Reference library

Unit conversion reference: formulas, base units, and system notes

Last updated: May 11, 2026

A compact reference for the 17 measurement dimensions covered on this site: each one's SI base unit, its most common US/imperial counterpart, the defining conversion factor where one exists, and a link to the dedicated dimension page for deeper formulas, common values, and FAQ entries.

The seven SI base units

The International System of Units (SI) is built on seven base units; every other physical quantity is derived from combinations of these. Most everyday conversions involve four of them:

Meter (m) for length. Defined as the distance light travels in vacuum during 1/299,792,458 of a second.
Kilogram (kg) for mass. Defined in terms of the Planck constant since the 2019 SI redefinition (replacing the platinum-iridium artifact in Sèvres).
Second (s) for time. Defined by the hyperfine transition of cesium-133 (9,192,631,770 cycles).
Kelvin (K) for temperature. Defined in terms of the Boltzmann constant since 2019.

The remaining three (ampere for electric current, mole for amount of substance, candela for luminous intensity) appear less often in casual conversion work but underpin derived units like the watt, the volt, and the lumen.

Dimension reference table

For each dimension supported on this site, the SI base unit, the most common US/imperial counterpart, and the conversion factor between them. Factors marked "exact" are defined by international agreement; the rest are accurate to six or more significant figures.

Dimension	SI base	Common US/imperial	Key conversion
Length	meter (m)	foot, mile, inch	1 ft = 0.3048 m (exact)
Mass	kilogram (kg)	pound (lb)	1 lb = 0.45359237 kg (exact)
Temperature	kelvin (K)	Fahrenheit (°F)	°F = (K − 273.15) × 9/5 + 32
Volume	cubic meter (m³)	US gallon, fl oz	1 US gal = 3.78541 L
Speed	m/s	mph	1 mph = 0.44704 m/s (exact)
Area	square meter (m²)	square foot, acre	1 acre = 4,046.86 m²
Time	second (s)	minute, hour, day	1 hour = 3,600 s (exact)
Pressure	pascal (Pa)	psi, atm, bar	1 atm = 101,325 Pa (exact)
Energy	joule (J)	BTU, kcal, kWh	1 BTU IT = 1,055.05585262 J
Power	watt (W)	horsepower (hp)	1 mech hp = 745.6999 W
Frequency	hertz (Hz)	(uses Hz globally)	1 GHz = 10⁹ Hz (exact)
Angle	radian (rad)	degree (°)	180° = π rad
Digital storage	byte (B)	(uses bytes globally)	1 KiB = 1,024 B; 1 KB = 1,000 B
Data rate	bit/s	(uses bps/Bps)	1 Mbps = 10⁶ bit/s (decimal)
Force	newton (N)	pound-force (lbf)	1 lbf = 4.44822 N
Fuel economy	L/100 km	MPG (US, UK)	L/100km = 235.215 / US MPG
Density	kg/m³	lb/ft³, g/cm³	1 g/cm³ = 1,000 kg/m³ (exact)

Conversion pitfalls worth knowing

US vs imperial (UK) gallons. They look the same in product specs but a US gallon is 3.78541 L and an imperial gallon is 4.54609 L (about 20% larger). Always check the source country before comparing fuel-efficiency numbers, recipes, or container volumes.

Decimal vs binary digital prefixes. Network speeds use decimal (1 Mbps = 1,000,000 bits/s). File sizes can use either: operating systems frequently use decimal MB (10⁶ bytes) while some legacy tools report MiB (2²⁰ = 1,048,576 bytes). The IEC introduced the kibi/mebi/gibi prefixes in 1998 to disambiguate.

Square and cubic prefix mistakes. When converting area or volume, square the linear prefix factor: 1 m² is 10,000 cm² (not 100), because the conversion factor between meters and centimeters (×100) is itself squared when applied to area. Forgetting to square or cube is a common source of off-by-100 mistakes.

Mass vs weight. In casual use, pounds and kilograms are interchangeable mass units. In strict physics, mass measures matter (kg, lb-mass) and weight is the gravitational force on that mass (newtons, lb-force). Body weight, food weight, and shipping weight are all mass measurements despite the label.

Authoritative references

For work that needs to be defensible against a primary standard:

NIST Special Publication 811: US guide for the use of the International System of Units.
BIPM SI Brochure (9th edition): the international SI definitions in their primary source.
NIST Metric System / SI: accessible reference for prefixes, base units, and derived units.

All dimension pages

Pair-specific conversion pages

Each pair page is a dedicated calculator for one specific conversion, with a citation-friendly lead answer, the conversion formula, a 10-row common-values table, related-conversions cross-links, and a FAQ: