Millimeters (mm) 
Inches (in) 

Swap < == > 

1 mm = 0.03937008 in  1 in = 25.4 mm 
1

*

1
10 
*

1 in
2.54 
=

0.03937007874 in 
1

*

1 in
25.4 
=

0.03937007874 in 
Inches  0.03937007874 
Feet  0.003280839895 
Yards  0.001093613298 
Miles  6.213711922e7 
Nautical Miles  5.399568035e7 
Lightyears  1.056998307e19 
Centimeters  0.1 
Meters  0.001 
Kilometers  0.000001 
We know 1 Inch is equal to 25.4 mm and Inch is a larger unit and mm smaller. So whenever we go to convert mm to inches, we have to remember that if we want to convert from small units to large units, we have to divide. And if you want large unit to small unit, we have to do multiple.
For example, we divided 15 by 25.4 to convert 15mm to inches. Since mm is a small unit and Inches large. And we know 1in = 25.4mm.
So to convert 15 Inches to mm, we need to multiply 25.4 by 15. Since we are going to convert from Inches to mm. We need to remember all the time to convert from large units to small units to multiply.
Convert 15 mm to inches:
d_{(″)} = 15mm / 25.4 = 0.5905″
Note: Fractions are rounded to the nearest 8th fraction. Values are rounded to 4 significant figures.
Millimeters  Inches (decimal)  Inches (fraction) 

1 mm  0.03937"  3/64" 
2 mm  0.07874"  5/64" 
3 mm  0.11811"  1/8" 
4 mm  0.15748"  5/32" 
5 mm  0.19685"  13/64" 
6 mm  0.23622"  15/64" 
7 mm  0.275591"  9/32" 
8 mm  0.314961"  5/16" 
9 mm  0.354331"  23/64" 
10 mm  0.393701"  25/64" 
11 mm  0.433071"  7/16" 
12 mm  0.472441"  15/32" 
13 mm  0.511811"  33/64" 
14 mm  0.551181"  35/64" 
15 mm  0.590551"  19/32" 
16 mm  0.629921"  5/8" 
17 mm  0.669291"  43/64" 
18 mm  0.708661"  45/64" 
19 mm  0.748031"  3/4" 
20 mm  0.787402"  25/32" 
21 mm  0.826772"  53/64" 
22 mm  0.866142"  55/64" 
23 mm  0.905512"  29/32" 
24 mm  0.944882"  15/16" 
25 mm  0.984252"  63/64" 
26 mm  1.0236"  1 1/32" 
27 mm  1.063"  1 1/16" 
28 mm  1.1024"  1 7/64" 
29 mm  1.1417"  1 9/64" 
30 mm  1.1811"  1 3/16" 
31 mm  1.2205"  1 7/32" 
32 mm  1.2598"  1 17/64" 
33 mm  1.2992"  1 19/64" 
34 mm  1.3386"  1 11/32" 
35 mm  1.378"  1 3/8" 
36 mm  1.4173"  1 27/64" 
37 mm  1.4567"  1 29/64" 
38 mm  1.4961"  1 1/2" 
39 mm  1.5354"  1 17/32" 
40 mm  1.5748"  1 37/64" 