Conversion Calculators
Units of length, weight, volume, temperature, speed and data.
- Acres to Square Feet Calculator Convert acres to square feet exactly, with square metres, hectares, square yards and the side of an equal square plot.
- Area Converter Convert areas between square millimetres, metres and kilometres, hectares, square inches, feet, yards and miles, and acres.
- Celsius to Fahrenheit Calculator Convert degrees Celsius to Fahrenheit with the formula shown step by step, plus kelvin and a quick mental estimate.
- cm to Inches Calculator Convert centimetres to inches, with feet and inches and the nearest sixteenth of an inch as on a tape measure.
- Data Storage Converter Convert data sizes between bits, bytes, KB, MB, GB, TB and PB, and binary KiB, MiB, GiB and TiB.
- Data Transfer Rate Converter Convert data rates between bps, kbps, Mbps and Gbps and bytes per second (kB/s, MB/s, GB/s, MiB/s), with transfer times.
- Energy Converter Convert energy between joules, kilojoules, calories, kilocalories, kWh, BTU, therms, foot-pounds and electronvolts.
- Fahrenheit to Celsius Calculator Convert degrees Fahrenheit to Celsius with the formula shown step by step, plus kelvin and a quick mental estimate.
- Feet to Meters Calculator Convert feet, or feet and inches such as a height, to metres, with centimetres, millimetres and yards.
- Fuel Economy Converter Convert fuel economy between US mpg, UK mpg, litres per 100 km and km per litre, with fuel for 100 km and 100 miles.
- Gallons to Liters Calculator Convert US or UK gallons to litres exactly, with millilitres, the other gallon, and a price per litre from a price per gallon.
- Grams to Ounces Calculator Convert grams to ounces exactly, with pounds, troy ounces for gold and silver, and the nearest quarter ounce for cooking.
- Hectares to Acres Calculator Convert hectares to acres exactly, with square metres, square kilometres, square feet and a price per hectare as a price per acre.
- Inches to cm Calculator Convert inches, or feet and inches such as a height, to centimetres, metres and millimetres exactly.
- Joules to Calories Calculator Convert joules to calories and kilocalories exactly, with kilojoules, International Table calories and watt-hours.
- kg to Pounds Calculator Convert kilograms to pounds exactly, with pounds and ounces, stone and pounds, and total ounces.
- km to Miles Calculator Convert kilometres to miles exactly, with yards or feet, nautical miles and walking and driving times.
- Knots to mph Calculator Convert knots to miles per hour and km/h exactly, with metres per second, distance per day and the Beaufort wind force.
- Length Converter Convert lengths and distances between millimetres, centimetres, metres, kilometres, inches, feet, yards and miles.
- Liters to Gallons Calculator Convert litres to US and UK gallons exactly, with quarts, pints and fluid ounces, and a price per gallon from a price per litre.
- Meters to Feet Calculator Convert metres to feet, and to feet and inches, inches and yards, using the exact factor of 0.3048 metres per foot.
- Miles to km Calculator Convert miles to kilometres exactly, with metres, nautical miles and walking and driving times.
- ml to oz Calculator Convert millilitres to US or UK fluid ounces exactly, with cups, tablespoons and teaspoons and the weight of the same volume of water.
- mph to km/h Calculator Convert miles per hour to kilometres per hour exactly, with metres and feet per second and the distance covered each second.
- Ounces to Grams Calculator Convert ounces to grams exactly, for everyday (avoirdupois) ounces or troy ounces used for gold and silver.
- Pounds to kg Calculator Convert pounds, or pounds and ounces, to kilograms and grams exactly, with total ounces and stone.
- Power Converter Convert power between watts, kilowatts, megawatts, horsepower, metric horsepower, BTU per hour and foot-pounds per second.
- Pressure Converter Convert pressure between psi, bar, kPa, pascals, atmospheres, millibars, mmHg and inches of mercury.
