What 1.8mm equals what fraction of an inch?

First convert 1.8 mm to inches. Since there are 25.4 mm in 1 inch, divide by 25.4:

1.8 / 2.54 = 0.070866 inches (although that's probably more significant figures than are warranted).

Now convert 0.07... to a fraction. Suppose you want to know it to the nearest 16th of an inch. You're trying to solve the equation

0.070866 = x / 16

where x is the numerator you want to know. To solve for x, multiply by 16 on both sides, and you get

x = 16 times 0.070866 = 1.1338...

So 1.8 mm is a bit more than 1/16 of an inch.

If you want to get it to the nearest 128th of an inch, multiply by 128:

128 time 0.070866 = 9.07...

So 1.8 mm is about 9/128ths of an inch. (The 1/16 we got before is the same as 8/128.)