Unit Converter
Convert between metric and imperial units across five categories.
What is a unit converter?
A unit converter translates measurements between different units of the same physical quantity. Length, weight, volume, area, speed, pressure, energy, and many other quantities can be expressed in multiple units. This tool handles conversions across dozens of categories, from everyday measurements (miles to kilometers, pounds to kilograms) to specialized engineering and scientific units.
The world uses two main measurement systems: metric (SI) used by most countries, and imperial/US customary used primarily in the United States. Converting between these systems is the most common need. Within the metric system, conversions are straightforward (multiply by powers of 10), but metric-to-imperial conversions require specific conversion factors.
How to use this tool
Select a category (length, weight, temperature, etc.), choose the source and target units, and enter a value. The conversion happens instantly. The tool shows the conversion factor used and supports both common units and specialized ones. You can quickly swap source and target units.
Conversion categories
- Length — millimeters, centimeters, meters, kilometers, inches, feet, yards, miles, nautical miles.
- Weight/Mass — milligrams, grams, kilograms, tonnes, ounces, pounds, stones, US tons.
- Volume — milliliters, liters, gallons (US and UK), cups, fluid ounces, tablespoons.
- Temperature — Celsius, Fahrenheit, Kelvin. The only category where conversion is not a simple multiplication.
- Speed — m/s, km/h, mph, knots, Mach number.
The metric system
The International System of Units (SI) is based on seven base units: meter (length), kilogram (mass), second (time), ampere (current), kelvin (temperature), mole (amount of substance), and candela (luminous intensity). Prefixes scale units by powers of 10: kilo (1,000), mega (1,000,000), milli (0.001), micro (0.000001). This consistent decimal system makes metric conversions simple compared to imperial units.
Worked example
To convert 5 miles to kilometres, multiply by 1.60934: 5 × 1.60934 = 8.05 km. To convert 68°F to Celsius, subtract 32 then multiply by 5/9: (68 − 32) × 5/9 = 20°C. For 10 US gallons to litres, multiply by 3.78541 = 37.85 L. Keep the factor’s direction straight — multiply when moving to smaller units, divide when moving to larger ones.
Common mistakes
The classic error is mixing up US and imperial units — a US gallon is 3.785 L but an imperial gallon is 4.546 L, a 20% difference that has caused real engineering failures. People also confuse mass and weight, and forget that temperature conversions are not simple multiplications because the scales have different zero points. Round only at the final step to avoid compounding errors.
Frequently asked questions
Why does the US still use imperial units?
The US attempted to switch to metric in the 1970s (Metric Conversion Act of 1975) but the change was voluntary and never gained widespread public adoption. The cost of changing road signs, industrial standards, and public habits was seen as too high. US science, medicine, and military already use metric. Everyday life (cooking, driving, weather) remains imperial, creating a dual-system reality.
Why is temperature conversion different from other unit conversions?
Most unit conversions are proportional (multiply by a constant). Temperature conversions require both multiplication and addition because the scales have different zero points. Zero degrees Celsius is 273.15 Kelvin and 32 degrees Fahrenheit. The formula F = C * 9/5 + 32 includes both a scaling factor (9/5) and an offset (32). This makes temperature the only common unit where doubling the number does not mean doubling the physical quantity.
What is the difference between US and imperial units?
They share names but differ in size: a US gallon is 3.785 litres while an imperial (UK) gallon is 4.546 litres, and US and imperial pints and fluid ounces differ too. Always confirm which system a figure uses before converting.