- psi to bar Calculator Convert psi to bar exactly, with kPa, MPa and atmospheres, and the absolute pressure for a gauge reading such as a tyre.
- Roman Numeral Converter Convert numbers from 1 to 3,999 to Roman numerals, or Roman numerals back to numbers, with the breakdown of each symbol.
- Shoe Size Converter Convert adult shoe sizes between UK, EU, US men's and US women's sizes, or from foot length, with the approximate foot length and Mondopoint.
- Speed Converter Convert speeds between km/h, mph, metres per second, knots and feet per second, with pace and travel time.
- Square Feet to Square Meters Calculator Convert square feet to square metres exactly, from an area or a room's length and width, with a price per square foot as per square metre.
- Stone to Pounds Calculator Convert stone, or stone and pounds, to pounds and kilograms, as body weight is often given in the UK and Ireland.
- Temperature Converter Convert temperatures between Celsius, Fahrenheit, kelvin and Rankine, as readings or as temperature differences.
- Time Converter Convert time between seconds, minutes, hours, days, weeks and years, with a days-hours-minutes breakdown.
- Tons to Yards Calculator Convert tons of gravel, sand, soil, mulch or other bulk material to cubic yards, with how much area that covers at common depths.
- Unit Converter Convert length, weight, volume, area, temperature, speed and time between metric and imperial units, with the exact factor.
- Volume Converter Convert volumes between millilitres, litres, cubic metres, US and UK gallons, pints, cups, fluid ounces and spoons.
- Watts to Horsepower Calculator Convert watts to mechanical, metric and electrical horsepower exactly, with kilowatts and the energy used in an hour.
- Weight Converter Convert weights between milligrams, grams, kilograms, tonnes, ounces, pounds, stone and US and UK tons.
- Yards to Tons Calculator Convert cubic yards of gravel, sand, soil or other bulk material to tons and pounds, with pickup and dump truck loads to haul it.
A Guide to Conversion Calculators
Unit conversion is the one kind of calculation where the arithmetic is trivial and the mistakes are expensive. Multiply by the wrong factor, or by the right factor in the wrong direction, and the answer is confidently wrong with nothing on the page to suggest it.
The forty-two converters here are built to make the direction unmistakable. Each states the factor it used, so the result can be checked at a glance, and the dedicated one-way converters are labelled in the direction people actually search for rather than asking you to work out which way round a general tool is pointing.
Why there are two calculators for each pair
You will notice cm to inches and inches to cm as separate pages, and the same for several other pairs. That is deliberate, not duplication.
Someone converting a 32-inch waist to centimetres and someone converting a 170 cm height to feet are doing opposite jobs, and they want different things on the page: a different example, a different reference table, a different set of common values. A single page serving both has to be vaguer than either, and a general converter with a direction toggle adds a step before you get an answer. The factor is the same; the page is not.
Temperature
Celsius to Fahrenheit and Fahrenheit to Celsius are the two directions, and the general Temperature Converter adds Kelvin.
Temperature is the awkward case in unit conversion, because the scales have different zeros as well as different sizes of degree. That means a temperature cannot be converted by multiplying alone, and a temperature difference converts differently from a temperature. A rise of 10 degrees Celsius is a rise of 18 degrees Fahrenheit, not 50.
Length and distance
cm to inches and inches to cm for the small end, feet to meters and meters to feet in the middle, and km to miles and miles to km for distance. The general Length Converter handles anything else, including the units that come up rarely enough that nobody remembers the factor.
Weight and mass
kg to pounds and pounds to kg are the common pair, with grams to ounces and ounces to grams for smaller quantities and stone to pounds for the unit still used for body weight in Britain and Ireland. The general Weight Converter covers the rest.
One thing worth knowing: the ounce in this group is the ounce of weight. The fluid ounce is a volume and is not related to it by any fixed factor, because that depends on what the liquid is. Converting a recipe's fluid ounces as though they were weight is a common and unrecoverable error.
