cm to inches divides by 2.54. The inch is defined as exactly 2.54 centimetres, so the conversion is not an approximation the page invented. One centimetre is 1/2.54 inches, which is 0.3937007874 inches when you keep ten significant figures. Ten centimetres is 3.937007874 inches. Thirty centimetres, a common ruler length, is 11.81102362 inches. A hundred centimetres, one metre, is 39.37007874 inches. The form on this page opens with 1 cm already chosen so the first press shows that 0.3937 figure. Change the amount, press Convert, and the same factor applies. Swap the dropdowns to go from inches back to centimetres: one inch must return exactly 2.54, and twelve inches, a foot, must return 30.48 cm.
People round this pair too early. Calling a centimetre 0.4 inches is fine for a rough glance and wrong for a hem, a screen bezel, or a drill size. Calling it 0.39 is closer and still short of 10 cm: 10 times 0.39 is 3.90, and the real length is just under 3.94. If you are copying a measurement into a form that only accepts two decimal places, round at the end, once. Do not convert, round, convert back, and round again. This page shows a longer figure so you can do that last rounding yourself. The unit converter is the same multiplication with the other units visible: millimetres, metres, feet, yards, miles.
A few lengths that show up in real arguments. 15 cm is 5.9055 inches, just under 6. 2.5 cm is 0.98425 inches, just under an inch, which is why a 25 mm drill is not a 1 inch drill. 91.44 cm is 36 inches, a yard. 160 cm is 62.992 inches, about 5 feet 3 inches if you then divide by 12 and keep the remainder. The height box on the BMI calculator wants centimetres, or feet plus leftover inches, and it uses this same 2.54. If you know your height in centimetres, you do not need this page before that one. If a form abroad asked for inches and you know centimetres, this is the page.
The factor is exact because the inch was tied to the metre, not the other way around in daily use. Older survey inches in some land records differ by a couple of parts per million. This page does not offer that survey inch. For cloth, furniture, and screens, 2.54 cm is the inch you want. Nothing you type is sent anywhere. If 2.54 cm does not come back as 1 inch, you are not on this conversion.