Unit Converter
Convert between units of temperature, length, weight, volume, and speed. Celsius to Fahrenheit, km to miles, kg to lbs, and more. Free unit converter.
Reviewed for accuracy by Marcus Webb and the Blueprint Dynamics editorial team (last updated July 2026). Our calculators use primary-source formulas and are cross-checked against IRS publications, Fannie Mae guidelines, and Federal Reserve data. Learn more about our methodology.
How unit conversion works
Unit conversion multiplies or divides a measurement by a conversion factor — the exact ratio between two units. All conversions within the same system (e.g., metric) derive from a base unit: the meter for length, kilogram for mass, second for time. Every other unit is defined as a multiple or fraction of that base. 1 kilometer = 1,000 meters, 1 centimeter = 0.01 meters, 1 millimeter = 0.001 meters. Converting between any two metric units is always a power of 10, which makes metric-to-metric conversions far simpler than the arbitrary ratios in the US customary system (12 inches per foot, 3 feet per yard, 5,280 feet per mile).
Temperature conversions — the exception
Temperature is the major exception to pure multiplication because Fahrenheit, Celsius, and Kelvin all have different zero points. Celsius to Fahrenheit: multiply by 9/5, then add 32. F = (C × 9/5) + 32. Fahrenheit to Celsius: subtract 32, then multiply by 5/9. C = (F − 32) × 5/9. Celsius to Kelvin: add 273.15. Kelvin is an absolute temperature scale — 0 K is absolute zero, the theoretical lowest temperature possible, at which molecular motion stops. There are no negative Kelvin values. Kelvin is used in scientific and thermodynamic calculations; Celsius for everyday use in most countries; Fahrenheit primarily in the US.
Common conversions worth memorizing
Length: 1 inch = 2.54 cm (exact). 1 foot = 0.3048 m. 1 mile = 1.609 km. 1 meter = 3.281 feet. Weight/mass: 1 pound = 453.6 grams. 1 kilogram = 2.205 pounds. 1 ounce = 28.35 grams. Volume: 1 US gallon = 3.785 liters. 1 liter = 33.81 fl oz. 1 cup = 236.6 mL. Speed: 1 mph = 1.609 km/h. 1 knot = 1.151 mph. Area: 1 acre = 4,047 m² = 0.4047 hectare. 1 square mile = 640 acres. Pressure: 1 psi = 6.895 kPa. 1 atmosphere = 14.696 psi = 101.325 kPa.
Why unit errors matter
Unit conversion errors have caused real-world disasters. The 1999 Mars Climate Orbiter was lost because one engineering team used metric units (Newton-seconds) while another used US customary units (pound-force-seconds) — a $327 million mistake. A 1983 Air Canada flight ran out of fuel mid-flight because ground crew confused kilograms and pounds when calculating fuel load. In medicine, dosing errors from mg/kg vs. mg/lb confusion can be life-threatening. This calculator eliminates unit-conversion errors for everyday tasks where precision matters.