Search and filter 149 units across 11 categories — length, mass, volume, area, pressure, energy, power, temperature, speed, data storage, and angle — with exact conversion factors to each category's SI base unit.
Sources verified • Last updated: August 2026
| Category | Unit | Symbol | System | Value in Base Unit | Type |
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Every row shows one unit's exact relationship to its category's own SI base unit — meters for length, kilograms for mass, Pascals for pressure, and so on. Multiply a quantity expressed in that row's unit by the "Value in Base Unit" figure to get the equivalent in the base unit; for example, a length of 5 inches equals 5 × 0.0254 = 0.127 meters. Temperature is the one category without a simple multiplier, since Celsius, Fahrenheit, and Kelvin don't share a common zero point — use the Unit Converter for temperature instead.
Most conversion factors on this page are exact by legal or mathematical definition — an inch has been defined as precisely 0.0254 meters since the 1959 International Yard and Pound Agreement, so no amount of additional precision changes that number. A handful of units are instead measured or derived physical constants (a parsec, a slug, Mach 1 at sea level) carried to a fixed number of significant figures — these are marked "Approx." in the Type column, and their true value technically has more digits than shown.
The US customary and UK imperial systems share a common historical root but split administratively, and several units of liquid volume and mass diverged in the process. A US gallon (3.785 liters) is meaningfully smaller than a UK/Imperial gallon (4.546 liters) — the same gap shows up in fluid ounces, pints, quarts, tons, and hundredweight. Always check the System column before assuming a shared unit name means a shared size; length and area units (inch, foot, yard, mile, acre) are identical between the two systems, but volume and some mass units are not.
Suppose a historical land survey lists a distance in furlongs and you need it in meters. Searching "furlong" in the table above shows 1 furlong = 201.168 meters exactly (660 feet by definition). A distance of 3.5 furlongs converts as 3.5 × 201.168 = 704.088 meters — no separate lookup table needed once you have the base-unit factor.
SI Base Unit — the single reference unit each category is normalized against (meter, kilogram, Pascal, etc.), chosen by the International System of Units.
Conversion Factor — the fixed multiplier that converts one unit into its category's base unit.
Customary Units — the US and UK non-metric systems (inches, pounds, gallons) still used for everyday measurement in a small number of countries.
CODATA — the Committee on Data for Science and Technology, which publishes the internationally recommended values for physical constants like the electronvolt and the speed of light used in this table.
The BIPM SI Brochure (9th edition), NIST Special Publication 811 and Handbook 44, the 1959 International Yard and Pound Agreement, the UK Weights and Measures Act 1985, ISO/IEC 80000, and CODATA fundamental physical constants — the same authorities that define these units legally and scientifically.
Exact means the conversion factor is defined precisely by law or mathematical definition (an inch is defined as exactly 0.0254 meters, for example), so the digits shown never round further. Approximate means the factor is a measured or derived physical constant (a parsec, a slug) carried to a fixed number of significant figures rather than an infinitely precise ratio.
The US and UK customary systems share a common ancestry but diverged for units of liquid volume and some units of mass after they split administratively. A US gallon (3.785 L) and a UK/Imperial gallon (4.546 L) are genuinely different sizes, as are the US and UK fluid ounce, pint, quart, ton, and hundredweight — always check the System column rather than assuming a shared unit name means a shared size.
The Unit Converter takes one value and instantly converts it between two chosen units. This page is a browsable, searchable reference of every unit's raw conversion factor to its category's SI base unit at once, useful for looking up an unfamiliar unit (a furlong, a slug, a therm) or comparing several related units side by side.