What is 4mm converted to inches?

1/4 inch??

Divide by 25.4

5/32" would be more accurate

Sounds a bit like a homework question. I usually help by giving clues rather than answers. 1" = 2.54 cm. Once you know this you can convert cm to mm and make your formula. Biggest clue is that rtw is correct.

Metric conversions (http://www.sciencemadesimple.com/conversions.html)