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Fractional Inch to Decimal and Millimetre Conversion Chart

Drawings, purchase orders and tooling catalogues still mix fractional inches, decimal inches and millimetres. This chart lists every 1/32 inch step with its decimal inch and exact millimetre value so you can check a size without opening a calculator.

Download:
Fraction (inch)Thirty-secondsDecimal inchMillimetres
1/32
1
0.0313
0.7938
1/16
2
0.0625
1.5875
3/32
3
0.0938
2.3812
1/8
4
0.125
3.175
5/32
5
0.1563
3.9688
3/16
6
0.1875
4.7625
7/32
7
0.2188
5.5562
1/4
8
0.25
6.35
9/32
9
0.2813
7.1438
5/16
10
0.3125
7.9375
11/32
11
0.3438
8.7313
3/8
12
0.375
9.525
13/32
13
0.4063
10.3188
7/16
14
0.4375
11.1125
15/32
15
0.4688
11.9063
1/2
16
0.5
12.7
17/32
17
0.5313
13.4938
9/16
18
0.5625
14.2875
19/32
19
0.5938
15.0813
5/8
20
0.625
15.875
21/32
21
0.6563
16.6688
11/16
22
0.6875
17.4625
23/32
23
0.7188
18.2562
3/4
24
0.75
19.05
25/32
25
0.7813
19.8438
13/16
26
0.8125
20.6375
27/32
27
0.8438
21.4313
7/8
28
0.875
22.225
29/32
29
0.9063
23.0187
15/16
30
0.9375
23.8125
31/32
31
0.9688
24.6063
1
32
1
25.4

Millimetres use the exact definition 1 inch = 25.4 mm. Values are rounded for display only. For fractions in 64ths, double the numerator and denominator ratio: 1/64 inch is 0.015625 inch or 0.3969 mm.

Check unit conversions in your ERP data

Send us a sample of your item and UoM tables and we will review where inch and metric conversions disagree.

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How to use the chart

Type a fraction, a decimal or a millimetre value into the search box to find the nearest row. Use the decimal column when programming a CNC control or filling a decimal-inch drawing, and the millimetre column when ordering from a metric supplier. Export to Excel or CSV if you want to paste it into a tool crib list or an ERP unit-of-measure review. Remember that a stock size like 5/8 inch bar is a nominal size and the supplier tolerance still applies to the actual diameter.

The method behind the numbers

Decimal inch equals the numerator divided by the denominator. Millimetres equal decimal inches multiplied by 25.4, which is exact since the 1959 international yard and pound agreement. Every value here is computed from the count of thirty-seconds, so there is no hand-typed rounding drift. The table therefore matches what a spreadsheet or a controller would compute, which matters when two departments each convert the same dimension.

  • 1/32 inch = 0.03125 inch = 0.79375 mm
  • 1/16 inch = 0.0625 inch = 1.5875 mm
  • 1/8 inch = 0.125 inch = 3.175 mm
  • 1/2 inch = 0.5 inch = 12.7 mm

Common mistakes

The most frequent errors are rounding a converted size before applying a tolerance, treating a nominal fraction as a precision dimension, and converting a fraction like 5/16 to 0.31 instead of 0.3125 and then losing 0.0025 inch. Another is mixing fractional drill sizes with letter and number drill series, which are different tables. Keep full precision during calculation and round only the final stated dimension to the number of places the drawing tolerance supports.

Frequently Asked Questions

What is 5/8 inch in decimal and mm?

5/8 inch is 0.625 inch, which is 15.875 mm. It is 20 thirty-seconds, so the chart shows it at row 20. Many metric suppliers will quote 16 mm as the nearest standard size, which is 0.125 mm larger.

Is the 25.4 factor exact?

Yes. One inch is defined as exactly 25.4 millimetres. Any rounding you see in the chart is a display choice only, not an approximation in the conversion factor itself.

Can I use this for drill sizes?

Only for fractional drills. Number drills (#1 to #80) and letter drills (A to Z) have their own decimal sizes that do not follow thirty-second steps, so use a drill size chart for those.

How do I get 64th inch values?

Divide the numerator by 64 for the decimal inch and multiply by 25.4 for millimetres. For example 5/64 inch is 0.078125 inch or 1.9844 mm. The 32nd values in this chart are every second 64th.

If your item master stores sizes in more than one unit and the conversions drift between purchasing and the shop floor, we can help audit and automate the check inside your ERP.

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