Convert between metric and imperial units: length, weight, temperature, volume, area & more. Fast, accurate & free.
Select a category (Length, Weight, Temperature, Volume, or Area), choose your input and output units, enter a value, and see the conversion instantly. The converter supports both metric and imperial systems.
| Category | Units |
|---|---|
| 📏 Length | Millimeters, Centimeters, Meters, Kilometers, Inches, Feet, Yards, Miles |
| ⚖️ Weight | Milligrams, Grams, Kilograms, Metric Tons, Ounces, Pounds, Stones |
| 🌡️ Temperature | Celsius, Fahrenheit, Kelvin |
| 💧 Volume | Milliliters, Liters, Cubic Meters, Teaspoons, Tablespoons, Cups, Pints, Gallons |
| 📐 Area | Square Millimeters, Square Centimeters, Square Meters, Hectares, Square Kilometers, Square Inches, Square Feet, Acres |
Length: 1 mile = 1.609 kilometers | 1 foot = 30.48 centimeters | 1 inch = 2.54 centimeters
Weight: 1 pound = 0.454 kilograms | 1 ounce = 28.35 grams | 1 stone = 6.35 kilograms
Temperature: 0°C = 32°F (freezing) | 100°C = 212°F (boiling) | 37°C = 98.6°F (body temp)
Volume: 1 gallon = 3.785 liters | 1 cup = 236.6 milliliters | 1 liter = 33.8 fluid ounces
Area: 1 acre = 4047 square meters | 1 hectare = 10,000 square meters | 1 square mile = 2.59 square kilometers
Unit conversion is one of those skills that looks trivial until it isn't — a recipe from an American website, a furniture listing in centimeters, a weather forecast in a scale you don't think in. Behind every conversion is a simple idea: units of the same quantity relate by fixed factors, defined today with extraordinary precision by the International System of Units (SI). Understanding a few of those relationships, and the one famous exception, makes every conversion predictable.
Since 2019, all SI base units have been defined by constants of nature rather than physical objects. The meter is the distance light travels in 1/299,792,458 of a second; the kilogram is fixed via Planck's constant. Every everyday unit is anchored to these definitions: an inch is exactly 25.4 mm, a pound is exactly 0.45359237 kg, a mile is exactly 1,609.344 m. "Exactly" matters — these aren't measured approximations but definitions, which is why conversion results can be trusted to as many decimal places as you need.
Every conversion above is a pure multiplication, because those scales share a zero point. Temperature doesn't — 0°C and 0°F are different physical temperatures — so converting requires an offset as well as a factor: °F = °C × 9/5 + 32, and °C = (°F − 32) × 5/9. This is why you can't say "double the Celsius" for anything except rough intuition. Handy anchors: 0°C = 32°F (freezing), 20°C = 68°F (room temperature), 37°C = 98.6°F (body temperature), 100°C = 212°F (boiling), and the two scales cross at −40°, the one temperature that reads identically in both.
A converter gives you many decimal places; the situation tells you how many to keep. Cutting a shelf? Millimeter precision matters. Following a soup recipe? Rounding 236.6 mL to "roughly 240 mL" changes nothing. A good rule is to keep one more significant figure than your measuring tool can actually resolve, then round. Beware compounding: converting inches → cm → back to inches with aggressive rounding at each step drifts noticeably, so convert once from the original figure whenever possible.
Travel: a 120 km/h motorway limit is about 75 mph, and 26°C beach weather is 79°F. Cooking: an American recipe's 350°F oven is 177°C — most bakers set 175 or 180. Fitness: a 5K run is 3.11 miles; a 70 kg body weight is 154 lbs. Shopping: a 55-inch TV is 140 cm diagonal, and that 8.5 × 11 inch "Letter" paper is 216 × 279 mm, subtly different from A4's 210 × 297 — the reason imported documents sometimes print with clipped margins.