Unit Converter Free Online

Free online unit converter. Convert length (km to miles), weight (kg to lbs), temperature (°C to °F) and more instantly.

Categories

  • Length — mm, cm, m, km, inch, ft, yard, mile
  • Weight — mg, g, kg, tonne, oz, lb, stone
  • Area — m², km², hectare, acre, sq ft
  • Speed — m/s, km/h, mph, knot
  • Temp — Celsius, Fahrenheit, Kelvin

Different countries and industries use different measurement systems - the United States uses the imperial system (miles, pounds, Fahrenheit, gallons) while most of the world uses the metric system (kilometres, kilograms, Celsius, litres). Engineers, scientists, cooks, travellers, and students all regularly encounter the need to convert between these systems. This free unit converter handles over 200 unit conversions across 10 measurement categories, computing results instantly as you type.

How to Use the Unit Converter

1

Select the measurement category

Choose from the available categories: Length, Weight/Mass, Temperature, Volume/Capacity, Area, Speed, Time, Digital Storage, Pressure, or Energy. Each category shows only the relevant units for that measurement type.

2

Enter your value and select units

Type the number you want to convert in the input field, then select the source unit (what you have) and the target unit (what you want) from the dropdowns. The converted value appears instantly without needing to press any button.

3

Read and copy the result

The converted value is displayed with appropriate decimal precision. For most categories, all equivalent units are shown simultaneously so you can see the value in all units at once - useful when you need multiple conversions from the same source value.

Common Unit Conversions Explained

Length conversions - the most common length conversions between metric and imperial: 1 kilometre = 0.621 miles; 1 mile = 1.609 kilometres; 1 metre = 3.281 feet; 1 foot = 0.305 metres; 1 inch = 2.54 centimetres; 1 centimetre = 0.394 inches.

Weight/mass conversions - 1 kilogram = 2.205 pounds; 1 pound = 0.453 kilograms; 1 stone = 6.35 kilograms (14 pounds); 1 tonne (metric) = 1,000 kg = 1.102 short tons; 1 ounce = 28.35 grams.

Temperature conversions - temperature cannot be converted with a simple multiplication factor because the scales have different zero points. The formulas are: Celsius to Fahrenheit: F = (C × 9/5) + 32. Fahrenheit to Celsius: C = (F - 32) × 5/9. Celsius to Kelvin: K = C + 273.15. Common reference points: 0°C = 32°F (freezing), 100°C = 212°F (boiling), 37°C = 98.6°F (body temperature), 20°C = 68°F (comfortable room temperature).

Volume conversions - 1 litre = 0.264 US gallons; 1 US gallon = 3.785 litres; 1 UK gallon = 4.546 litres; 1 US fluid ounce = 29.57 ml; 1 UK fluid ounce = 28.41 ml; 1 cup (US) = 236.6 ml.

Speed conversions - 1 km/h = 0.621 mph; 1 mph = 1.609 km/h; 1 knot = 1.852 km/h; 1 m/s = 3.6 km/h = 2.237 mph.

Metric vs Imperial - Which System Is Which?

The metric system (International System of Units, or SI) is the official measurement system in most of the world. It uses base-10 relationships between units (1 kilometre = 1,000 metres = 100,000 centimetres), making calculations straightforward.

The imperial system (also called the US customary system in the US) is used in the United States and to varying degrees in the UK. It uses irregular conversion factors (1 mile = 1,760 yards = 5,280 feet = 63,360 inches) that are harder to work with mathematically but deeply embedded in everyday life in these countries.

The UK officially uses metric for most purposes but retains imperial for road distances (miles) and some everyday quantities (pints for beer and milk, stones for body weight). This mix makes UK unit conversion particularly important.

Frequently Asked Questions

What measurement categories does the converter support?
The converter supports: Length (mm, cm, m, km, in, ft, yd, mi), Weight/Mass (mg, g, kg, tonne, oz, lb, stone), Temperature (°C, °F, K), Volume (ml, L, fl oz, cup, pint, quart, gallon), Area (cm², m², km², in², ft², yd², acre, hectare), Speed (m/s, km/h, mph, knot), Time (seconds through years), Digital Storage (bits, bytes through terabytes), and more categories depending on the tool version.
How do I convert Celsius to Fahrenheit?
Multiply the Celsius value by 9/5 (or 1.8) and then add 32. For example: 25°C × 1.8 = 45, then +32 = 77°F. To reverse (Fahrenheit to Celsius): subtract 32, then multiply by 5/9. For example: (77°F - 32) = 45, then × 5/9 = 25°C. The converter handles this calculation automatically - just select Temperature, enter your value, and choose your units.
How many kilometres are in a mile?
Exactly 1 mile = 1.60934 kilometres (often rounded to 1.609). To convert miles to km, multiply by 1.609. To convert km to miles, multiply by 0.621 (or divide by 1.609). For rough mental arithmetic: 5 miles ≈ 8 km, 10 miles ≈ 16 km, 100 km ≈ 62 miles. The converter calculates the precise value to full floating-point precision.
How accurate are the unit conversions?
Conversions use the exact SI-defined conversion factors published by the International Bureau of Weights and Measures (BIPM). Results are accurate to the floating-point precision of modern JavaScript (approximately 15-17 significant decimal digits). For practical purposes this is far more precision than any real-world measurement requires - physical measurements themselves are limited by instrument precision, not calculation accuracy.
Does the converter work offline?
Yes. Once the page has loaded, all unit conversion calculations run entirely in your browser without requiring an internet connection. This makes it useful for travellers who may not always have reliable mobile data, or in environments with restricted internet access. The page needs to load once initially, after which all conversions work locally.