inverse measure: it counts fuel per distance, while mpg counts distance per fuel. Converting between them is a division, so the scale is not linear — the tool says so on screen when you cross between the two kinds.
Are the cooking measurements reliable for baking?
For liquids, yes — a cup is a fixed volume. For dry ingredients, no converter can help: a cup of flour is anywhere between 120 and 150 g depending on how it was scooped, which is why serious baking recipes give weights. Use the Cooking category for volumes and a scale for solids.
Does it work offline?
Once the page has loaded, yes. Nothing is fetched while you convert — the factors are in the page.