The correlation between mm and Inch has the same ratio than between cm and Inch. The reason therefore is that one centimeter is exactly comparable to 10 millimeter. So if you take one Inch you can always be sure that it has the same length than 2.54 centimeter or even 25.4 millimeter. If you now want to convert from mm to Inches the ratio is still the same than from converting from cm to Inches. As far as one centimeter is 0.3937 Inches you can say that one millimeter is 0.03937 Inches. As you see the only difference is that the point moves one digit closer to the left. So if you know any value either in inch, cm or mm you always can convert instantly.

When you want to buy some pipes, tubes etc. in same cases you quite have to be careful instead of just looking at the indicated sizes. The problem is that the wall of steel or aluminium of course also needs some space. So the information on the products usually refer on the inner and just in some cases refer on the outer diameter. In order to always get the right product you should look carefully what the descriptions says. So for example if you buy a pipe it has a inner diameter of 25.4 millimeter but in real you need enough space according to the outer diameter which is 33mm. Of course the same procedure could be happen in reverse.