As a rule of thumb it is 2 1/2 cm to one inch. Funny, that expression - a rule of thumb. It actually comes from the measurement of an inch.The nail joint of the average person's thumb is about an inch long, so measuring quickly one would use their thumb to measure and this was termed a thumb rule, or a ruler made by using the thumb joint. This has since become known as a rule of thumb, equating to a good approximation - one that will do for rough measurements. I have no doubt that acceleration due to gravity is as happy being 10mss instead of 9.81, so an inch is just as pleased to be 2 1/2 cm.