Unit Converter

What Is a Unit Converter and How Does It Work?

A unit converter is a tool that translates a quantity expressed in one unit into the equivalent value in another unit — for example, kilometers to miles, kilograms to pounds, or Celsius to Fahrenheit. This free online unit converter covers 8 families and 80+ units: length, mass, temperature, volume, area, time, speed and digital storage. Every conversion runs locally in your browser using exact mathematical formulas, with no external API calls and no signup.

Under the hood, most unit conversions follow the same simple rule: each unit is defined relative to a base unit (meter for length, kilogram for mass, second for time, and so on). To convert a value, the calculator multiplies it by the source unit's factor and divides by the destination unit's factor. Temperature is the exception — it uses an affine transformation because each scale has a different zero point, not just a different size.

How to Use This Unit Converter

The converter is designed to be instant and frictionless:

Step 1 — Pick a family. Use the tabs at the top to switch between length, mass, temperature, volume, area, time, speed and digital storage. The unit dropdowns automatically refresh to show only the relevant options.

Step 2 — Enter a value. Type any number in the "From" field. Decimals are supported with the dot separator. The result updates instantly in the "To" field as you type.

Step 3 — Choose source and target units. Use the dropdowns below each field. The default pair for length is kilometers to miles, but you can change it at any time. Click the ⇅ swap button between the two boxes to invert the conversion.

Step 4 — Copy or compare. Use the "Copy result" button to grab the converted value, or scroll to the "Common conversions" table below for a quick reference of the most popular pairs in the current family.

Metric vs Imperial: The Two Big Families

The world uses two main measurement systems. The metric system (officially the SI — Système International d'Unités) is the global standard used by almost every country: meters, kilograms, liters, Celsius. It is decimal-based, so converting within the system is just a matter of moving the decimal point (1 km = 1,000 m = 100,000 cm).

The imperial system — also called US customary in the United States — is still used in the US, UK (partially), and a handful of other countries. It uses miles, pounds, gallons and Fahrenheit. Imperial units are historical and don't follow a clean decimal pattern (1 mile = 1,760 yards = 5,280 feet = 63,360 inches). This is why a converter is essential when working between systems.

Length, Mass, Volume, Area

For the most common conversions: 1 kilometer ≈ 0.621 miles, 1 inch = 2.54 cm exactly, 1 pound ≈ 0.4536 kilograms, 1 US gallon ≈ 3.785 liters (UK gallons are slightly larger at 4.546 liters), and 1 acre ≈ 4,047 square meters. The "Common conversions" table below shows the full reference for the active family.

Common Conversion Formulas Explained

The math behind unit conversion is straightforward once you see the pattern:

• Length, mass, volume, area, time, speed: result = value × (from-factor / to-factor). Both factors express how many base units fit in one of that unit.
• Temperature (Celsius ↔ Fahrenheit): °F = °C × 9/5 + 32, and the inverse °C = (°F − 32) × 5/9. The +32 offset is what makes temperature special: 0°C is not 0°F.
• Temperature (Celsius ↔ Kelvin): K = °C + 273.15. Kelvin shares the same step size as Celsius but starts at absolute zero (−273.15°C).

Digital Storage: Decimal vs Binary (MB vs MiB)

Digital units have a famous trap: kilobyte (KB) and kibibyte (KiB) are not the same. The decimal definition uses powers of 10: 1 KB = 1,000 bytes, 1 MB = 1,000,000 bytes. The binary definition uses powers of 2: 1 KiB = 1,024 bytes, 1 MiB = 1,048,576 bytes. Storage manufacturers typically use decimal (so a "500 GB" SSD is 500,000,000,000 bytes), while operating systems often display the binary equivalents (which is why your SSD shows as "465 GiB" in macOS or Linux). Both units are supported by this converter — pick the one that matches the context.

Bits vs Bytes

A bit is the smallest unit of digital information (0 or 1). A byte is 8 bits. Network speeds are quoted in bits per second (Mbps, Gbps), while file sizes are quoted in bytes (MB, GB). A "100 Mbps" internet plan delivers a maximum theoretical download speed of 12.5 MB/s — the factor 8 is the bit-to-byte ratio.