How to Convert Units
One method handles every conversion: multiply by a fraction that equals one. Learn it once, convert anything.
Every unit conversion is multiplication by a conversion factor written as a fraction equal to one, arranged so the old units cancel. To convert 5 miles to kilometers: 5 mi x (1.609344 km / 1 mi) = 8.047 km. Chain factors for multi-step conversions, and convert directly from source to target to avoid rounding error.
The factor-label method
Write the conversion factor as a fraction equal to one, with the unit you want on top and the unit you have on the bottom, then multiply. Converting 5 miles to kilometers: 5 mi x (1.609344 km / 1 mi). The 'mi' cancels, leaving 8.047 km. Because you multiplied by something equal to one, the quantity did not change, only the units.
This works for every conversion because every defined factor is an identity in disguise: 2.54 cm / 1 in equals one, since they are the same length. Arranging the fraction so units cancel is the whole skill.
Chaining conversions
For multi-step conversions, chain factors and let every intermediate unit cancel. Miles per hour to meters per second: 60 mi/h x (1609.344 m / 1 mi) x (1 h / 3600 s) = 26.82 m/s. Each fraction equals one, so the value is preserved through every step.
Check your setup by watching the units: if anything fails to cancel, a factor is upside down. The units are a built-in error check that catches most mistakes before they become answers.
Temperature: the exception
Temperature scales have different zero points, so they need offset formulas, not pure multiplication. Celsius to Fahrenheit: F = C x 9/5 + 32. The 9/5 handles the degree size; the +32 handles the shifted zero. Kelvin is simpler: K = C + 273.15, a pure shift with the same degree size.
For temperature differences (not absolute temperatures), the offsets cancel: a 10-degree Celsius rise is an 18-degree Fahrenheit rise, with no +32 involved. Mixing up absolute and difference conversions is the classic temperature error.
Avoiding rounding error
Convert directly from source to target in one step using the full-precision factor, and round only the final displayed result. Chaining rounded intermediates, miles to feet (rounded), feet to inches (rounded), inches to cm, accumulates error at every step.
Keep two extra digits through intermediate steps and round at the end. And sanity-check every answer: converting should never change the order of magnitude, so if 5 miles becomes 80 kilometers, a factor is upside down.
Skip the arithmetic
Let the tool do the factor-label work: try the free unit converter.
Conversion method questions
What is the easiest way to convert units?
Use the factor-label method: multiply by the conversion factor written as a fraction equal to one, with the target unit on top and the starting unit on the bottom so the old units cancel. Example: 5 miles x (1.609344 km / 1 mi) = 8.047 km.
How do I convert units without a calculator?
Use one-multiplication shortcuts with rounded factors: miles x 1.6 gives km, kg x 2.2 gives pounds, inches x 2.5 gives cm, Fahrenheit minus 30 halved gives Celsius. These land within a percent or two, fine for travel, cooking, and weather, but use exact factors for engineering or medicine.
Why do my chained conversions lose accuracy?
Because every rounded intermediate step injects error that later steps amplify. Convert directly from the source unit to the target unit in a single multiplication with the full-precision factor, keep extra digits through any intermediate math, and round only the final displayed answer.