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.
Why diameters are often not the same?
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.
Formula to convert between mm and Inches
If you would like to convert between the two units we recommend to use our calculator. It is quite faster, you can easily convert between a lot of values and don't have to type in the calculation manually. If you prefer to use a formula here we have the right ones for you: Millimeter = Inch * 25.4
Inch = Millimeter / 25.4
Millimeter = Centimeter * 10
Centimeter = Millimeter / 10
We recommend to always use the unit of length which is common in the country you are actually located. Otherwise the locals will get problems.