Inches to centimetres
One inch is 2.54 centimetres, and this shows the working rather than just the answer. Nothing you type here leaves your browser — it is worked out on your own device, so there is nothing to send and nothing to log.
The factor
One inch is 2.54 centimetres. That is the whole of the conversion: multiply by it, and nothing else happens.
And it is exact
Not rounded, not measured, not a convention that might be revised. The factor above is the definition, so an answer from it is as exact as the number you typed.
The inch is exactly 25.4 mm, and has been since 1 July 1959.
The number in your head
Most people use 2.5 for this, which is the factor to two significant figures. It is low by 1.5748% — 15.748 centimetres too few in every thousand, and 157.48 in every ten thousand.
Going the other way
The reverse is not this sum backwards in your head. It is a different factor, 0.393700787402, and dividing by one is not the same as multiplying by a rounded version of the other.
Rounded, the way back is 0.394, which is high by 0.076%. So the two roundings do not cancel: go out and back with both and you do not land where you started.
How many of those decimals mean anything
The box above will give you ten decimal places because it can. Whether they mean anything is decided by your measurement, not by the arithmetic.
If you measured to the nearest inch, your figure is already uncertain by half of one — so the answer is uncertain by about 1.27 centimetres, and every digit past that is decoration. Quoting 3.1357824 when you measured with a tape is claiming a precision the tape never had.
Where these two sit
Inches and centimetres both measure distance, which is why they can be converted at all. A conversion between families — a length into a weight — is not a conversion, it is a question with a missing quantity in it.
Other conversions from inches
millimetres · feet · metres · yards · kilometres · miles · micrometres · nautical miles