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📏 Unit Converter

Convert between length, mass, and volume units in one place. Metric and imperial supported.

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Understanding Metric and Imperial Unit Systems

The metric system (SI units) is the international standard used by the vast majority of the world. It's built on base-10, meaning conversions always involve multiplying or dividing by powers of 10: 1 metre = 100 centimetres = 1,000 millimetres. This makes arithmetic straightforward and is why metric has been adopted for science, engineering, medicine, and commerce globally.

The imperial system — feet, inches, miles, pounds, gallons — is the legacy system still used primarily in the United States and to varying degrees in the United Kingdom. The UK has been officially metric since the 1970s but retains imperial units informally for certain things: road distances in miles, pints in pubs, and heights in feet and inches in everyday conversation.

uses the metric system for all official purposes. However, because a large volume of imported goods, recipes, workout programmes, and online content originates from the US, unit conversions remain practically relevant for consumers on a regular basis. A US recipe calling for 2 cups of flour, a US workout plan prescribing 135 lb on the barbell, or a US product listing a TV as 65 inches — all require metric conversion for practical use.

Length conversions: 1 inch = 2.54 cm. 1 foot = 30.48 cm. 1 yard = 0.9144 m. 1 mile = 1.609 km. Mass conversions: 1 pound = 453.6 g = 0.4536 kg. 1 ounce = 28.35 g. 1 US ton = 907.2 kg. 1 metric tonne = 1,000 kg. Volume conversions: 1 US cup = 236.6 ml. 1 US fluid ounce = 29.57 ml. 1 US gallon = 3.785 litres. 1 imperial pint (UK) = 568 ml.

This converter handles the most common length, mass, and volume conversions. For temperature, use the dedicated Temperature Converter on CalForge.

How to Convert Between Units of Measurement

Unit conversion multiplies a value by a conversion factor — a ratio that expresses how many of one unit equal one of another. All valid conversion factors equal exactly 1 when the units are included: 2.54 cm/inch means 2.54 centimetres per inch, and using this factor changes the units while keeping the quantity the same.

The key to accurate unit conversion: always track the units through your calculation. If you want to end up in km/h from m/s, multiply by 3.6. If you get km/s instead, you know the factor was applied incorrectly.

Frequently Asked Questions

What are the most commonly needed unit conversions? +
Length: inches to cm (×2.54), miles to km (×1.609), feet to metres (×0.3048). Weight: lbs to kg (×0.4536), ounces to grams (×28.35). Volume: gallons to litres (US: ×3.785; UK: ×4.546), cups to ml (×240). Temperature: °F to °C = (°F−32)×5/9.
Why do conversion factors have so many decimal places? +
Most unit conversions involve irrational or non-terminating ratios. Some, like inches to centimetres (exactly 2.54), are defined. Others, like miles to kilometres, are derived from the definitions and result in non-repeating decimals. More decimal places = more precision, which matters in engineering and science.
What is SI and why does it matter? +
SI (Système International d'Unités) is the modern metric system, the global standard for science, medicine, and most commerce. It uses base units (metre, kilogram, second, etc.) with decimal prefixes (kilo-, milli-, micro-). Its coherence — each unit derived from others without conversion factors — makes it far simpler for calculation than imperial units.