Disclaimer & sources

Last updated: September 2026

The short version: the conversion factors here are the exact, internationally defined ones wherever an exact definition exists, and they are checked by an automated test suite. But this is a general-purpose tool, not a certified instrument. Do not use it as the sole basis for anything safety-critical — flight planning, medication dosing, structural loads, or regulatory filings — without checking against the authoritative source for your field.

Where the numbers come from

Most of the units on this page are exact by definition, not measured approximations. Since the 1959 international yard and pound agreement, the inch is exactly 25.4 mm and the pound is exactly 0.45359237 kg, and everything built on them (feet, miles, acres, gallons, ounces) follows exactly. The same is true of the nautical mile (exactly 1852 m), the liter, and the binary data units.

The rows that are conventions, not exact definitions

Unit	What this page uses, and why
Mach	340.294 m/s, the speed of sound in the ISA standard atmosphere at sea level. Real mach number depends on air temperature and falls with altitude, so this row is an approximation and is labeled as one. Never use it for flight planning.
inHg	3386.389 Pa, the 0 °C convention used for altimeter settings.
inH₂O	249.0889 Pa, the 4 °C convention. Some HVAC references use a 60 °F value that differs by about 0.1%.
BTU / therm	The International Table BTU (1055.05585262 J). Other BTU definitions (thermochemical, 39 °F, 59 °F) differ by a few tenths of a percent.
Calorie	The thermochemical calorie (4.184 J), which is the one nutrition labeling uses.
Month / year	A flat 30 days and 365.25 days. Calendar months and years vary — use these for rough duration math, not for contract or interest dates.
Ton of refrigeration	Exactly 12,000 BTU/h, the HVAC industry convention.

Units that share a name but are not the same thing

Several rows are deliberately duplicated and labeled because the same word means different quantities in different places. US and Imperial gallons, pints, quarts and fluid ounces; decimal and binary data units (GB vs GiB); avoirdupois and troy ounces; short, long and metric tons; mechanical and metric horsepower. Picking the wrong one is the single most common source of conversion errors, which is why they sit next to each other here instead of being hidden behind a setting.

Torque is not energy. The foot-pound appears in two different categories on this page: as a unit of torque (a twisting force, what a torque wrench measures) and as a unit of energy (work done). They have the same name and the same numeric factor to SI, but they are different physical quantities and are not interchangeable. Likewise, pound-force and pound-mass are different things; this page's weight table treats the pound as a mass.

Rounding

Results are rounded for display to the number of significant digits you choose (3, 6 or 10). The underlying math is done in full double precision, so a value you copy at 10 digits is accurate to essentially the limit of the format. Converting a rounded result again can compound that rounding — convert from your original figure rather than from a displayed one when precision matters.

No warranty

This tool is provided free, as-is, without warranty of any kind, express or implied. It is offered for general information and convenience. The author is not liable for any loss or damage arising from its use. If a conversion matters, verify it independently.

Not professional advice

Nothing here constitutes engineering, medical, aviation, legal, financial or other professional advice.