Every unit at once
Assumptions & sources
- Conversions are exact where the definition is exact. The inch is 25.4 mm, the pound is 0.453 592 37 kg, the barrel is 42 US gallons of 231 in³ each, and pounds-force follow from standard gravity, 9.806 65 m/s². Everything derived from those is exact too, and the self-checks below assert it rather than trusting the typing.
- Gauge pressure needs an atmosphere. barg, psig and kPag are absolute pressure less whatever the atmosphere is taken to be, and that is a choice, not a constant — it is selectable above and defaults to the standard atmosphere of 101.325 kPa. Real gauges read against the actual local barometric pressure, which moves with the weather and with altitude, so a gauge reading converted to absolute is only ever as good as that assumption. The unsuffixed units — bar, psi, kPa — are absolute throughout.
- Negative absolute values are refused rather than converted. A temperature below −273.15 °C, or a gauge pressure more negative than the atmosphere, has no absolute counterpart, and the earlier version of this tool would cheerfully report one.
- mmHg and torr are not the same unit. A millimetre of mercury is defined from the density of mercury and standard gravity; a torr is exactly 1/760 of a standard atmosphere. They differ by about one part in seven million, which never matters in practice and is preserved here anyway.
- The awkward ones, spelled out. The calorie here is the thermochemical calorie of exactly 4.184 J, not the International Table calorie of 4.1868 J. The BTU is the International Table BTU. The therm is the UK legal value, which is very slightly not 10⁵ BTU. Horsepower is mechanical — 550 ft·lbf/s — not metric PS (735.5 W) or electrical (746 W). The inch of water is at 4 °C; the 60 °F definition is about 0.1% smaller.
- Some units are listed twice on purpose. The centipoise and the millipascal second are the same unit, as are the centistokes and mm²/s, and kg/L and g/cm³. Both spellings appear because both turn up on datasheets. The self-checks assert that each pair really is identical.
- API gravity is 141.5/SG − 131.5, where SG is relative to water at 60 °F, taken here as 999.016 kg/m³. It is defined for liquids near the density of water and goes to nonsense outside that range: below −131.5 °API the formula implies a negative density, and that is refused. 10 °API is exactly the density of water by construction.
- Standard gas volumes are a temperature and a pressure, not a volume. A standard cubic foot is taken at 60 °F and 14.696 psia, a standard cubic metre at 15 °C and 101.325 kPa, and a normal cubic metre at 0 °C and 101.325 kPa. Converting between them is the ideal gas law on those conditions, so 1 Nm³ is 1.0549 sm³ purely because 0 °C is colder than 15 °C. Contracts sometimes specify other bases — 14.73 psia is common in US gas sales — and the difference is real money, so check the contract rather than this page. Compressibility is ignored, which is fine at these conditions and not at reservoir ones.
- Gas–oil ratio follows from the same standard conditions, with the oil volume in stock tank barrels or cubic metres. It inherits every assumption above.
- Rounding is display only. Results are shown to the number of significant figures selected above, with the digits grouped for readability, but every conversion is done in full double precision. Swapping the units carries the unrounded result into the input rather than the figure on screen, which is why the box can fill with digits after a swap and why swapping twice returns you exactly where you started.
- Not covered: anything needing a fluid property to convert — volumetric to mass flow, pressure to head, mass to mole — because that is a calculation with inputs rather than a conversion. Currency, and any unit whose definition depends on a substance not named above.
A reference tool, not a design document. The definitions and assumptions above are stated so you can check them against whatever standard or contract actually governs your work.