Unit Converters

Most unit conversions are a single multiplication by a fixed factor: feet to meters, pounds to kilograms, miles per hour to meters per second. The factors are not arbitrary; since the late twentieth century they are defined exactly, so one inch is exactly 25.4 millimeters and one pound is exactly 0.45359237 kilograms. Our converters pass every value through a common base unit, which keeps the results consistent no matter which pair of units you choose.

A few conversions behave differently and are worth understanding. Temperature scales have different zero points, so Celsius to Fahrenheit needs both a multiplication and an offset, while Celsius to Kelvin is addition only. Area and volume factors are the length factor squared or cubed, which is why the numbers look surprising — a square meter is about 10.76 square feet, not 3.28. And fuel economy is a ratio that can be written two ways: miles per gallon measures distance per fuel, while liters per 100 kilometers measures fuel per distance, so converting between them is an inverse relationship rather than a simple multiple.

Because conversions are exact, they are also easy to check by hand. We show the formula and a worked example on each tool page so you can confirm the result or reuse the factor in a spreadsheet.

Seven converters and the units each one covers

Each converter is built around a family of related units, and knowing what is in the family saves a search. The Length Converter handles millimeters, centimeters, meters, kilometers, inches, feet, yards and miles. The Weight Converter covers milligrams, grams, kilograms, metric tons, ounces, pounds, stone and US short tons. The Temperature Converter moves between Celsius, Fahrenheit and Kelvin. The Area Converter spans square inches through square miles, plus acres and hectares. The Volume Converter pairs milliliters, liters and cubic meters with cubic feet and the US customary units from gallons down to teaspoons. The Speed Converter works in miles per hour, kilometers per hour, meters per second and knots, and the Fuel Economy Converter relates US MPG, liters per 100 kilometers and kilometers per liter.

Most of these tools show a value box and two drop-downs, one for the unit you have and one for the unit you want. That is deliberate: a converter that asks only two questions is hard to use incorrectly. Where several units sit in a single drop-down, the tool treats them as one scale, so you can go from millimeters to miles or from teaspoons to cubic meters in one step without chaining anything by hand.

If you meet a unit that is missing, use a nearby one to bridge the gap. A UK long ton, for instance, is not in the weight list, but it equals 2,240 pounds, so you can convert from pounds. The matching guides on this site explain metric versus imperial systems and the Celsius, Fahrenheit and Kelvin scales for readers who want the background.

Why a common base unit keeps the answers consistent

Behind each converter is a single base unit: meters for length, grams for weight, square meters for area, liters for volume, meters per second for speed and kilometers per liter for fuel economy. Your value is converted into that base, then out to the unit you picked. The benefit is that every pair of units in a family is reachable with just one table of factors, and the results agree with each other. Convert feet to meters and meters to yards, and you will land on the same answer as converting feet to yards directly.

That consistency is also why you should avoid rounding in the middle of a chain of conversions. If you round a result to three digits and then convert it again, the small error carries forward. When you need a result in a unit the tool lists, enter the original number and pick the target unit directly, rather than copying an intermediate answer. For a quick sanity check on any result, a few rules of thumb are enough to tell whether it is the right size.

• Inches to centimeters: multiply by 2.54, so 70 inches is 177.8 centimeters.
• Kilometers to miles: multiply by about 0.6214, so 100 kilometers is about 62.1 miles.
• Kilograms to pounds: multiply by about 2.2046.
• Liters to US gallons: divide by about 3.785.
• Meters per second to kilometers per hour: multiply by 3.6.
• Square meters to square feet: multiply by about 10.764.

Worked conversions from everyday situations

A recipe from a US cookbook calls for a cup, and your measuring jug is in milliliters. A US cup is one sixteenth of a gallon, which works out to about 236.6 milliliters, and a US fluid ounce is about 29.6. In the kitchen, a recipe at 350 degrees Fahrenheit corresponds to about 176.7 degrees Celsius, and normal body temperature of 98.6 degrees Fahrenheit is exactly 37 degrees Celsius. Absolute zero, 0 Kelvin, is minus 273.15 degrees Celsius, and the Temperature Converter blocks values below it.

On the road, 65 miles per hour is about 104.6 kilometers per hour, and a speed limit of 100 kilometers per hour is about 62.1 miles per hour. In a weather report or on a boat, 30 knots is about 34.5 miles per hour, because the knot is built on the nautical mile of 1,852 meters. A wind of 10 meters per second equals 36 kilometers per hour, or roughly 22.4 miles per hour.

For body weight, a person who weighs 11 stone weighs 154 pounds, since a stone is 14 pounds, or about 69.9 kilograms. For land, one acre is 43,560 square feet, about 4,047 square meters or 0.405 hectares, and one hectare is about 2.47 acres. A fuel tank of 50 liters holds about 13.2 US gallons.

Look-alike units that cause real mistakes

Several unit names overlap in ways that catch people out. The word ton covers at least three values: the metric ton is 1,000 kilograms, or about 2,204.6 pounds, while the US short ton is 2,000 pounds, or about 907.2 kilograms. Both appear in the Weight Converter and they differ by more than 10 percent, so be sure which one a shipping quote or a materials supplier means. An ounce measures weight and a fluid ounce measures volume. They are not interchangeable, because a fluid ounce of water, oil or honey each weighs a different amount.

Kilograms are a unit of mass, though everyday speech calls them weight, and the converters follow everyday usage. Area units cause a different slip: a mile is 1,609.344 meters, but a square mile is not 1,609 square meters. It is the length factor squared, about 2.59 million square meters, which is why area and volume numbers often look surprising. The Volume Converter lists US customary units only, so gallons, quarts and cups there are not their larger British imperial counterparts; if a recipe or a fuel receipt is British, convert from the imperial figure with a known factor first.

Fuel economy has its own trap. Because MPG measures distance per fuel and liters per 100 kilometers measures fuel per distance, improvements are not symmetric. Going from 25 to 30 MPG saves about 0.67 gallons per 100 miles, while going from 15 to 20 saves about 1.67, and going from 40 to 45 saves only 0.28. The Fuel Economy Converter makes these comparisons easy: 30 MPG is about 7.84 liters per 100 kilometers, and 8 liters per 100 kilometers is about 29.4 MPG.

Rounding, precision and what the display tells you

The conversion factors in these tools are fixed by definition, so the arithmetic is exact apart from the rounding applied when the result is displayed. That does not make every digit meaningful. If you measured a room as 12 feet, converting it to 3.6576 meters implies a precision your tape measure never had. A good habit is to keep about as many significant digits in the answer as you had in the input. A length known to the nearest foot becomes about 3.7 meters, not 3.6576.

The reverse is also worth remembering. When you are converting a spec, such as a machine part or a medication label, keep every digit given and round only at the end, in the way the spec allows. For anything that matters for safety, such as dosing, structural loads, aviation or pressure vessels, check the result against the documentation for the product and do not rely on a general web tool alone.