Glossary & Sources

Authoritative references for the unit definitions, conversion factors, and historical context used throughout AllUnits.io. Click any converter name to go directly to that converter.

Methodology

Conversion factors on AllUnits.io are sourced from primary metrology references in this priority order:

BIPM SI Brochure (9th ed.) for SI base and derived units.
NIST SP 811 for exact US customary / imperial conversions (e.g., inch = 25.4 mm exactly, pound = 0.45359237 kg exactly).
IEC / ISO 80000 for international quantities and digital storage prefixes (KiB vs KB).
Wikipedia as a cross-check and for historical context, never as a primary source for the factor itself.

Calculations run client-side at IEEE 754 double precision (≈15–17 significant digits) and route through a base unit per category to keep all pair conversions consistent. See About AllUnits.io for the full methodology.

Length ↗

BIPM SI Brochure, 9th ed. metre defined as distance light travels in 1/299,792,458 s
NIST SP 811 inch = 25.4 mm exactly; imperial-to-metric conversion factors
Wikipedia: Metre

Weight & Mass ↗

BIPM SI Brochure, 9th ed. kilogram redefinition via Planck constant h (2019)
NIST SP 811 pound and other customary mass units
Wikipedia: Kilogram

Temperature ↗

BIPM SI Brochure, 9th ed. kelvin defined via Boltzmann constant; thermodynamic temperature
Wikipedia: Celsius
Wikipedia: Fahrenheit
Wikipedia: Kelvin

Volume ↗

BIPM SI Brochure, 9th ed. litre = 10⁻³ m³
NIST Handbook 133 US customary volume units: fluid ounce, cup, quart, gallon
Wikipedia: US Customary Units US vs. imperial gallon differences

Area ↗

NIST SP 811 square metre and SI derived area units
Wikipedia: Hectare
Wikipedia: Acre

Time ↗

BIPM SI Brochure, 9th ed. second defined via caesium-133 hyperfine transition frequency
IERS: Coordinated Universal Time UTC definition and leap seconds
Wikipedia: Second

Speed ↗

BIPM SI Brochure, 9th ed. metre per second as SI derived unit of speed
NIST SP 811
Wikipedia: Knot 1 nautical mile per hour; used in aviation and maritime

Cooking ↗

NIST Handbook 133 US customary cooking measures: teaspoon, tablespoon, cup
Wikipedia: Cooking weights and measures US vs. UK tablespoon and cup size differences

Fuel Efficiency ↗

US EPA: fueleconomy.gov MPG standards and official fuel economy labelling
Wikipedia: Fuel economy in automobiles L/100 km vs. MPG; inverse relationship

Pressure ↗

BIPM SI Brochure, 9th ed. pascal = 1 N/m²; SI derived unit
Wikipedia: Pascal
Wikipedia: Atmosphere (unit) standard atmosphere = 101,325 Pa exactly

Energy ↗

BIPM SI Brochure, 9th ed. joule = 1 kg·m²/s²; SI derived unit
Wikipedia: Calorie thermochemical vs. International Table calorie
Wikipedia: British thermal unit

Power ↗

BIPM SI Brochure, 9th ed. watt = 1 J/s; SI derived unit
Wikipedia: Horsepower mechanical vs. metric hp; James Watt's original definition

Force ↗

BIPM SI Brochure, 9th ed. newton = 1 kg·m/s²; SI derived unit
NIST SP 811
Wikipedia: Pound-force lbf vs. lbm distinction; gravitational vs. mass units

Torque ↗

BIPM SI Brochure, 9th ed. newton-metre as SI unit of torque
Wikipedia: Torque τ = F × r; distinction between torque and energy (work)

Angle ↗

Wikipedia: Radian SI supplementary unit; arc length equals radius at 1 rad
Wikipedia: Degree (angle) Babylonian 360° system and its divisibility
Wikipedia: Gradian metric-era surveying unit; 400 grad = full circle
Wikipedia: Milliradian optics and ballistics: 1 mrad ≈ 1 m at 1 km distance

Frequency ↗

BIPM SI Brochure, 9th ed. hertz = 1 cycle per second; SI derived unit
Wikipedia: Hertz
NIST CODATA: Planck constant h = 6.626 × 10⁻³⁴ J·s; E = hf links frequency to photon energy

Digital Storage ↗

IEC 80000-13:2008 official standard defining KiB, MiB, GiB, TiB, PiB binary prefixes (1998)
Wikipedia: Binary prefix SI decimal vs. binary prefix disambiguation; OS display discrepancy

Number Base ↗

Wikipedia: Numeral system
Wikipedia: Hexadecimal base-16; each hex digit represents exactly 4 binary bits
Wikipedia: IEEE 754 64-bit floating-point format; 2⁵³ − 1 safe integer precision limit

Currency ↗

IMF Data International Monetary Fund exchange rate reference data
European Central Bank: Euro reference rates
Wikipedia: Reserve currency US dollar as global reserve currency; Bretton Woods system history

Pace ↗

World Athletics official road race distance standards; min/km and min/mile pace references
Wikipedia: Pace (speed) pace as inverse of speed; relationship to time-per-distance metrics
NIST SP 811 mile = 1,609.344 m exactly; basis for min/mile ↔ min/km conversion

Density ↗

BIPM SI Brochure, 9th ed. kg/m³ as SI derived unit of density (kilogram per cubic metre)
NIST SP 811 lb/ft³ and lb/in³ conversion factors
Wikipedia: Density ρ = m/V; Archimedes' principle; reference densities for common materials

Flow Rate ↗

BIPM SI Brochure, 9th ed. m³/s as SI derived unit of volumetric flow rate
NIST SP 811 US gallon = 3.785411784 L; gallon per minute (GPM) conversion
Wikipedia: Volumetric flow rate

Data Transfer Rate ↗

IEC 80000-13:2008 SI prefixes for data rates: kilo=10³, mega=10⁶, giga=10⁹ (decimal)
Wikipedia: Data-rate units bits vs. bytes; ISP Mbps vs. OS MB/s; factor of 8 conversion
IEEE 802 Ethernet standards 10/100/1000 Mbps and 10/40/100 Gbps Ethernet definitions

Illuminance ↗

BIPM SI Brochure, 9th ed. lux = 1 lm/m²; SI derived unit of illuminance; candela as SI base unit
Illuminating Engineering Society (IES) recommended illuminance levels for workplaces, roadways, and interiors
Wikipedia: Lux 1 foot-candle = 10.7639 lux; typical illuminance values for common environments

Typography Units ↗

W3C CSS Values and Units Level 4 official CSS unit definitions: px, pt, pc (pica), em, rem, in, cm, mm; 96px = 1in
Wikipedia: Point (typography) 1 CSS pt = 1/72 in; history of the typographic point from Didot to PostScript
Wikipedia: Pica (typography) 1 pica = 12 points = 1/6 inch; used in print layout for column and margin widths

Electric Units ↗

BIPM SI Brochure, 9th ed. ampere as SI base unit; volt and ohm as derived units; V = I × R (Ohm's Law)
NIST SP 811 SI electrical unit definitions and prefixes (µ, m, k, M)
Wikipedia: Ohm's Law V = IR; relationship between voltage, current, and resistance