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.
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.