Unit Converter

Unit systems — metric, US customary, and imperial

Three measurement systems still appear in daily software. The International System of Units (SI) is the metric system most of the world standardized on, anchored at seven base units (metre, kilogram, second, ampere, kelvin, mole, candela) and using decimal prefixes for every other quantity. US customary units are a separate system grounded in old English measures — inches, pounds, gallons, Fahrenheit — and used almost exclusively inside the United States. Imperial units are a third system, used historically in the British Commonwealth and still surviving in UK road signs, pub measures, and weather forecasts; they overlap with US customary in name but not always in value.

The tool above lets you flip between them. Two practical points follow. Multiplying within one system is safe (1 km is 1,000 m, always). Crossing systems is not — 1 fl oz of milk in a US recipe is not the same volume as 1 fl oz in a British one, and software that ignores the difference can drift far enough in recipe scaling or freight pricing to break the result.

SI prefixes — strict powers of 1,000

SI prefixes scale a base unit by powers of 1,000. The same prefix attaches to any base — kilometre, kilogram, kilowatt, kilobyte (when used in the SI sense) — and means the same multiplier everywhere. This regularity is the practical reason SI dominates scientific publishing.

Units that sound the same but are not

These pairs catch teams that build internationally — same name on each side of an ocean, different value. The numbers below match what the tool above uses internally.

Why temperature does not divide and multiply like length

Temperature is not a linear quantity in the way length or mass are. "Twice as cold" has no fixed meaning because the scales start from different points. Celsius zero is the freezing point of water; Fahrenheit zero is roughly the freezing point of a salt brine the 18th-century instrument-maker Daniel Fahrenheit calibrated against; Kelvin zero is absolute zero, the lower limit of thermodynamic temperature.

That means conversion is affine, not multiplicative. The formula C → F is F = C × 9/5 + 32; C → K is K = C + 273.15. Multiplying a Celsius temperature by a factor produces nonsense (3 × 25°C is not "three times hotter," just a new Celsius value that may not match any physical quantity). When subtracting temperatures to get a difference, however, the offset cancels: the temperature difference 10°C → 25°C is 15 K, full stop.

Computing units — kB vs KiB, and why your hard drive looks smaller

For decades "kilobyte" meant 1,024 bytes inside an operating system and 1,000 bytes on a hard-drive box. The International Electrotechnical Commission resolved the ambiguity in 1998 by introducing binary prefixes — KiB, MiB, GiB — that explicitly mean 2^10, 2^20, 2^30 bytes. Strict usage today: kB/MB/GB for powers of 1,000, KiB/MiB/GiB for powers of 1,024. The discrepancy grows with size.

In practice, storage and network speeds advertise SI values (a "1 TB drive" is 10^12 bytes), while RAM and operating systems mostly report binary (Linux and macOS show file sizes in binary, Windows reports binary but labels the numbers with the SI suffix "GB," which is what causes the "my new 500 GB drive only has 465 GB" complaint — it's 500 × 10^9 bytes shown as 465 × 2^30, not missing space).

How to use

Pick a category (length / weight / temperature / time / area / volume / speed / data size), enter a number in any unit, and every other unit in that category updates simultaneously. Swap input units freely — the math is symmetric, so entering 1 meter or 39.37 inches produces the same row of conversions, just with the focus on a different cell.

Reach for this for everyday conversions where remembering the constant is overkill: cm ↔ inches for a screen size, kg ↔ lbs for a recipe scaled across regions, °C ↔ °F for travel weather, m² ↔ ft² for apartment listings, MB ↔ MiB ↔ GB when reading both decimal and binary storage figures. The conversion factors are based on the SI definitions (2019 revision); for currency, exchange rates are not included because they change minute-to-minute and a static value would be misleading — use a dedicated FX tool that pulls live rates.

Examples

category: length
input:    5.75 ft  (or 69 inches)

meters:        1.7526
centimeters:   175.26
millimeters:   1752.6
inches:        69.0
yards:         1.917

category: temperature
input:    100 °F

°C:  37.78
K:   310.93
°R:  559.67

category: data
input:    1 GiB

bytes:       1,073,741,824
KB (decimal):  1,073,741.824
KiB (binary):  1,048,576
MB (decimal):  1,073.741
MiB (binary):  1,024
GB (decimal):  1.074
TB (decimal):  0.001074

FAQ

Related concepts

Unit systems are conventions for measuring physical quantities, and the world runs two big ones plus stragglers. The **SI** (Système international d'unités, 1960, updated 2019) is the international scientific standard, used by every country except the US, Liberia, and Myanmar for everyday measurement, and used by every country including the US for scientific work. The **US customary** and **imperial** systems share roots but diverge — the US "gallon" is smaller than the British, the US "ton" (2000 lbs short ton) differs from the British "ton" (2240 lbs long ton) and from the metric tonne (1000 kg). Conversion tools mostly bridge these three families.

The **2019 SI redefinition** quietly fixed a long-standing scientific embarrassment: until then, the kilogram was defined by a literal platinum-iridium cylinder in a vault in Sèvres, France. Variations in the cylinder's mass over time were detectable. Since 2019, all seven SI base units are defined in terms of fundamental physical constants — the Planck constant, the speed of light, the cesium atomic transition frequency, Avogadro's number, the elementary charge, the Boltzmann constant, and the luminous efficacy. Conversions between SI units are now exact rational numbers tied to these constants rather than tied to an artifact.

Three adjacent concepts shape the day-to-day. **Decimal vs binary prefixes**: kilo / mega / giga are 10^3 / 10^6 / 10^9 in SI, while kibi / mebi / gibi (KiB / MiB / GiB, IEC 60027-2, 1998) are 2^10 / 2^20 / 2^30 in computing. Drive manufacturers use decimal, operating systems use binary, network engineers use decimal — explaining "why my 1 TB drive only has 931 GB". **Temperature offset vs scale**: most unit conversions are scalar multiplications, but temperature requires an affine transformation because the zero points differ (0°C is not 0°F is not 0 K). **Currency** lives outside this — exchange rates fluctuate, so currency belongs in its own category with live data, not in a static unit converter.