Unit Converter
Convert between any two units in the same dimension — metric, imperial, US customary, and SI — with conversion factors that match NIST SP 811 and the 1959 International Yard and Pound Agreement.
1 m =
3.28084 ft
Converted via the length base unit. Imperial factors are exact under the 1959 International Yard and Pound Agreement; SI factors follow NIST SP 811.
How to use this calculator
Type the value you want to convert, pick the "from" unit, then pick the "to" unit. Both units must belong to the same category — converting metres to kilograms is not physically meaningful, so the result pane shows a message instead. The dropdowns group units by category (Length, Mass, Volume, Temperature, Area, Speed, Time, Energy) and each entry shows the full name and symbol so there is no ambiguity between, for example, a US gallon and an imperial gallon.
How the calculation works
Linear units (everything except temperature) are converted through a base unit: metre for length, kilogram for mass, litre for volume, square metre for area, metres per second for speed, second for time, and joule for energy. The result of converting value × from_factor / to_factor is exact when both factors are exact, which is true for every imperial and metric pairing on this page. Temperature uses a kelvin pivot because the Celsius and Fahrenheit scales have non-zero offsets: the calculator first converts the input to kelvin (K = °C + 273.15 or K = (°F − 32) × 5/9 + 273.15), then converts kelvin back out to the target scale.
Worked example
Convert 100 miles to kilometres. The mile is defined as exactly 1609.344 metres under the 1959 International Yard and Pound Agreement, and the kilometre is exactly 1000 metres. So 100 mi × 1609.344 / 1000 = 160.9344 km. As a second example, 100 °C in fahrenheit: K = 100 + 273.15 = 373.15, then °F = (373.15 − 273.15) × 9/5 + 32 = 100 × 1.8 + 32 = 212 — the boiling point of water at standard pressure.
Frequently asked questions
Which conversion factors does this calculator use?
Length, area, mass, and volume use the exact 1959 International Yard and Pound Agreement values (1 inch = 0.0254 m exactly, 1 pound = 0.45359237 kg exactly, 1 US gallon = 3.785411784 L exactly). The imperial gallon is the 1985 redefinition (4.54609 L exactly). Energy uses the thermochemical calorie (4.184 J exact) for "cal" and the International Table BTU (1055.05585262 J) for "BTU". Every factor matches NIST SP 811.
Why can I only convert within a category?
Converting metres to kilograms is dimensionally meaningless — there is no physical relationship between length and mass. Same for litres to seconds, or fahrenheit to acres. The calculator checks that the "from" and "to" units share a category (both Length, both Mass, etc.) and shows an error if they do not. Pick units from the same category to get a result.
What is the difference between a US gallon and an imperial gallon?
They are completely different volumes. A US gallon is 3.785 411 784 L exactly (defined as 231 cubic inches). An imperial (UK) gallon is 4.546 09 L exactly — about 20% larger. The same is true for pints and fluid ounces, but in opposite directions: a US fluid ounce (29.57 mL) is slightly larger than an imperial fluid ounce (28.41 mL) because an imperial gallon contains 160 imperial fluid ounces while a US gallon contains only 128 US fluid ounces. Always check which gallon you mean.
Is the "calorie" here a food calorie?
No — "cal" is the small calorie (4.184 J), used in chemistry. The food calorie is one kilocalorie (kcal), which is 1000 small calories or 4184 J. A 200-calorie snack on a nutrition label is 200 kcal = 836.8 kJ. Pick "kcal" if you are converting a nutrition-label value; pick "cal" only if you really mean the small calorie used in physics and chemistry.
Why does the converter use kelvin internally for temperature?
Celsius and Fahrenheit are scales with offsets, not just different units — 0 °C is not the absence of temperature, and 0 °F is colder still. Converting directly between them needs both a multiplication (×9/5 or ×5/9) and an addition (+32 or −32). Going through kelvin keeps the maths uniform: kelvin starts at absolute zero, so every other temperature unit is a simple linear transform of it. The end result for a user is identical, but the implementation is cleaner.
How precise are the results?
All factors are stored to full IEEE 754 double precision (~15–17 significant digits). The displayed result is rounded to 6 decimal places to keep the output readable, but the underlying computation is exact for any pair of exact factors. Converting 1 inch to centimetres returns 2.54 exactly; converting 1 pound to kilograms returns 0.45359237 exactly. Floating-point rounding only appears in displayed values, never in the computation pipeline.