Unit Converter - Length, Area, Volume, Temperature, Pressure, Force & Power

Length BIPM SI Brochure (metric) · NIST Handbook 44 (US) · historical measurement tables (regional)

The meter is the SI base unit, defined since 1983 via the speed of light (1/299,792,458 light-second)
The nautical mile (1852 m) is not the same as the statute mile (1609.34 m) - about 15% difference
Regional historical units (ell, mile, foot, klafter) were never standardized - values vary strongly by region and era
1 inch = exactly 2.54 cm (internationally fixed since 1959)

"Ell" is not a fixed length - the Prussian ell (66.69 cm) and the Viennese ell (77.7-89.6 cm) differ by up to 34%.

Area BIPM SI Brochure · NIST · historical measurement tables

The hectare (10,000 m²) is an officially accepted non-SI unit still common in everyday use
The acre historically derives from the area a yoke of oxen could plow in one day
"Morgen" is regionally inconsistent just like the ell (Prussian Morgen 2553 m², others differ substantially)
Area units are always derived from squaring a length unit (m² = m × m)

1 hectare ≈ 2.47 acres - a rough 1:2.5 rule of thumb quickly causes noticeable rounding errors over large areas.

Volume BIPM SI Brochure · NIST · UK Weights and Measures Act

US and UK units sharing the same name are different sizes (gallon, pint, fl oz, ...)
1 m³ = 1000 L is the only purely metric anchor relationship without an awkward factor
"Bushel" and "peck" are historical dry measures, not directly comparable to liquid measures of the same name
"Barrel" means different volumes depending on context (oil barrel 158.99 L ≠ beer barrel)

US gallon (3.785 L) ≠ UK gallon (4.546 L) - about a 20% difference, one of the most common conversion mistakes.

Weight BIPM SI Brochure · NIST Handbook 44 · historical measurement tables

The kilogram is the SI base unit of mass, defined since 2019 via the Planck constant
The avoirdupois pound (453.59 g) is the everyday pound used in the US and UK - not to be confused with the troy pound (373.24 g) used for precious metals
The carat (0.2 g) is the international standard for gemstone weight since 1907
US short ton (2000 lb = 907.18 kg) and UK long ton (2240 lb = 1016.05 kg) differ by about 12%

The US hundredweight (cwt) is 100 lb, the UK hundredweight is 112 lb - the same abbreviation means different weights on each side of the Atlantic.

Temperature BIPM SI Brochure · historical temperature scale literature

Celsius, Fahrenheit and Kelvin are NOT a pure multiplication - converting needs an offset, not just a factor
Kelvin and Rankine are absolute zero-point scales - negative values are physically impossible
Réaumur, Delisle, Rømer and the Newton scale are historical, virtually unused today
The Delisle scale runs inverted: higher values mean colder temperatures

0 °C ≠ 0 - anyone who only multiplies by a factor instead of adding an offset gets it fundamentally wrong.

Pressure BIPM SI Brochure · NIST

1 atm is a fixed reference value (101,325 Pa), not a measured physical constant
psi (pound-force per square inch) is the US standard for tire pressure and engineering
mmHg (Torr) historically comes from the mercury barometer and is still used in medicine (blood pressure)
bar and Pascal differ by a factor of 100,000 (1 bar = 100,000 Pa)

Tire pressure readings are usually "gauge pressure" (relative to atmospheric pressure), not absolute pressure - this converter only performs plain unit conversion, no gauge/absolute distinction.

Force BIPM SI Brochure · CGS/MTS system literature

kg is a unit of mass, not force - kgf/kilopond is the force equivalent (mass × standard gravity)
Newton is the SI unit, defined via kg·m/s²
Poundal and dyne come from older physical unit systems (dyne from the CGS system)
Sthène is a historical French unit from the MTS system (1 sthène = 1000 N)

"1 kg of force" is common colloquial usage but physically imprecise - what's usually meant is kgf/kilopond, not the mass unit kg itself.

Power BIPM SI Brochure · NIST

Watt is the SI unit, defined as 1 joule per second
Metric horsepower (PS) and imperial horsepower (hp) are defined differently
BTU/h comes from US thermal engineering (air conditioning, heating)
There are three different "hp" definitions: mechanical, electrical and boiler horsepower

PS (735.5 W) ≠ hp (745.7 W) - metric and imperial horsepower differ by about 1.4%, yet are constantly used interchangeably.

Angle BIPM SI Brochure · ISO 80000-3 · NATO STANAG 2934

The radian is the SI unit for angles - 1 rad = 180/π degrees ≈ 57.296°
The grade/gon (400 per full circle) was introduced with the metric system for surveying
Arcminute and arcsecond are used in navigation, astronomy and geodesy (1° = 60' = 3600")
NATO mil (6400/circle) and Soviet mil (6000/circle) are different - they are NOT interchangeable

"Grad" in German means degree (°), not the metric grade/gon - mixing these up causes significant errors in surveying.