Why an Inch Is Exactly 2.54 Centimeters

ficklestudio26 · Facts checked October 25, 2026

US customary units are not measured against old metal bars any more. They are defined from metric units, with exact numbers.

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The exact values

1 yard = 0.9144 meter, exactly
1 inch = 2.54 centimeters (25.4 millimeters), exactly
1 foot = 0.3048 meter, exactly
1 avoirdupois pound = 0.45359237 kilogram, exactly

Because these are definitions and not measurements, a converter can use them with no rounding error. Rounding only happens when the answer is displayed.

The 1959 agreement

On July 1, 1959, representatives of six countries (Australia, Canada, New Zealand, South Africa, the United Kingdom and the United States) agreed on common values for the yard and the pound, so that precise measurements would be identical in all of them. Before that, the American and British yards and pounds differed slightly. The international yard turned out to be about two millionths of a meter longer than the old imperial yard, and the international pound about six ten-millionths of a kilogram lighter than the old imperial pound.

Where the inch value came from

The 25.4 millimeter inch had been adopted by the British Standards Institution in 1930, and a British Commonwealth scientific conference recommended the same value in 1946. Dividing the yard definition by 36 gives the same number: 0.9144 ÷ 36 = 0.0254 meter.

What this means for conversions

cm to inches: divide by 2.54. 170 cm is about 66.93 inches.
Miles to km: 1 mile is 1,760 yards, which is 1,609.344 meters, so multiply miles by 1.609344.
Pounds to kg: multiply by 0.45359237. 150 lb is about 68.04 kg.

Note that gallons and cups here are US units. The imperial gallon used in the UK is a different size.