Unit Converter

What a unit converter does and why exact factors matter

A unit converter changes a measurement expressed in one unit into the equivalent value in another unit — miles into kilometers, pounds into kilograms, Celsius into Fahrenheit — without changing the actual quantity being measured. The tricky part isn't the concept, it's the exact conversion factor, because small rounding errors compound quickly in recipes, construction measurements, travel planning, or scientific work.

This page lists the exact factors used for length, weight, temperature, volume, area, and speed, along with worked examples so you can double-check any conversion by hand. Every factor here is a defined constant, not a rounded approximation, which is what separates a reliable converter from one that quietly drifts off after a few decimal places [1](https://www.nist.gov/pml/owm/metric-si/unit-conversion).

Length conversions

The base relationship for most everyday length conversions is 1 inch = 2.54 centimeters exactly, and 1 mile = 1.609344 kilometers exactly. From those, 1 foot = 30.48 cm (12 inches × 2.54), and 1 yard = 91.44 cm (3 feet × 30.48).

Example: converting 5 miles to kilometers is 5 × 1.609344 = 8.04672 km. Converting 100 centimeters to inches is 100 ÷ 2.54 = 39.3701 inches. Converting 6 feet 2 inches to centimeters: 6 feet = 182.88 cm, plus 2 inches = 5.08 cm, for a total of 187.96 cm — a common one for describing someone's height across metric and imperial systems.

Weight and mass conversions

The standard factor is 1 kilogram = 2.20462262 pounds, and its inverse, 1 pound = 0.45359237 kilograms exactly (this is the internationally agreed definition of the pound). For ounces, 1 pound = 16 ounces, so 1 kilogram = 35.27396 ounces.

Example: a 70 kg adult weighs 70 × 2.20462262 = 154.32 lb. A 10-pound bag of flour weighs 10 × 0.45359237 = 4.5359 kg. For a recipe calling for 8 ounces of butter: 8 ÷ 35.27396 × 1000 = 226.8 grams, since 1 kg = 1000 g.

Temperature conversions

Temperature is different from length and weight because it uses an offset, not just a multiplier — you can't just multiply Celsius by a factor to get Fahrenheit. The formula is F = C × 9/5 + 32, and the reverse is C = (F − 32) × 5/9.

Example: converting 25°C to Fahrenheit: 25 × 9/5 = 45, plus 32 = 77°F, a comfortable room temperature. Converting 98.6°F (normal body temperature) to Celsius: (98.6 − 32) × 5/9 = 66.6 × 5/9 = 37°C. Converting 0°C, the freezing point of water, gives 0 × 9/5 + 32 = 32°F, and 100°C, the boiling point, gives 100 × 9/5 + 32 = 212°F.

For Kelvin, the scale used in science, there's no offset multiplication at all — you just add or subtract 273.15: K = C + 273.15. So 25°C equals 298.15 K, and absolute zero, 0 K, equals −273.15°C.

Volume conversions

For volume, 1 liter = 1.05669 US quarts, and 1 US gallon = 3.785411784 liters exactly. A US gallon is different from a UK (imperial) gallon, which equals 4.54609 liters — mixing these up is one of the most common conversion errors, especially in fuel economy and recipe conversions between American and British sources [2](https://convr.tools/guides/common-unit-conversion-mistakes/).

Example: a 2-liter soda bottle holds 2 × 1.05669 = 2.113 US quarts, or about 0.528 US gallons (2 ÷ 3.785411784). Converting 12 US fluid ounces (a standard can) to milliliters: 12 × 29.5735 = 354.88 mL. Cooking measurements often trip people up too: 1 US cup = 236.588 mL, while 1 UK cup (used in older British recipes) is closer to 284 mL, so check which system a recipe was written in before converting.

Area conversions

Area conversions square the linear factor, which is where a lot of mistakes happen — people forget to square the number and just reuse the length factor. Since 1 meter = 3.28084 feet, 1 square meter = 3.28084² = 10.7639 square feet, not 3.28084 square feet.

For land measurement, 1 acre = 4,046.86 square meters, and 1 hectare = 10,000 square meters = 2.47105 acres. Example: a 2-hectare plot of land equals 2 × 2.47105 = 4.9421 acres. A 1,500-square-foot house equals 1,500 ÷ 10.7639 = 139.35 square meters.

Speed conversions

Speed conversions combine a length factor with a time factor, so the two most useful constants are 1 mph = 1.609344 km/h (the same number as the mile-to-kilometer factor, since both are per hour), and 1 m/s = 3.6 km/h.

Example: a car traveling at 60 mph is going 60 × 1.609344 = 96.56 km/h. A world-record sprint speed of 12 m/s converts to 12 × 3.6 = 43.2 km/h, or roughly 26.85 mph (43.2 ÷ 1.609344). Wind speed reports sometimes use knots: 1 knot = 1.852 km/h, so a 20-knot wind is 20 × 1.852 = 37.04 km/h.

How the linear conversion method works

Most conversions in this tool (everything except temperature) follow the same pattern: convert the input value to a base unit, then convert from the base unit to the target unit. For length, the base unit is typically the meter; for weight, the kilogram; for volume, the liter. The general formula is: Result = Input Value × (Source Factor ÷ Target Factor), where each factor expresses how that unit relates to the shared base unit [5](https://supercalc.dev/tools/unit-converter/).

This two-step approach (through a common base) is why a good converter can handle dozens of units without needing a separate formula for every possible pair — it only needs one factor per unit relative to the base, and the base cancels out in the middle of the calculation.

Common unit conversion mistakes

Forgetting to square or cube factors for area and volume conversions is one of the most frequent errors, along with confusing US and UK versions of gallons, cups, pints, and tons (a US short ton is 2,000 lb; a UK long ton is 2,240 lb; a metric tonne is 1,000 kg) [2](https://convr.tools/guides/common-unit-conversion-mistakes/).

Rounding too early in a multi-step conversion is another common issue — carry full precision through intermediate steps and round only the final answer. And with temperature, the most common mistake by far is trying to apply a straight multiplication factor the way you would for length or weight, when Celsius and Fahrenheit actually use different zero points and require the offset formula shown above.

Finally, always double-check which specific unit is meant when a name is ambiguous — "ton," "gallon," "ounce" (weight vs. fluid ounce), and "mile" (statute vs. nautical) all have more than one definition depending on the country or context, and picking the wrong one can throw off a result by a meaningful margin.