Engineering Unit Converter
Convert between common piping and process engineering units. Select a category, enter a value, and see all conversions instantly.
Pipe design requires working across unit systems daily. Specifications may arrive in metric (bar, m/s, kg/m3) while vendor datasheets use imperial (psi, ft/s, lb/ft3). A missed conversion in pressure drop, flow rate, or pipe diameter can cascade into incorrect sizing, over-spec'd pumps, or unsafe operating conditions.
This converter covers the units most frequently encountered in hydraulic design: pressure (including lesser-used units like inH2O and mmHg for instrument engineers), volumetric flow rate, dynamic and kinematic viscosity for fluid characterisation, and density for compressible-flow calculations.
SI (International System): The global standard for engineering. Base units include metres (m), kilograms (kg), seconds (s), and Kelvin (K). Pressure in Pascals (Pa), flow in m3/s, viscosity in Pa-s.
Imperial / US Customary: Still dominant in North American industry. Pressure in psi, flow in GPM, pipe sizes in inches, temperature in Fahrenheit. ASME and API standards primarily use these units.
SimuPipe stores all values internally in SI and converts at the user interface, so you can work in whichever system you prefer without losing precision.
One bar entered in the Pressure category, with the exact factors the converter applies (each unit is defined by its value in the SI base unit, pascals; a conversion is a direct ratio of the two stored factors, so no intermediate rounding is applied).
- 1 bar = 100,000 Pa by definition = 100 kPa; in psi, 100,000 / 6894.757 = 14.5038 psi (1 psi = 6,894.757 Pa, from the exact pound-force and inch).
- 100,000 / 101325 = 0.98692 atm (the standard atmosphere is exactly 101,325 Pa); 100,000 / 133.322 = 750.06 mmHg (conventional millimetre of mercury, 133.322 387 Pa, stored rounded to 133.322).
- Head units: 100,000 / 9806.65 = 10.197 m of water (conventional, 9,806.65 Pa per metre at 1,000 kg/m³ and standard gravity) and 401.5 inH₂O — the inch of water at 39.2 °F / 4 °C, 249.082 Pa (NIST also lists a "conventional" inch of water of 249.0889 Pa; the two differ by 0.003 %).
These figures are computed from the same factor table the converter uses; entering 1 bar reproduces them. The factors for the other categories are exact by definition (inch 25.4 mm, pound 0.453 592 37 kg, US gallon 3.785 411 784 L, UK gallon 4.546 09 L, cubic foot 0.028 316 846 592 m³, barrel 158.987 294 928 L) or the NIST SP 811 values where a unit is conventional.
- Pure scale conversions: every unit is a fixed multiple of its SI base unit, so a conversion is exact to the precision of the factor. No offsets are applied except in temperature, where °C, °F, K and °R are converted by the defining relations (K = °C + 273.15, °F = 1.8·°C + 32, °R = °F + 459.67).
- Pressure units are converted as pressure differences: no gauge / absolute offset is added. To convert a gauge reading to absolute, add the local atmospheric pressure (101.325 kPa standard) before or after converting — the converter does not know which basis a value is on.
- Head and column units are conventional, not physical: inH₂O and ftH₂O are the inch and foot of water at 4 °C (249.082 and 2,989.067 Pa), mH₂O uses 1,000 kg/m³ with standard gravity (9,806.65 Pa), mmHg and inHg are the conventional mercury units at 0 °C (133.322 and 3,386.389 Pa). An actual water or mercury column at another temperature differs by its density ratio (about 0.2 % for water at 20 °C).
- Gas flow units tied to a reference state — SCFM, Nm³/h, Sm³/h, ACFM — are deliberately absent from the flow categories, because they are not pure scales: ft³/min here is a volume rate at whatever state the gas is in.
- Dynamic viscosity (Pa·s; cP = centipoise) and kinematic viscosity (m²/s; cSt = centistokes) are separate categories; converting between them needs the fluid density (ν = μ / ρ), which is not part of a unit conversion. cP = mPa·s and cSt = mm²/s exactly.
- The US gallon (231 in³) and the UK (imperial) gallon (4.546 09 L) differ by 20 %; GPM and lb/gal are offered in both, and the label says which. "Ton" is the US short ton (907.184 74 kg exactly); the metric tonne is listed separately as t. The slug (14.593 902 94 kg) is the US customary mass unit that makes lbf = slug·ft/s².
For flows that carry a reference condition, use the gas flow converter, which applies the ideal-gas density ratio between states instead of a scale factor.
Definitions and conversion factors behind this page:
- NIST Special Publication 811 (2008). Guide for the Use of the International System of Units (SI), Appendix B: Conversion factors. nist.gov — the conversion factors for every non-SI unit offered, including the conventional water and mercury columns.
- BIPM (2019). The International System of Units (SI Brochure), 9th ed. bipm.org — the SI base and derived units and the 2019 redefinition that makes the factors exact.
- ISO 80000-1:2022 and ISO 80000-4:2019. Quantities and units — General; Mechanics — quantity and unit symbols and the conventions for pressure, viscosity and density.
- ASME B36.10M-2018 and API/ASTM conventions — the customary units (psi, gpm, inches, lb/ft³) that ASME and API documents are written in.
Frequently Asked Questions
What is the difference between dynamic and kinematic viscosity?
How do I convert between gauge and absolute pressure?
What is the difference between SCFM and Nm3/h for gas flow?
Why are there so many pressure units in engineering?
How do I convert flow rate between volumetric and mass flow?
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