Engineering unit converter: SI and imperial units, pressure, energy, flow, viscosity, with a free-form expression converter

Convert between SI, metric and US/imperial units for length, area, volume, mass, force, pressure, energy, power, temperature, flow, viscosity, density, torque, heat transfer and more, or type any expression such as 3 kN*m to lbf*ft and get the answer with a dimension check that catches impossible conversions.

Keep it one tap away for everyday use.

Type
Dimensions
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Tables

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Convert anything

Quantity, then "to" and the unit you want

Multiply with * or a space, divide with /, powers with ^ or digits (m^2, m2, m², s-1), and use brackets: W/(m*K). Without "to" you get SI base units. Units are case-sensitive where it matters (mm and Mm, t and T). °C and °F are absolute on their own and temperature differences inside compound units.

Unit tables

Quantity Value From

Every factor is an exact definition: the 1959 international inch, foot and pound, standard gravity 9.80665 m/s², the US gallon of 231 in³, the imperial gallon of 4.54609 L, the 42-gallon oil barrel, the thermochemical calorie (4.184 J), the international table calorie and Btu, and the conventional mercury and water densities for mmHg, inHg and inH₂O. Values are shown to 10 significant figures.

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What it does

Engineering work mixes unit systems all the time: a pump curve in US gallons per minute, a pipe in millimetres, a pressure in psi, a heat exchanger datasheet in Btu per hour per square foot per °F. This converter does two jobs:

Unit tables: pick a quantity (24 of them, from length and pressure to kinematic viscosity, thermal conductivity and specific heat), type a value, choose the unit, and see it in every common SI, metric and US/imperial unit at once.
Convert anything: type a quantity and the unit you want, such as 3 kN*m to lbf*ft or 450 bbl/d to m^3/h. The tool works out the dimensions of both sides (length, mass, time, temperature and so on) and refuses a conversion that is physically impossible, such as metres per second to newtons, telling you what each side actually is.

All factors are exact definitions, not rounded table values: the inch is 25.4 mm, the pound 0.45359237 kg, the pound-force a pound times standard gravity (9.80665 m/s²), the US gallon 231 cubic inches, and the Btu and calorie follow their international table definitions. Everything runs in your browser.

How to use the expression box

Multiply with *, · or a space (kN m), divide with /, and raise to powers with ^, **, digits or superscripts: m^2, m2, m², s-1.
Brackets group: W/(m*K). Without brackets, / and * read left to right, so W/m*K means (W/m)·K.
Any SI unit takes an SI prefix: km, µm, MPa, kWh, mN, GJ. Units are case-sensitive where it matters: mm is a millimetre and Mm a megametre; t is a tonne and T a tesla.
Leave out “to” to see the value in SI base units and its dimension.
Temperatures: 20 °C to K gives 293.15 (absolute). Inside a compound unit such as Btu/(h*ft*°F), °F is a temperature difference (1 °F = 5/9 K).

Worked examples

3 kN*m to lbf*ft gives 2212.686448 lbf·ft, with the dimension L²·M·T⁻² (energy, work or torque) and 3000 in SI base units.
450 bbl/d to m^3/h gives 2.98101178 m³/h.
25 MPa to psi gives 3625.943443 psi.
1 W/(m*K) to Btu/(h*ft*°F) gives 0.5777893165.
100 °C to °F gives 212 °F.
5 m/s to N is refused: “5 m/s” is L·T⁻¹ (speed), but “N” is L·M·T⁻² (force).
In the tables, 1 bar is 100,000 Pa, 14.50377377 psi, 0.9869232667 atm and 750.0616827 Torr.

Tips

Angles are treated as dimensionless, so torque × speed gives power: 100 N*m * 3000 rpm to kW is 31.42 kW. For a shaft, use rpm or rev/s rather than Hz: 1 rev/s is 60 rpm, while Hz is just 1/s.
inH₂O, mmH₂O and inHg use conventional densities (water 1000 kg/m³, mercury 13.5951 g/cm³) and standard gravity; instruments calibrated at another water temperature differ slightly.
The year is the Julian year of 365.25 days; the therm is 100,000 IT Btu.

FAQ

How accurate is it? The factors are exact definitions; results are shown to 10 significant figures. Every table entry was checked against the open-source pint package.

Can it convert currencies or US survey feet? No: currencies change daily (see the currency converter), and the US survey foot was retired in 2023, so the international foot is used.

Is anything sent anywhere? No. It works offline once the page has loaded.

Where the numbers come from

SI units and prefixes: BIPM, The International System of Units (SI brochure, 9th edition). Inch, foot, yard, mile and pound: the 1959 international yard and pound agreement. US gallon (231 cubic inches) and imperial gallon (4.54609 litres) by definition; standard gravity 9.80665 m/s² (CGPM 1901); thermochemical calorie 4.184 J; international table calorie and Btu.
Each factor is written by fibub as arithmetic on those definitions. Checked against the Python pint package (BSD licence), used only for testing; no code copied.

Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.

Last updated 2026-11-09.