Free Unit Converter Online
Our free unit converter makes it easy to convert between different units of measurement. Whether you need to convert length, weight, temperature, area, volume, speed, or time, this tool provides instant and accurate results. Perfect for students, professionals, travelers, and anyone working with different measurement systems.
Supported Categories
Length: Meters, feet, inches, miles, kilometers, yards, and moreWeight: Kilograms, pounds, ounces, grams, stones, tonsTemperature: Celsius, Fahrenheit, KelvinArea: Square meters, square feet, acres, hectaresVolume: Liters, gallons, cups, milliliters, fluid ouncesSpeed: km/h, mph, m/s, knotsTime: Seconds, minutes, hours, days, weeks
Metric vs Imperial
The world uses two main measurement systems:
Metric System: Used by most countries. Based on meters, grams, and liters with decimal prefixes (kilo, centi, milli)Imperial System: Used primarily in the US, UK, and a few other countries. Based on feet, pounds, and gallons
Common Conversion Tips
1 inch = 2.54 centimeters (exactly)
1 mile = 1.609 kilometers
1 pound = 0.4536 kilograms
Water freezes at 0C = 32F
1 gallon (US) = 3.785 liters
Frequently Asked Questions
How accurate are the conversions? Our converter uses precise conversion factors and displays results with appropriate decimal precision for practical use.
What's the difference between US and UK gallons? A US gallon is approximately 3.785 liters, while a UK (Imperial) gallon is approximately 4.546 liters.
How do I convert Celsius to Fahrenheit? Multiply by 9/5 (or 1.8) and add 32. For example, 20C = (20 x 1.8) + 32 = 68F.
Why are there different ton measurements? There are metric tons (1000 kg), short tons (2000 lbs), and long tons (2240 lbs). Our converter uses metric tons by default.
Can I use this offline? Yes! Once the page loads, all conversions are calculated in your browser without needing an internet connection.
How do I convert between square and cubic units? Use our Area category for square units (area) and Volume category for cubic units (capacity).
Why does 1 inch = 2.54 cm exactly, but other conversions have decimals? The inch was formally redefined in 1959 by international agreement to equal precisely 2.54 centimetres, making it an exact conversion. Most other imperial-metric conversions (miles to kilometres, pounds to kilograms) were never redefined this way, so they remain irrational or long recurring decimals that get rounded for practical use.
Where rounding matters
For everyday use — a recipe, a DIY measurement, a rough travel-distance estimate — rounding a conversion to two or three decimal places is invisible in practice. It matters more in fields with tight tolerances: engineering drawings, medical dosing, and scientific measurement typically specify how many significant figures a conversion should carry, because compounding a rounded conversion factor across multiple calculation steps can introduce meaningful drift. This converter carries results to several decimal places internally and displays a sensible number of significant figures, but for regulated or safety-critical work always use the conversion factor and precision your governing standard specifies rather than a general-purpose tool.
Sources & further reading
BIPM — the SI Brochure, the authoritative definition of the International System of Units. bipm.org
NIST Special Publication 811 — Guide for the Use of the International System of Units, source of the conversion factors used here. nist.gov
Written by Alex van den Berg · Last reviewed 4 July 2026 · How we test our calculators · Report an error
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