Unit conversions look trivial but go wrong in expensive ways — pilots and engineers have crashed projects over feet vs meters, doctors have misdosed over micrograms vs milligrams, recipes get ruined over cups vs grams. The math is simple once you know the conversion factor, but the factors aren't always memorable (1 inch = 2.54 cm exactly, but 1 mile = 1.60934 km), and rounding at the wrong step compounds errors fast.
These unit converters give you exact conversion factors and consistent precision across distance, length, weight, temperature, volume, and speed. Convert miles to kilometers, kilograms to pounds, Celsius to Fahrenheit, gallons to liters, or numbers to Roman numerals — instantly, with the conversion shown both ways. Each converter uses the modern definition of the unit, not approximations. Browse the categories below.
Length
- Inches to CM / CM to Inches
Convert between inches and centimeters.
- Inches to Feet / Feet to Inches
Convert between inches and feet.
- Feet and Inches to CM / CM to Feet and Inches
Convert between feet and cm.
- Meters to Feet / Feet to Meters
Convert between meters and feet.
- MM to Inches / Inches to MM
Convert between millimeters and inches.
Distance
- Miles to KM / KM to Miles
Convert between miles and kilometers.
- Feet to Miles
Convert between feet and miles.
Speed
- MPH to KPH / KPH to MPH
Convert between miles per hour and kilometers per hour.
Weight
- Pounds to KG / KG to Pounds
Convert between pounds and kilograms.
- Stone to KG / KG to Stone
Convert between stones and kilograms.
- Grams to Ounces / Ounces to Grams
Convert between grams and ounces.
Volume
- Liters to Gallons / Gallons to Liters
Convert between liters and US gallons.
- Liters to Quarts / Quarts to Liters
Convert between liters and US quarts.
- Cups to ML / ML to Cups
Convert between US cups and milliliters.
Other
- Celsius to Fahrenheit / Fahrenheit to Celsius
Convert between celsius and fahrenheit.
- Roman numerals to numbers / Numbers to Roman numerals
Convert between Roman numerals and numbers.
FAQs
The mile was originally a Roman unit (mille passus, 1,000 paces) and evolved differently from the metric kilometer, which was defined in 1799 based on the Earth's circumference. The conversion factor 1.609344 km/mile was set exactly by international agreement in 1959, but it isn't a round number because the two units were defined independently for centuries.
Multiply by 9/5 and add 32. So 0°C is 32°F, 100°C is 212°F, and 37°C (body temperature) is 98.6°F. To go the other way (Fahrenheit to Celsius), subtract 32 and multiply by 5/9.
They use the official SI conversion factors and don't round intermediate steps, so the precision is limited only by floating-point arithmetic — accurate to roughly 15 significant figures. For everyday use that's far more precision than needed. For scientific work, the conversion factors themselves (e.g., 2.54 cm per inch) are exact by definition.
A stone is 14 pounds, or about 6.35 kilograms. It's still used in the UK and Ireland for body weight (people say things like 12 stone 4), though kilograms are common too. It isn't used in the US or in most of the rest of the world — it's mostly a British Isles convention.
Roman numerals use letters where I=1, V=5, X=10, L=50, C=100, D=500, M=1000. Generally letters are added together (LX = 60), but when a smaller value comes before a larger one it's subtracted (IV = 4, IX = 9, XL = 40). The Roman Numerals Converter handles both directions automatically, including compound numerals like MCMLXXXIV (1984).