Fraction, Decimal, and Millimeter Chart

Fraction	Decimal inch	Millimeters
1/64	0.015625	0.3969
1/32	0.031250	0.7937
3/64	0.046875	1.1906
1/16	0.062500	1.5875
5/64	0.078125	1.9844
3/32	0.093750	2.3812
7/64	0.109375	2.7781
1/8	0.125000	3.1750
9/64	0.140625	3.5719
5/32	0.156250	3.9688
11/64	0.171875	4.3656
3/16	0.187500	4.7625
13/64	0.203125	5.1594
7/32	0.218750	5.5562
15/64	0.234375	5.9531
1/4	0.250000	6.3500
17/64	0.265625	6.7469
9/32	0.281250	7.1437
19/64	0.296875	7.5406
5/16	0.312500	7.9375
21/64	0.328125	8.3344
11/32	0.343750	8.7312
23/64	0.359375	9.1281
3/8	0.375000	9.5250
25/64	0.390625	9.9219
13/32	0.406250	10.3187
27/64	0.421875	10.7156
7/16	0.437500	11.1125
29/64	0.453125	11.5094
15/32	0.468750	11.9062
31/64	0.484375	12.3031
1/2	0.500000	12.7000
33/64	0.515625	13.0969
17/32	0.531250	13.4937
35/64	0.546875	13.8906
9/16	0.562500	14.2875
37/64	0.578125	14.6844
19/32	0.593750	15.0812
39/64	0.609375	15.4781
5/8	0.625000	15.8750
41/64	0.640625	16.2719
21/32	0.656250	16.6687
43/64	0.671875	17.0656
11/16	0.687500	17.4625
45/64	0.703125	17.8594
23/32	0.718750	18.2562
47/64	0.734375	18.6531
3/4	0.750000	19.0500
49/64	0.765625	19.4469
25/32	0.781250	19.8438
51/64	0.796875	20.2406
13/16	0.812500	20.6375
53/64	0.828125	21.0344
27/32	0.843750	21.4312
55/64	0.859375	21.8281
7/8	0.875000	22.2250
57/64	0.890625	22.6219
29/32	0.906250	23.0187
59/64	0.921875	23.4156
15/16	0.937500	23.8125
61/64	0.953125	24.2094
31/32	0.968750	24.6062
63/64	0.984375	25.0031
1	1.000000	25.4000

Sage pointing while explaining something

Use the chart as a starting point

Reference values help compare sizes and plan a job. Product specifications, codes, tolerances, and the actual material still control the final choice.

Sage's guided lesson

Why measuring tapes stop at 64ths

Inch fractions look arbitrary until you notice the pattern: every denominator is a power of two. This lesson covers why that pattern exists, how to move between fractions, decimals, and millimeters without losing accuracy, and where the habit came from in the first place.

Sage dressed as an ancient Greek philosopher

Halves of halves of halves

Why 64ths became the standard

Every denominator on this chart, 2, 4, 8, 16, 32, 64, is produced by repeatedly cutting the previous size in half. That is exactly how a measuring tape, a ruler, or a set of calipers with fractional markings is physically laid out, and exactly how a woodworker with dividers or a folding rule would have marked a board without doing any arithmetic at all.

Decimal inches and millimeters are a completely different counting system, based on tens instead of twos. That is why a fraction like 1/3 inch has no exact home on a standard tape measure. It does not divide evenly into any power of two, so the chart, and the tape, skip straight past it.

Sage's rule

If a measurement will not simplify to a fraction with a power-of-two denominator, it is not going to land on a mark on a standard tape. Round to the nearest 64th, or switch to a metric or decimal tool instead.

Using the chart in the shop

Reading a tape measure against this chart

Count the small marks between whole inches on a standard tape. Sixteen marks means you are reading in 16ths, thirty-two means 32nds. Match what you see against the fraction column, then read across for the decimal or millimeter equivalent if that is what a tool, caliper, or spec sheet needs instead.

Worked example

11/16 in = 0.6875 in decimal0.6875 in × 25.4 = 17.4625 mm

Going the other direction, the Decimal to Fraction Calculator finds the nearest standard fraction for a decimal reading automatically, which is faster than working it out by hand at the bench.

I know this is off topic, but this reminds me of a joke I heard when teaching this to one of my interns, Why did the spider go to the computer? To check his website. Yeah, I don’t see the connection either, but I think it is cute!

Before you cut

Tips and common mistakes

Always simplify a fraction before comparing it to a chart or a print. 8/16 and 1/2 are the same length, but they will not visually match a chart row unless reduced first.
Do not round a decimal to a fraction and then round again to a coarser denominator. Rounding twice compounds the error.
When mixing fraction-marked tools with decimal calipers on the same project, convert once and record the decimal value, rather than converting back and forth repeatedly.

Do one reality check

A stack of small rounding errors across many parts can add up to a visible gap or overlap at final assembly. Round only as much as the actual tolerance of the job allows.

Where the habit came from

A little history

Dividing a length in half, then in half again, is one of the oldest layout tricks available, since it needs nothing more than a straightedge or a folding rule and no arithmetic at all. Early builders, surveyors, and instrument makers relied on this halving method long before decimal notation was in common use for everyday measurement.

Decimal and metric measurement came later, built on a base-ten counting system that is easier to add and multiply by hand or by machine. Workshops kept the older fractional habit for hand tools and tapes because it matches how a ruler is physically marked, even after decimal and metric systems became standard for engineering drawings and precision instruments.

Sage dressed as an ancient Egyptian scribe

That measures up exactly

Class is over. It is officially cheese fries time.

You now know why tape measures stop at 64ths, and how to move between fractions, decimals, and millimeters without stacking up rounding errors. That earns cheese fries, Sage's favorite way to celebrate a finished lesson.

Quick answers

Fraction and decimal questions

Why do fractions on a tape measure only use 2, 4, 8, 16, 32, and 64?

Each of those denominators comes from repeatedly folding or halving the previous size, which is how tape measures and rulers are physically marked without needing decimal math.

How do I convert a fraction to millimeters?

Convert the fraction to decimal inches first, then multiply by 25.4. For example, 3/8 in equals 0.375 in, and 0.375 × 25.4 is 9.525 mm.

Why won't a decimal like 0.3 inch land exactly on a tape measure mark?

0.3 inch does not simplify to a fraction with a power-of-two denominator, so it falls between two marks on a standard tape. The chart rounds it to the closest available fraction instead.

Is it more accurate to work in decimal inches or fractions?

Neither is inherently more accurate; the tool you are reading determines which is more practical. Digital calipers read decimal directly, while a standard tape or folding rule reads in fractions.