Which conversion factors are exact rather than approximate?

Several are defined exactly: 1 inch equals 2.54 cm, 1 pound equals 0.45359237 kg, 1 US gallon equals 3.785411784 litres, and 1 mile equals 1.609344 km. These were fixed by international agreement, so they are not rounded approximations but definitions.

Why is temperature conversion different from the others?

Length, mass, and volume conversions are simple multiplications because their zero points coincide. Temperature scales have different zero points, so converting Celsius to Fahrenheit needs both a scale factor of 9/5 and an offset of 32 degrees: F equals C times nine fifths plus thirty-two.

Is a US gallon the same as a UK gallon?

No. A US liquid gallon is about 3.785 litres, while a UK (imperial) gallon is about 4.546 litres, roughly 20 percent larger. This reference uses US gallons for volume; always confirm which gallon a recipe or spec means, especially for fuel economy figures.

What is the difference between mass and weight in these conversions?

Strictly, the kilogram and pound here are units of mass, and the conversion 1 lb equals 0.4536 kg is a mass equivalence. In everyday use weight and mass are treated as interchangeable on Earth's surface, which is fine for cooking and shipping but not for physics calculations involving gravity.

How precise are the results shown?

The tool computes with full double-precision factors and rounds the displayed result to a sensible number of digits. For engineering or legal use, apply the exact factor yourself and round only at the final step to avoid accumulated rounding error.

Metric–Imperial Conversion Reference

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The metric and imperial systems coexist across the world, and a handful of exact conversion factors cover most everyday needs. This reference lists those factors for length, mass, volume, and temperature, and provides a live two-way converter that applies each one — including the offset that makes temperature special.

How it works

Most unit conversions are a single multiplication because the two scales share a zero point: 0 cm is 0 inches. So inches equal centimetres divided by 2.54, and kilograms equal pounds times 0.45359237. Many of these factors are defined exactly by international agreement rather than measured.

Temperature is the exception. Celsius and Fahrenheit have different zero points, so the conversion needs both a slope and an offset:

F = C × 9/5 + 32
C = (F − 32) × 5/9

Example

To convert 100 km to miles, divide by the exact factor 1.609344, giving about 62.14 miles. To convert 20 °C to Fahrenheit, multiply by 9/5 to get 36, then add 32 for 68 °F.

Notes

Watch the gallon: a US liquid gallon (3.785 L) is about 20 percent smaller than a UK imperial gallon (4.546 L), which quietly distorts fuel-economy and recipe figures. Keep full precision through a calculation and round only at the final step.