A unit converter translates a measurement from one unit to another — meters to feet, kilograms to pounds, Celsius to Fahrenheit — using exact, internationally agreed conversion factors. This converter handles six measurement types: length, weight (mass), temperature, volume, area, and speed, covering the metric and US customary units people search for most.
Enter a value, pick a From unit and a To unit of the same type, and the result appears instantly along with the conversion factor used and a table showing the same value in every related unit. All factors are exact values published by NIST, so results are as precise as your input.
How Unit Conversion Works
Every unit of a given type is defined as a fixed multiple of a base unit — the meter for length, the kilogram for mass, the liter for volume. To convert, the calculator first expresses your value in the base unit, then divides by the target unit’s factor:
converted = value × (factor of From unit) ÷ (factor of To unit)
For example, 3 miles to kilometers: 3 × 1,609.344 m ÷ 1,000 m = 4.828 km. Temperature is the exception — Celsius, Fahrenheit, and Kelvin have different zero points, so they need formulas with an offset rather than a simple multiplier: °F = °C × 9/5 + 32, and K = °C + 273.15.
Exact Conversion Factors Worth Memorizing
Since the 1959 international yard and pound agreement, imperial-to-metric factors are exact by definition, not measured approximations:
- 1 inch = 25.4 mm exactly; 1 foot = 0.3048 m exactly
- 1 mile = 1.609344 km exactly; 1 yard = 0.9144 m exactly
- 1 pound = 453.59237 g exactly; 1 ounce = 28.349523125 g
- 1 US gallon = 3.785411784 L exactly; 1 US cup = 236.588 mL
- 1 acre = 4,046.8564224 m²; 1 square foot = 0.09290304 m²
- 1 mph = 0.44704 m/s exactly; 1 knot = 1.852 km/h exactly
Every other factor in this converter is derived from these definitions.
Worked Examples: Miles to Km and °F to °C
Convert 5 miles to kilometers. One mile is exactly 1.609344 km, so 5 × 1.609344 = 8.04672 km. Going the other way, divide: 10 km ÷ 1.609344 = 6.21371 miles.
Convert 350 °F (a common baking temperature) to Celsius. Using °C = (°F − 32) × 5/9: (350 − 32) × 5/9 = 318 × 5/9 = 176.67 °C — which is why recipes list 350 °F as about 175–180 °C.
Convert 75 kg to pounds. One kilogram is 2.20462 lb, so 75 × 2.20462 = 165.35 lb.
Frequently Asked Questions
What is the difference between metric and imperial units?
The metric (SI) system uses base-10 units — meters, kilograms, liters — and is the official standard in nearly every country. Imperial and US customary units — feet, pounds, gallons — descend from older English measures. Since 1959 they have been defined exactly in metric terms, e.g. 1 inch = 25.4 mm.
How do I convert Celsius to Fahrenheit?
Multiply the Celsius temperature by 9/5 (1.8) and add 32: °F = °C × 9/5 + 32. For example, 20 °C × 1.8 + 32 = 68 °F. To go the other way, subtract 32 and multiply by 5/9. The two scales meet at −40°, where −40 °C equals −40 °F.
How many feet are in a meter?
One meter equals 3.28084 feet, or about 3 feet 3.37 inches. The relationship is exact in reverse: 1 foot is defined as exactly 0.3048 meters, so to convert meters to feet you divide by 0.3048 (or multiply by 3.28084).
Why can’t I convert ounces to fluid ounces?
They measure different things: ounces measure weight (mass), while fluid ounces measure volume. This converter blocks cross-type conversions because there is no universal factor — 8 fl oz of water weighs about 8.35 oz, but 8 fl oz of flour weighs only about 4.4 oz.
Are the conversion factors in this calculator exact?
Yes. All factors come from NIST Special Publication 811 and the 1959 international yard and pound agreement, which define units like the inch (25.4 mm), pound (453.59237 g), and US gallon (3.785411784 L) exactly. Any rounding you see happens only in the displayed digits, not in the calculation.