Unit conversion reference

The conversion factors product engineers reach for daily — length, force, pressure, torque, energy, power, temperature, viscosity and rotation — SI ↔ imperial in one place.

Revision1.0

IssuedJune 2026

OwnerIdeambox engineering

CompanionPDF reference

units conversion SI imperial pressure torque viscosity temperature mechanical

Abstract

A single-page conversion reference for the units that come up in mechanical and electromechanical design. SI is the working system; the imperial equivalents are here because suppliers, datasheets and legacy drawings still use them.

Section 1 is SI basics and prefixes. Sections 2–6 are the conversion tables: length/area/volume, mass/force, pressure/stress, torque/energy/power, and temperature/viscosity/rotation. Convert by multiplying through unit factors and watch significant figures.

SI prefixes step by ×1000 — micro, milli, kilo, mega are the engineering staples. The tables below cover the SI ↔ imperial factors you actually use on hardware.

1.SI basics and prefixes

SI is built on base units (metre, kilogram, second, ampere, kelvin) and coherent derived units (newton = kg·m/s², pascal = N/m², joule = N·m, watt = J/s). Prefixes scale by powers of ten — each common step is ×1000:

µ (micro)

10⁻⁶ — microns (µm) for tolerances and surface finish

m (milli)

10⁻³ — millimetres, the default mechanical length unit

k (kilo)

10³ — kN, kPa, kg

M (mega)

10⁶ — MPa (the working stress unit), MJ

G (giga)

10⁹ — GPa (modulus of elasticity)

Convert by chaining unit factors (multiply by ratios equal to 1) and keep significant figures honest — a conversion can't add precision the measurement didn't have. 1 MPa = 1 N/mm² is the handy identity for stress.

2.Length, area, volume

Convert	× factor	To
inch (in)	25.4	mm
foot (ft)	0.3048	m
mil (0.001 in)	25.4	µm
inch²	645.16	mm²
foot²	0.0929	m²
inch³	16.387	cm³
US gallon	3.785	litre
foot³	28.317	litre

3.Mass and force

Convert	× factor	To
pound-mass (lb)	0.4536	kg
ounce (oz)	28.35	g
pound-force (lbf)	4.448	N
kilogram-force (kgf)	9.807	N
N	0.2248	lbf
metric ton	1000	kg

4.Pressure and stress

Convert	× factor	To
psi	6.895	kPa
ksi	6.895	MPa
MPa	145.0	psi
bar	100	kPa
bar	14.50	psi
atm	101.325	kPa
MPa	1.0	N/mm²

5.Torque, energy, power

Convert	× factor	To
lbf·ft	1.356	N·m
lbf·in	0.1130	N·m
N·m	0.7376	lbf·ft
calorie	4.184	J
BTU	1055	J
kWh	3.6	MJ
horsepower (hp)	745.7	W
W	0.001341	hp

6.Temperature, viscosity, rotation

Convert	Formula / factor	To
°F → °C	(°F − 32) / 1.8	°C
°C → °F	°C × 1.8 + 32	°F
°C → K	°C + 273.15	K
dynamic visc. cP	1	mPa·s (= 0.001 Pa·s)
kinematic visc. cSt	1	mm²/s (= 10⁻⁶ m²/s)
density g/cm³	1000	kg/m³
lb/in³	27.68	g/cm³
rpm	0.10472	rad/s
m/s	3.600	km/h
m/s	3.281	ft/s
radian	57.296	degree

Kinematic viscosity ν = dynamic µ / density ρ. Surface speed v = ω·r (ω in rad/s) — handy for the bearing ndm and plain-bearing PV checks.