units calculator
Unit Converter
Convert length, mass, temperature, time, area, volume, and speed using sourced definitions and no intermediate rounding.
7 dimensions · 25 units
Convert measurement units
Converted value
Enter a value and choose Convert.
Formula version 1.0.0 · NIST SP 811 and BIPM SI definitions · No intermediate rounding.
Defined factors
Supported dimensions and conversion method
The converter supports length, mass, temperature, time, area, volume, and speed. Values convert through one base unit per dimension without intermediate rounding.
Factor conversions
Length, mass, time, area, volume, and speed multiply by a documented factor to the base unit, then convert from that base.
Temperature
Temperature uses affine scale and offset formulas. For example, °C=(°F−32)×5/9.
Exact definitions
Examples include 1 inch = 0.0254 metre exactly and 1 US gallon = 0.003785411784 cubic metre exactly.
Scope
Absolute-temperature physical validity is not enforced; the tool performs mathematical scale conversion. It does not cover currency or contextual units.
How to use this calculator
- Choose a Dimension so the From and To menus contain compatible units.
- Enter a finite integer or decimal in Value.
- Choose the source unit in From and destination unit in To, or select Swap units to reverse them.
- Select Convert, then read Converted value and open Show conversion path to inspect the base-unit calculation.
Symbols and units
| Symbol | Meaning | Unit or domain |
|---|---|---|
m, km, cm, in, ft, mi | Supported length units | metre, kilometre, centimetre, inch, foot, mile |
kg, g, lb | Supported mass units | kilogram, gram, pound |
°C, °F, K | Supported temperature scales | Celsius, Fahrenheit, kelvin |
s, min, h, d | Supported time units | second, minute, hour, day |
m², ft², ha | Supported area units | square metre, square foot, hectare |
m³, L, US gal | Supported volume units | cubic metre, litre, US gallon |
m/s, km/h, mph | Supported speed units | metres per second, kilometres per hour, miles per hour |
Checked examples
Miles to kilometres
Inputs: Dimension: length; Value: 1; From: miles; To: kilometres
Calculation: 1 × 1609.344 m ÷ 1000 m/km
Result: 1.609344 km
One international mile equals exactly 1.609344 kilometres.
Check: unit-conversion.test.ts — converts length through one base unit
Fahrenheit to Celsius
Inputs: Dimension: temperature; Value: 68; From: °F; To: °C
Calculation: (68 − 32) × 5/9
Result: 20 °C
Temperature conversion uses a scale and offset rather than a factor alone.
Check: unit-converter-formula.test.ts — converts affine temperature scales
US gallons to litres
Inputs: Dimension: volume; Value: 2; From: US gal; To: L
Calculation: 2 × 0.003785411784 m³ × 1000 L/m³
Result: 7.570823568 L
The US liquid gallon definition is used; imperial gallons are not supported.
Check: unit-conversion.test.ts — validates sourced volume factors
Assumptions, rounding, and limits
- Only units within the selected dimension can be converted; currency and contextual units are outside scope.
- Factor-based dimensions convert through one base unit without intermediate rounding.
- Temperature uses affine scale-and-offset formulas; it is not valid to multiply a temperature by a simple unit ratio.
- The converter accepts finite JavaScript numbers and displays at most 12 decimal places using half-up formatting.
- Absolute-temperature physical validity is not enforced; values below absolute zero are mathematically converted but may be physically impossible.
- US gallon means the US liquid gallon, not the imperial gallon.
Frequently asked questions
Why can’t I convert between two different dimensions?
Length, mass, temperature, time, area, volume, and speed describe different quantities. A conversion is defined only within one compatible dimension.
Are the displayed digits all measured precision?
No. The calculator preserves working precision and rounds the display, but real measurements cannot gain precision beyond their source data.
Why does temperature use a different formula?
Celsius and Fahrenheit have different zero points as well as different interval sizes, so an offset and a scale factor are both required.
Sources and review
- National Institute of Standards and Technology: NIST Guide to the SI, Appendix B — Conversion factors and exact definitions; accessed 2026-10-25
- BIPM: The International System of Units — SI base and derived unit definitions; accessed 2026-10-25
- Formula version
- unit.converter@1.0.0
- Formula review
- Equatara formula review
- Editorial review
- Equatara editorial review
- Last verified
- 2026-10-25
- Applicability
- Global measurement definitions; US gallon is explicitly US liquid gallon.
- Next review
- On material definition/source change, or by 2028-10-25.
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Found a problem?
Report a formula, result, example, source, or accessibility issue through the calculator correction process. Do not include sensitive input values.