Volume and capacity
Gallons to liters and liters to gallons carry a trap: the US gallon and the imperial gallon are different sizes, by about twenty percent. These calculators ask which you mean rather than guessing from your location, and show both.
ml to oz handles the smaller end, and the general Volume Converter covers cubic measures as well as capacity.
Area
Acres to square feet, hectares to acres and square feet to square meters cover land and floor measurement, with the general Area Converter for the rest.
Area conversion is where people most often convert correctly and still get the wrong answer, because a linear factor has to be squared. Three feet make a metre to within a few percent, but nine square feet do not make a square metre: the factor is about 10.76.
Speed
mph to km/h for road speeds and knots to mph for marine and aviation use, with the general Speed Converter alongside. A knot is one nautical mile per hour, and the nautical mile is defined from the Earth's circumference rather than from the statute mile, which is why the factor looks arbitrary.
Energy, power and pressure
Joules to calories spans the physics and nutrition conventions, which differ by a factor of a thousand: the dietary Calorie is a kilocalorie. Watts to horsepower converts power, and psi to bar pressure.
The general Energy Converter, Power Converter and Pressure Converter cover wider ranges of units. Pressure needs care about whether a figure is absolute or gauge, since the difference is one atmosphere and both conventions are in everyday use.
Digital units
The Data Storage Converter and Data Transfer Rate Converter both handle the distinction that causes most confusion here. Storage is often quoted in powers of ten by manufacturers and powers of two by operating systems, which is why a 1 TB drive shows as about 931 GB. Transfer rates are quoted in bits per second while file sizes are in bytes, a factor of eight, so a 100 Mbps connection moves about 12.5 MB each second.
Fuel economy and time
The Fuel Economy Converter handles miles per gallon, kilometres per litre and litres per 100 km. The last of these is inverted relative to the others, so improving efficiency makes the number go down rather than up, and conversions between the two conventions are reciprocal rather than proportional. That also means the difference between 15 and 20 mpg is a much larger saving than the difference between 40 and 45 mpg, despite being the same five units.
The Time Converter covers seconds through years, and the general Unit Converter is the single page that handles every category if you would rather not pick one first.
Specialty converters
Two that do not fit a physical dimension. The Roman Numeral Converter works in both directions and enforces the subtractive rules, so it will tell you that IIII is not valid rather than accepting it. The Shoe Size Converter maps between UK, US and EU sizing, which is not a conversion in the mathematical sense at all: the systems are not linearly related and differ by manufacturer, so the result is the nearest standard size rather than an exact figure, and the page says so.
Tons and yards
Tons to yards and yards to tons convert between weight and volume, which is only possible if you know the density of the material. These calculators therefore ask what you are converting, because gravel, topsoil, sand and mulch all give different answers from the same volume. Any tool that offers a single tons-to-yards factor without asking is averaging over materials that differ by a factor of two.
When a conversion is not really a conversion
Three of the categories above look like unit conversion and are not, which is worth knowing before you trust a single factor.
Weight to volume needs a density, so it is a property of the material rather than of the units. Shoe and clothing sizes are conventions that were never defined against each other, so the mapping is approximate by nature. And currency, which this site deliberately does not convert, changes by the minute; a figure cached in a page would be wrong before you read it, so currency here is a display choice applied to your own figures rather than a conversion.
In each of those cases a tool that hands back a precise-looking number is overstating what it knows. The honest output is a range, a nearest standard value, or a request for the one piece of information that makes the conversion possible.
Checking a conversion
The quickest check on any conversion is a rough mental estimate first. A metre is a bit more than a yard, a kilogram is a bit more than two pounds, a litre is a bit more than a quart, and a kilometre is a bit more than half a mile. If the answer is not close to what the estimate suggests, the factor has probably been applied upside down, and that is the single most common conversion error there is.