A unit converter turns a measurement in one system into its equivalent in another, whether that means inches to centimeters, pounds to kilograms, or Celsius to Fahrenheit. This tool uses the exact reference factors published by the National Institute of Standards and Technology, not rounded approximations. Choose a category and units, then enter a value to see the result and its formula.
Enter your values to see the result.
1 inch to centimeters → 2.54 cm
NIST defines 1 in = 2.54 cm exactly, so this conversion has no rounding error.
100 °C to Fahrenheit → 212 °F
Temperature conversion is affine, not a multiplication: °F = °C × 9/5 + 32.
1 pound to kilograms → 0.45359237 kg
NIST defines 1 lb = 0.45359237 kg exactly, the modern legal definition of the pound.
Most of the world measures length in meters and mass in kilograms, following the International System of Units, commonly called SI or the metric system. The United States, and to varying degrees the United Kingdom and a handful of other countries, still rely on the US customary system for everyday use: inches, feet, miles, ounces, and pounds. Anyone who cooks from a British recipe, reads a US engineering drawing, or checks the weather in a country that uses the other scale runs into the same problem, converting a number from one system into the other without losing accuracy along the way. This tool exists to make that conversion instant and exact, across the four families of units people ask about most: length, mass, temperature, and volume.
Every non-temperature conversion in this tool relies on a fixed factor: how many base units — a meter, a kilogram, or a liter — one unit of the source is worth. These factors are not measured approximations; they are legal definitions published by the National Institute of Standards and Technology (NIST) in Special Publication 811, "Guide for the Use of the International System of Units." The inch, for example, is defined as exactly 2.54 centimeters, a relationship fixed by international agreement in 1959 specifically so that scientific instruments and manufactured parts would mean the same thing on both sides of the Atlantic. The pound is defined as exactly 0.45359237 kilograms under the same agreement. Because these are definitions rather than measurements, there is no "true" value hiding behind them waiting to be discovered more precisely — 1 inch will always equal 2.54 centimeters, by agreement, the same way a mile will always equal exactly 1.609344 kilometers.
This matters because many online converters silently round their factors, which introduces small errors that compound when you convert back and forth or work with large numbers. Converting 37.5 meters to feet and immediately converting the result back to meters should return 37.5, not 37.499998 or 37.500003. This tool keeps its stored factors at full precision and only rounds the final displayed result, so round-trip conversions stay accurate and repeated conversions do not drift.
Length and mass conversions share a convenient property: their scales both start at zero. Zero meters is no length, zero kilograms is no mass, and zero feet or zero pounds mean exactly the same thing. That shared starting point is what makes a simple multiplication work — multiply by a single factor and you are done.
Celsius and Fahrenheit do not share a zero point. Zero degrees Celsius, the freezing point of water, corresponds to 32 degrees Fahrenheit, not zero. Because the two scales are offset from each other as well as stretched to different degree sizes, converting between them needs two operations, not one: a multiplication to account for the different degree size, and an addition or subtraction to account for the offset. The formula from Celsius to Fahrenheit is °F = °C × 9/5 + 32, and the reverse is °C = (°F − 32) × 5/9. Mathematicians call this kind of relationship affine rather than linear, precisely because of that added offset term. Kelvin, used throughout science, is linear with Celsius — it is simply Celsius shifted so that zero falls at absolute zero instead of the freezing point of water — but converting kelvin to Fahrenheit still needs the same two-step treatment as Celsius does, because Fahrenheit's offset does not disappear.
This tool converts every temperature through kelvin as a common reference point, applying the correct offset and scale for whichever pair of units you choose, rather than trying to maintain a separate formula for every possible pair. It also refuses any input below absolute zero — 0 kelvin, equal to -273.15 °C or -459.67 °F — because no physical temperature can go lower, and showing a computed answer for an impossible input would be misleading rather than useful.
Length, mass, temperature, and volume are different physical quantities, and no number converts one into another without extra information the converter does not have. A meter measures distance; a kilogram measures how much matter something contains. You could relate the two if you knew the density of a specific material — a cubic meter of water weighs roughly 1,000 kilograms, but a cubic meter of feathers does not — but that relationship depends entirely on what is being measured, not on the units themselves. Because there is no universal factor connecting length to mass, or either of them to temperature, this converter groups every unit into one of four families and only allows conversions within the same family. Asking to convert meters to kilograms produces a clear explanation instead of a nonsense number, which is far more useful than an answer that looks precise but means nothing.
This converter covers the situations people run into most often: reading a US recipe that lists cups and Fahrenheit when you think in milliliters and Celsius, checking a UK product's weight in pounds against a kilogram figure on a nutrition label, converting a US highway distance in miles into kilometers for a trip planner, or translating a scientific temperature reading in kelvin into something more familiar. Whatever the direction, the displayed formula lets you verify the arithmetic yourself and understand exactly how the result was reached, rather than trusting a black box.
Because that is its definition, not a measurement. In 1959, the United States, the United Kingdom, and several other countries signed the International Yard and Pound Agreement, which fixed the inch at exactly 2.54 centimeters so that scientific and engineering work using either system would agree perfectly. Before that agreement, slightly different inch definitions existed in different countries, which caused small but real discrepancies in precision manufacturing. Since 1959, 2.54 has been exact by definition, with no rounding involved at any number of decimal places.
The reference factors themselves are exact, taken directly from NIST Special Publication 811, which lists the legally defined relationships between US customary units and the metric system. The arithmetic performed on your input can still produce a long decimal, for example when converting an irregular number of miles to kilometers, and the display rounds that result to twelve significant digits purely for readability. The underlying factor used to get there, such as 1 mile = 1.609344 km, is never approximated.
Because meters measure length and kilograms measure mass, and no factor exists to turn one into the other without additional information, such as the density of a specific material. This converter groups units into families — length, mass, temperature, and volume — and only allows conversions within the same family, so a request to cross families is refused with an explanation rather than producing a meaningless number.
Length and weight conversions are simple multiplications because both scales share a common zero point: zero meters and zero feet both mean no length at all. Celsius and Fahrenheit do not share a zero point — 0 °C is 32 °F, not 0 °F — so converting between them requires both a multiplication and an addition or subtraction. This kind of relationship is called affine rather than linear, and it is also why you cannot express a Celsius-to-Fahrenheit relationship as a single "how many of one equals one of the other" factor the way you can for inches and centimeters.
The converter refuses the input and explains why instead of returning a number. Absolute zero, 0 kelvin, is the lowest temperature that is physically possible — equal to -273.15 °C or -459.67 °F — because it represents the point where particles have the minimum possible thermal energy. Any value colder than that does not correspond to anything that can exist, so showing a computed result for it would be misleading rather than helpful.
Last updated on 2026-09-20 · Written by UtiliaTools