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Free Unit Conversion Calculators

Length, area, volume, weight, temperature, energy, pressure, speed and data units.

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Unit conversion is multiplication by a defined ratio — with two exceptions that trip people up: temperature, which needs an offset as well as a scale, and data storage, where decimal and binary prefixes disagree by up to 7% at the terabyte level.

The tools here cover the SI base and derived quantities alongside their imperial and US customary counterparts, because most real work involves crossing between systems rather than staying inside one.

Precision matters. Conversion factors such as 1 inch = 25.4mm are exact by definition; others, like pounds to kilograms, are rounded. Carrying too many decimals through a chain of conversions implies accuracy the original measurement never had.

Derived units follow from base units. Speed is length over time, pressure is force over area, energy is force times distance. Understanding the derivation makes it obvious why converting a derived unit requires converting each component consistently.

For engineering and scientific work, convert once at the start into a single coherent system, do the mathematics, and convert back at the end. Converting repeatedly mid-calculation is where rounding error accumulates.

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Key concepts in unit conversion tools

SI base unit
The seven defined units — metre, kilogram, second, ampere, kelvin, mole, candela — from which all others derive.
Conversion factor
The fixed ratio between two units of the same quantity.
Derived unit
A unit built from base units, such as newton (kg·m/s²) or pascal (N/m²).
Offset scale
A scale whose zero point differs, such as Celsius to Fahrenheit, requiring addition as well as multiplication.
Binary prefix
Kibi, mebi and gibi denote powers of 1024, distinct from the decimal kilo, mega and giga.
Significant figures
The digits in a measurement that carry meaning, limiting how precisely a converted value can be reported.

How this topic works

How conversion factors are defined

Most imperial-to-metric factors are now defined exactly against SI: 1 inch = 25.4mm, 1 pound = 0.45359237kg, 1 gallon (US) = 3.785411784 litres. Because these are definitions rather than measurements, conversions carry no inherent error — any error comes from rounding the result or from the precision of the original measurement.

Temperature is different

Celsius to Fahrenheit is F = C × 9/5 + 32, and Celsius to Kelvin is K = C + 273.15. The offsets mean you cannot convert a temperature difference the same way you convert a temperature: a 10°C rise is an 18°F rise, not 50°F. This distinction between absolute values and intervals matters in HVAC and thermodynamics work.

Decimal versus binary data units

Storage manufacturers use decimal prefixes (1 TB = 10¹² bytes) while operating systems often report binary ones (1 TiB = 2⁴⁰ bytes). The gap grows with scale: a 1 TB drive shows as roughly 931 GiB. Neither figure is wrong; they measure with different prefix conventions.

Converting derived quantities

To convert a derived unit, convert each component. Miles per hour to metres per second means converting miles to metres and hours to seconds, giving a factor of 0.44704. Pressure conversions combine a force and an area conversion, which is why psi to pascal is 6894.76 rather than a round figure.

Frequently asked questions

Is 1 inch exactly 25.4 mm?

Yes. Since the 1959 international yard and pound agreement, the inch is defined as exactly 25.4 millimetres.

Why does my hard drive show less space than advertised?

Manufacturers count in decimal terabytes (10¹² bytes) while many operating systems display binary tebibytes (2⁴⁰ bytes), a difference of about 9%.

How do I convert a temperature difference?

Apply only the scale factor, not the offset: a difference of 1°C equals 1.8°F and exactly 1 K.

How many decimal places should a converted value keep?

Match the significant figures of the original measurement. Converting a 2-significant-figure measurement into a 6-decimal answer implies false precision.

What is the difference between mass and weight units?

Kilograms measure mass; newtons and pounds-force measure weight, which is mass times gravitational acceleration. Everyday usage conflates them because gravity is effectively constant on Earth.

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