How we ensure accuracy
Accuracy is the one thing a converter cannot compromise on. This page explains where our numbers come from and how they are checked.
Sources
- SI Brochure, 9th edition (BIPM, 2019) — definitions of the SI base units and the exact values of the defining constants (c, h, e, k, NA).
- NIST Special Publication 811 and NIST Handbook 44, Appendix C — conversion factors for US customary, imperial and other non-SI units.
- International yard and pound agreement (1959) — 1 yd = 0.9144 m and 1 lb = 0.45359237 kg exactly.
- IAU resolutions for the astronomical unit and parsec; CODATA 2022 for the dalton.
Method
Every unit is stored once, with its relationship to the SI base unit of its category. Derived factors are computed from exact definitions in code (for example the US gallon is 231 × (0.0254 m)³) rather than typed in as rounded decimals. The engine supports four conversion types:
- Linear — a single factor (inches to centimetres).
- Affine — a factor plus an offset (Celsius to Fahrenheit).
- Reciprocal — inverse relationships (mpg to L/100 km, frequency to wavelength).
- Logarithmic — decibel levels (dB SPL, dBm).
Conversions are only allowed between units of the same quantity: torque and energy share dimensions but are kept apart. Compound units typed into the universal converter are checked dimensionally before conversion.
Automated tests
Before every release an automated suite runs 38,740 round-trip checks — every unit converted to every other unit in its category and back, at several magnitudes — plus more than 190 reference values from NIST SP 811 and exact definitions, and checks for domain errors such as temperatures below absolute zero.
Precision and display
Calculations use IEEE-754 double precision (about 15–17 significant digits). Results are rounded only for display, to 12 significant figures by default; you can choose fixed decimals, significant figures or scientific notation in any converter. Values marked as approximate (such as Mach, which depends on air temperature) are labelled on their pages.
Currency
Exchange rates are not built into pages. They are requested live from the European Central Bank reference rates via the Frankfurter API and always shown with their publication date. If the rate cannot be loaded, no number is shown.