Trust & Governance

Unit Definitions & Reference Guide

Last updated: June 16, 2026

A high-level reference for the measurement systems supported across AllKindsOfConverters. For category-specific deep dives, see the matching converter pages and learning guides.

The International System of Units (SI)

SI is the modern form of the metric system, maintained by the Bureau International des Poids et Mesures (BIPM). Since 2019, all seven base units are defined in terms of fixed physical constants:

  • metre (m) — length, defined via the speed of light.
  • kilogram (kg) — mass, defined via the Planck constant.
  • second (s) — time, defined via the caesium-133 hyperfine transition.
  • ampere (A) — electric current, via the elementary charge.
  • kelvin (K) — thermodynamic temperature, via the Boltzmann constant.
  • mole (mol) — amount of substance, via the Avogadro constant.
  • candela (cd) — luminous intensity.

Imperial & US customary units

Modern Imperial (UK) and US customary units are now defined in terms of SI. For example, the international yard equals exactly 0.9144 metres, the international pound equals exactly 0.45359237 kg, and the US survey foot has been phased out in favor of the international foot.

Be careful: the same name can mean different things. The US gallon (≈3.785 L) is not the Imperial gallon (≈4.546 L); a US fluid ounce differs from a UK fluid ounce; the short ton and long ton are not the same as the metric tonne.

Metric prefixes

SI prefixes scale a base unit by powers of ten — from quecto (10⁻³⁰) through quetta (10³⁰). The most common are: nano (10⁻⁹), micro (10⁻⁶), milli (10⁻³), kilo (10³), mega (10⁶), giga (10⁹), tera (10¹²), peta (10¹⁵).

Temperature scales

  • Kelvin (K) — absolute scale, no degree symbol.
  • Celsius (°C) — 0 °C at the freezing point of water, 100 °C at the boiling point at 1 atm.
  • Fahrenheit (°F) — 32 °F freezing, 212 °F boiling.
  • Rankine (°R) — absolute scale in Fahrenheit-sized degrees.

Internally we follow the International Temperature Scale of 1990 (ITS-90) for high-precision use.

Digital storage

We follow IEC 80000-13:

  • Decimal prefixes (kB, MB, GB, TB) use powers of 1000.
  • Binary prefixes (KiB, MiB, GiB, TiB) use powers of 1024.

Pressure, energy & power

  • Pressure: pascal (Pa), bar, atm, psi, mmHg, Torr.
  • Energy: joule (J), calorie, electron-volt (eV), BTU, kWh.
  • Power: watt (W), horsepower (mechanical, metric, electrical).

Area, volume, weight, speed and force

Each of these categories has its own converter on the platform with full per-unit definitions, authoritative factors and worked examples. Visit the corresponding tool for the canonical reference.