Conversion methodology
Convert confidently. See the result, formula, definition, precision and source.
Read the guide to unit systems, SI prefixes and symbols
Definition sources
We use BIPM for SI definitions and prefixes, NIST for conversion factors, and NIST’s account of IEC binary prefixes for digital units. Derived units use these same definitions. Competitor tables are not authoritative sources.
- NIST: Household Weights and Measures (SP 430)
- NIST: Metric cup and spoon conventions
- NIST: SI units: gray, sievert and becquerel
- W3C: CSS Values and Units: absolute lengths
- NIST: SP 811 Chapter 8: logarithmic quantities and units
- BIPM: SI prefixes
- NIST: US survey foot: definition and retirement
- IAU: Measuring the Universe: astronomical unit
- BIPM: International System of Units
- NIST: SP 811 Appendix B.8: conversion factors
- NIST: IEC binary prefixes and decimal comparisons
- NIST: 2022 CODATA recommended constants
- US Naval Observatory: Leap years and the Gregorian calendar
- NASA: Speed of sound
How the engine works
Multiplicative units convert by input × source factor ÷ target factor. Absolute temperatures use a scale and offset through Celsius. Fuel economy converts through liters per 100 km, using reciprocal transformations where needed. These formulas also generate reference tables.
Exact definitions and displayed precision
An exact relationship describes a definition, not the accuracy of a physical measurement. The engine uses IEEE 754 binary floating point, approximately 15–17 significant digits. It is not arbitrary precision. Recurring fractions and π are represented approximately even when a relationship is exact.
Standard display uses up to six decimal places. You can choose two, four or six places, full stored precision, or scientific notation. Small nonzero values use scientific notation instead of appearing as zero. Copy unrounded value copies the stored number, without adding precision. Display rounding follows JavaScript toFixed/toExponential on the stored binary value; decimal halfway cases can therefore differ from decimal arithmetic.
Variants and boundaries
US liquid gallons and fluid ounces differ from Imperial units and dry volume measures. Cups use US customary definitions. Tonnes, US short tons and UK long tons are separate. Pounds measure mass; pound-force measures force. Feet use the international definition, not survey feet.
MB and GB use powers of 1,000; MiB and GiB use powers of 1,024. Bytes and bits differ by eight. Temperature intervals must not be entered as absolute temperatures. Values below absolute zero, nonpositive fuel economy, unsupported negative quantities, overflow and underflow are rejected.
Feet-and-inches output rounds to the nearest hundredth of an inch. Fractional inches round to the nearest sixteenth, with positive ties upward. Compound time rounds to the nearest second. Average months and years are conventions, not calendar arithmetic.
Privacy and corrections
Conversion arithmetic runs in your browser. The converter keeps input in component memory and does not put it into URLs, browser storage or custom analytics events. Site analytics and configured third-party scripts have separate behavior described in the privacy policy; we do not make an unconditional claim about third-party collection.
The correction link includes the page path, category, unit IDs and engine/definition versions. It omits entered values and URL parameters. Sending a report is your choice.
Common, specialist and legacy units
Unit selectors list common units first, then specialist units, followed by clearly labeled legacy units. Legacy definitions are retained only for interpreting existing records. The US survey foot is separate from the international foot; its retirement does not change the meaning of historical measurements.
Review policy
Where a unit shows a source-check date, that records an implementation review of its definition and a known-value test against the named reference. It does not claim independent scientific or editorial approval. Units without a recorded check show no review date.
Source coverage and arithmetic are checked automatically. Individual definition review dates are recorded only after an actual review; a code-change date is not a scientific review. Unreviewed definitions retain an empty review date. Before release, definitions, authoritative test vectors, mobile and keyboard behavior, canonical links and sitemap changes must pass review. New pair pages need independent intent evidence and at least 85/100, with no correctness or provenance failure.
Existing URLs are preserved. Alias, value-specific and arbitrary pair pages are not generated. Later technical categories require demonstrated demand and a separate release review.