You divide 3 into 16 (3/16) and you get .1875″. I use calipers to measure my guitar parts most all the time. The Dial Calipers give precise measurements especially when measuring diameter of magnets. Some calipers measure in ruler increments, some measure metric and the best is digital read out. The chart below will give you a quick reference and better measurements than using a standard ruler.

Inch Decimal Inch Decimal Inch Decimal Inch Decimal
1/64 .0156 17/64 .2656 33/64 .5156 49/64 .7656
1/32 .0312 9/32 .2812 17/32 .5312 25/32 .7812
3/64 .0468 19/64 .2968 35/64 .5468 51/64 .7968
1/16 .0625 5/16 .3125 9/16 .5625 13/16 .8125
5/64 .0781 21/64 .3281 37/64 .5781 53/64 .8281
3/32 .0937 11/32 .3437 19/32 .5937 27/32 .8437
7/64 .1093 23/64 .3593 39/64 .6093 55/64 .8593
1/8 .125 3/8 .375 5/8 .625 7/8 .875
9/64 .1406 25/64 .3906 41/64 .6406 57/64 .8906
5/32 .1562 13/32 .4062 21/32 .6562 29/32 .9062
11/64 .1718 27/64 .4218 43/64 .6718 59/64 .9218
3/16 .1875 7/16 .4375 11/16 .6875 15/16 .9375
13/64 .2031 29/64 .4531 45/64 .7031 61/64 .9531
7/32 .2187 15/32 .4687 23/32 .7187 31/32 .9687
15/65 .2343 31/64 .4843 47/64 .7343 63/64 .9843
1/4 .25 1/2 .5 3/4 .75 1 1