ORIGINAL: LT-40
A very easy way to do it that includes no calculations is to get a ruler with both cm and inches on it, and then line up the values with a straight edge or carpenters square. Using this method, I get 10 cm = 3 and 15/16" Which calculates to 3.9375. That is exactly the same as the google calculator. Sorry for the fuzzy pics, but you get the point.
LT-40
Just a note but this type of ruler is almost the only type you can buy in England, so if you know anyone over there or going over there then that could be a viable little present for you.
Good idea, I used to use it alot in England.
James