how long is an inch?
1 inch equals 25.4 millimeters or 2.54 centimeters or 0.0254 meters or 0.0000254 kilometers. It's about the diameter of a US quarter dollar coin. 2.5 cm. Your thumbnail, or a hairpin. Originally IT WAS BASED on the length of an English King's thumbnail. This is absolutely true. + 22 others found this useful.