About AllUnits.io
AllUnits.io is a free, ad-light unit converter covering everyday, scientific, digital, and financial units. The goal is simple: instant, accurate conversions with the formula and source visible, so you can verify the answer instead of trusting it.
Who builds it
AllUnits.io is operated by BT Enterprises, a small independent web publisher. The site is built and maintained by Bryan Telford. Questions, corrections, or unit requests are welcome at support@allunits.io or via the feedback form.
Where the numbers come from
Every conversion factor on this site is traceable to a primary metrology source. We do not invent factors or copy them from other converters. The hierarchy we use:
- BIPM SI Brochure (9th edition) — the definitive reference for SI base and derived units (metre, kilogram, second, kelvin, ampere, mole, candela).
- NIST Special Publication 811 — exact conversion factors for US customary and imperial units (inch = 25.4 mm exactly, pound = 0.45359237 kg exactly, etc.).
- IEC 80000 / ISO 80000 — international standard for quantities and units, including digital storage prefixes (KiB, MiB, GiB) distinguished from decimal (KB, MB, GB).
- Currency rates — fetched live from a public exchange-rate API; rates are mid-market reference values, not transactional. Always confirm with your bank or broker before sending money.
Per-converter source links are listed on the Glossary & Sources page.
How the converters work
All conversions run client-side in your browser using a single shared JavaScript module. Each converter:
- Stores conversion factors as exact decimal constants (e.g.,
0.45359237for pounds-to-kilograms), not floating-point approximations of approximations. - Performs conversions through a base unit (e.g., metres for length, kilograms for mass) so any pair routes through one well-defined factor instead of a 56-cell pair table that can drift.
- Shows the live formula above the result so you can sanity-check the math.
Temperature is the one exception: Celsius, Fahrenheit, and Kelvin have offset zero points, not a single multiplicative factor, so they use explicit per-pair formulas (e.g., °F = °C × 9/5 + 32).
Accuracy & rounding
Internal calculations are performed at full IEEE 754 double precision (≈15–17 significant digits). Display results are rounded to a reasonable number of significant figures based on the unit pair — typically 6–10 — to avoid the false precision of showing 17 digits for an inch-to-centimetre conversion.
If you spot an inaccurate factor or an unexpected result, please report it. Corrections are taken seriously and changelog'd.
Privacy & ads
AllUnits.io does not require an account, does not track personal information, and stores no data about who is using which converter. Converter inputs never leave your browser. The site is supported by minimal display advertising; we do not use intrusive interstitials or popups.
What's not here yet
Some unit categories are intentionally limited to the most-used pairs rather than every historical variant. If you need a unit that isn't supported (apothecary, troy weights, archaic regional units, niche scientific units like Rydberg energy), let us know and we'll consider adding it.