Unit Converter Online Free
Unit converter online free — convert between length, weight, temperature, and time units instantly. Metric and imperial supported. No signup, everything runs in your browser.
All Conversions
| Unit | Value |
|---|---|
| Millimeter (mm) | — |
| Centimeter (cm) | — |
| Meter (m) | — |
| Kilometer (km) | — |
| Inch (in) | — |
| Foot (ft) | — |
| Yard (yd) | — |
| Mile (mi) | — |
| Nautical Mile (nmi) | — |
How to Convert Units
- 1Select the category: Length, Weight (Mass), Temperature, or Time from the tabs at the top.
- 2Enter a value in the input field and select your source unit. All equivalent values appear instantly in the other units.
- 3Select a target unit for a direct two-field conversion with more decimal precision. Use decimal inputs (e.g. 1.5 metres, 72.5 kg) for higher accuracy.
Metric, Imperial, and Scientific Units
Unit conversion is a daily need across engineering, cooking, fitness, travel, and academic work. This converter supports the most commonly needed categories with immediate results. All calculations happen in your browser using standard conversion formulas, so there is no dependency on an internet connection after the page loads.
The tool supports both metric (SI) and imperial units, covering the needs of users in countries that use different measurement systems. Temperature conversion handles all three common scales: Celsius (metric/scientific), Fahrenheit (US everyday use), and Kelvin (scientific absolute scale). Note that temperature conversion requires a formula, not a simple multiplication, because the scales differ in both interval size and zero-point location.
Dedicated Converter Pages
Each category also has its own focused page with an extended reference table, tips, and FAQ.
Length Converter
mm, cm, m, km, inches, feet, yards, miles, nautical miles — metric and imperial.
Weight Converter
mg, g, kg, metric tons, ounces, pounds, stone, US tons.
Temperature Converter
Celsius, Fahrenheit, Kelvin, Rankine — with formula display for each conversion.
Time Converter
Nanoseconds to years — covers ns, ms, s, min, hr, day, week, month, year.
Common Conversion Reference
| Conversion | Factor or Formula | Example |
|---|---|---|
| Inches → Centimetres | × 2.54 | 12 in = 30.48 cm |
| Feet → Metres | × 0.3048 | 6 ft = 1.83 m |
| Miles → Kilometres | × 1.60934 | 10 mi = 16.09 km |
| Kilograms → Pounds | × 2.20462 | 70 kg = 154.32 lb |
| Stone → Kilograms | × 6.35029 | 11 st = 69.85 kg |
| Celsius → Fahrenheit | × 9/5 + 32 | 20 °C = 68 °F |
| Celsius → Kelvin | + 273.15 | 20 °C = 293.15 K |
| Hours → Minutes | × 60 | 2.5 hr = 150 min |
Quick Mental Conversion Shortcuts
Kilometres to Miles
Multiply by 0.6 for a quick estimate. More precisely, multiply by 0.621. So 100 km ≈ 62 miles. Useful for reading road signs in countries you don't usually drive in.
Kilograms to Pounds
Multiply by 2.2 for a quick estimate. So 70 kg ≈ 154 lb. The exact factor is 2.20462, but 2.2 works for everyday purposes like reading luggage weight limits.
Celsius to Fahrenheit
A rough mental shortcut: double the Celsius value and add 30. So 20 °C ≈ 70 °F. For exact conversions, use the calculator — the doubling method introduces a ~4°F error at high temperatures.
Feet and Inches
There are exactly 12 inches in a foot and 3 feet in a yard. When converting height (e.g. 5 ft 10 in), convert to total inches first (70 in), then multiply by 2.54 to get centimetres (177.8 cm).
Temperature Quick Check
Body temperature: 37 °C = 98.6 °F. Room temperature: 20 °C = 68 °F. Boiling point: 100 °C = 212 °F. Freezing point: 0 °C = 32 °F. These anchors help you sense-check other conversions.
Use Decimals for Precision
Enter decimal values for more accurate results: 1.75 metres instead of 1 metre, 72.5 kg instead of 72 kg. The converter accepts any number with up to several decimal places.
Frequently Asked Questions
What unit categories does the converter support?
The converter supports Length (mm, cm, m, km, inches, feet, yards, miles), Weight/Mass (mg, g, kg, tonnes, oz, lb, stone), Temperature (Celsius, Fahrenheit, Kelvin), and Time (seconds, minutes, hours, days, weeks).
How accurate are the conversions?
The conversions use standard internationally-recognised factors (SI and US/UK customary). Results are accurate to at least 5 significant figures. The exact defined values are: 1 inch = 2.54 cm exactly, 1 pound = 0.45359237 kg exactly.
Why does temperature conversion use a formula instead of a simple multiplier?
Celsius, Fahrenheit, and Kelvin differ in both scale interval and the location of their zero points. A 1-degree increase in Celsius equals a 1.8-degree increase in Fahrenheit, but 0 °C ≠ 0 °F. The conversion formula (°C × 9/5 + 32) accounts for both differences.
What is the difference between weight and mass?
In physics, mass is the amount of matter (kilograms, pounds), and weight is the gravitational force acting on that mass (Newtons). In everyday language, "weight" usually refers to mass. The units in this converter — kg, lb, stone — are technically mass units.
Why isn't my converted value exactly what I expected?
Floating-point arithmetic in JavaScript can produce results like 2.9999999 instead of exactly 3 for some conversions. The display rounds to a reasonable number of decimal places, but very long decimal results indicate a recurring fraction rather than a rounding error.
Does the converter work offline?
Yes. All conversion formulas are built into the JavaScript running in your browser. After the page loads, you can use the converter without an internet connection. Reload the page to get any updates.
What is Kelvin and when is it used?
Kelvin is the SI base unit of temperature used in scientific and engineering calculations. 0 K is absolute zero (−273.15 °C), the coldest theoretically possible temperature. Unlike Celsius and Fahrenheit, Kelvin values are always positive. It is used in thermodynamics, astronomy, and physics.