Unit Converter for Mechanical Engineering
Convert length, mass, force, pressure/stress, torque, power, energy and angular velocity — SI and imperial/US units side by side, with SI prefixes (k, M, m, µ…) where they apply.
Pick a physical quantity below, then a "From" and "To" unit — the result updates as you type. Every other unit for the same quantity is listed underneath so you can see all equivalent values at once, not just the one pair you picked.
Length
mm, cm, m, km, in, ft, thou…
All equivalent units
Every other unit for this quantity, computed from the value above. Click one to set it as "To".
Glossary
- SI prefix
- A standard multiplier combined with an SI unit to make it more convenient at a given scale — e.g. kilo (k, ×1000) in "kN", milli (m, ×0.001) in "mm", mega (M, ×1,000,000) in "MPa". Only units that support SI prefixes offer them in the dropdown.
- Base unit
- The unit each quantity's conversions are pivoted through internally (Newton for force, Pascal for pressure, metre for length, Kelvin for temperature, and so on) — never shown directly as a special case, just the first entry in the picker.
- Exact vs. standard conversion
- Some factors are exact by definition (1 in = 0.0254 m). Others rely on a standard reference value that's fixed by convention but not a law of physics — e.g. kgf and lbf both use standard gravity, 9.80665 m/s².
- Affine conversion
- A conversion with both a scale factor and an offset — only temperature works this way here, since 0 on one scale isn't 0 on another.
Frequently asked questions
How accurate are these conversions?
All factors come from defined, exact relationships (e.g. 1 in = 0.0254 m exactly, 1 lbf = 4.4482216152605 N exactly) or standard reference values (e.g. standard gravity 9.80665 m/s² for kgf, standard atmosphere 101325 Pa). Results are computed in double precision and are accurate to at least 6 significant figures.
What are the small letters like k, M, m, µ next to a unit?
Those are SI prefixes — standard multiples of ten you can combine with SI units. k = kilo (×1000), M = mega (×1,000,000), m = milli (×0.001), µ = micro (×0.000001), and so on. Pick a unit that supports prefixes (Newton, Pascal, metre, Joule, Watt, Newton-metre) and choose the prefix from the same dropdown, e.g. "kN" or "MPa".
Why does temperature convert differently from the other quantities?
Length, force, pressure and the other quantities here are proportional — double the value in one unit and you double it in any other (e.g. 2 m is always exactly twice 1 m in any unit). Temperature scales are affine, not proportional: they have an offset as well as a scale factor, because 0 °C isn't the same physical state as 0 °F or 0 K. That's why 0 °C converts to 32 °F, not 0 °F.
What is the difference between kgf and kg, or lbf and lb?
kg and lb are units of mass. kgf ("kilogram-force") and lbf ("pound-force") are units of force — the force exerted by that mass under standard gravity (9.80665 m/s²) on Earth. In mechanical design, forces, loads and torques should be in force units (N, kgf, lbf), not mass units, even though kgf and lbf are colloquially used as if they were weights.
What is the difference between metric horsepower (CV) and mechanical horsepower (hp)?
They're two historically different definitions of the same idea. Metric horsepower (CV, also PS or CH depending on the country) is defined as 75 kgf·m/s = 735.49875 W. Mechanical horsepower (hp, used in the US/UK) is defined as 550 ft·lbf/s = 745.69987158227 W. The difference is small (about 1.4%) but matters when comparing published power ratings between regions.
References & further reading
- NIST Special Publication 811 — Guide for the Use of the International System of Units (SI) — the US federal reference for SI unit names, symbols and conversion factors.
- The International System of Units (SI Brochure) — BIPM — the official definition of the SI, published by the International Bureau of Weights and Measures.
- Conversion of units — Wikipedia — background on unit systems and common conversion factors.
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