Unit Converter
Quick US ↔ metric conversions — pick a pair, type a number, copy the result.
Length
Speed
Weight
Volume
Temperature
Metric, US customary, and how these conversions work
Most of these conversions move between two measurement systems: the metric system (SI), which scales by powers of ten, and US customary units like inches, miles, pounds, and gallons. The two are linked by factors that are exact by definition, not approximations. One inch is exactly 2.54 centimeters, and one mile is exactly 1.609344 kilometers — these values were fixed by the 1959 international yard and pound agreement, so they never round.
Because the factors are exact, the only imprecision you ever see comes from rounding the final result for readability. Each converter does the arithmetic at full precision and then rounds the displayed number to a sensible number of decimal places for that unit. If you need every digit for an engineering or scientific use, multiply by the exact factor yourself.
Temperature is the exception. Celsius and Fahrenheit don't share a zero point, so you can't convert them with a single multiplier — you need a formula: °F = °C × 9/5 + 32, and °C = (°F − 32) × 5/9. That offset is why 0 °C maps to 32 °F rather than to zero.
Unit conversion FAQ
- How accurate are these conversions?
- Length, mass, and volume conversions here use the exact factors defined by international agreement, so the only imprecision comes from rounding the displayed result. Wherever a figure is rounded-md for readability, the underlying calculation uses the full-precision factor first.
- What's the difference between metric and US customary units?
- The metric system (SI) is decimal — units scale by powers of ten (millimeter, meter, kilometer) — and is the official standard in most of the world. US customary units (inches, feet, miles, pounds, gallons) descend from older English measures and use mixed ratios like 12 inches per foot and 5,280 feet per mile. Since 1959 the customary units have themselves been defined in terms of metric units, which is why exact conversion factors exist.
- How do I convert Celsius to Fahrenheit?
- Temperature is not a simple multiplication because the two scales have different zero points. Use °F = °C × 9/5 + 32. To go the other way, °C = (°F − 32) × 5/9. For example, 20 °C is 20 × 9/5 + 32 = 68 °F.
- Is 1 inch exactly 2.54 cm?
- Yes. Since the 1959 international yard and pound agreement, 1 inch is defined as exactly 2.54 centimeters — it is a definition, not a measured approximation. The same agreement fixes 1 mile at exactly 1.609344 kilometers and 1 pound at exactly 0.45359237 kilograms.
- Why do some results show different numbers of decimal places?
- Each conversion is rounded-md to a precision that's useful for the unit involved — there's no value in showing a kilometer figure to nine decimals when the input was a whole number. The math is always done at full precision and only the final display value is rounded-md.
Full data sources and formulas: /sources.