Fractional Inches to Millimeters — Chart, Mixed Numbers and Tool Sizes

The one rule: multiply by 25.4

Since 1959 the international inch has been defined as exactly 25.4 millimeters, so every inch-to-millimeter conversion is a single multiplication. The only extra step with a fraction is turning it into a decimal first: divide the top number by the bottom number, then multiply by 25.4.

Example: 5/32 inch. 5 ÷ 32 = 0.15625, and 0.15625 × 25.4 = 3.96875 mm, which most people round to 3.97 mm. Example: 3/8 inch. 3 ÷ 8 = 0.375, and 0.375 × 25.4 = 9.525 mm.

Going the other way, divide millimeters by 25.4 to get decimal inches, then find the nearest sixty-fourth: multiply the decimal by 64 and round. 10 mm ÷ 25.4 = 0.3937 in; 0.3937 × 64 = 25.2, so the closest fraction is 25/64 inch.

Fraction to millimeter chart (1/64 to 1 inch)

1/64 in = 0.397 mm · 1/32 in = 0.794 mm · 3/64 in = 1.191 mm · 1/16 in = 1.588 mm · 5/64 in = 1.984 mm · 3/32 in = 2.381 mm · 7/64 in = 2.778 mm · 1/8 in = 3.175 mm.

5/32 in = 3.969 mm · 3/16 in = 4.763 mm · 7/32 in = 5.556 mm · 1/4 in = 6.35 mm · 9/32 in = 7.144 mm · 5/16 in = 7.938 mm · 11/32 in = 8.731 mm · 3/8 in = 9.525 mm.

13/32 in = 10.319 mm · 7/16 in = 11.113 mm · 15/32 in = 11.906 mm · 1/2 in = 12.7 mm · 17/32 in = 13.494 mm · 9/16 in = 14.288 mm · 19/32 in = 15.081 mm · 5/8 in = 15.875 mm.

21/32 in = 16.669 mm · 11/16 in = 17.463 mm · 23/32 in = 18.256 mm · 3/4 in = 19.05 mm · 25/32 in = 19.844 mm · 13/16 in = 20.638 mm · 27/32 in = 21.431 mm · 7/8 in = 22.225 mm · 29/32 in = 23.019 mm · 15/16 in = 23.813 mm · 31/32 in = 24.606 mm · 1 in = 25.4 mm.

Each value is the exact fraction × 25.4, rounded to three decimals. Because 25.4 has only one decimal place, any fraction with 2, 4, 8, 16, 32 or 64 on the bottom converts to a terminating decimal in millimeters — there is no hidden repeating error.

Mixed numbers: 1 1/4 inch, 2 3/8 inch and so on

Convert the whole inches and the fraction separately and add them. 1 1/4 in = 25.4 mm + 6.35 mm = 31.75 mm. 2 3/8 in = 50.8 mm + 9.525 mm = 60.325 mm. 3 1/2 in = 76.2 mm + 12.7 mm = 88.9 mm.

Searches often lose the space, so "11/4 inches" usually means 1 1/4 inches (31.75 mm), not eleven quarters (69.85 mm). If a result looks far too big, check whether a mixed number was typed as a single fraction.

Decimal inches work the same way: 1.25 in × 25.4 = 31.75 mm, 0.050 in × 25.4 = 1.27 mm, 0.125 in × 25.4 = 3.175 mm (that is 1/8 inch). Machinists measure in thousandths of an inch, and one thou is 0.0254 mm, so 0.001 in and 25 micrometers are almost the same thing.

Drill bits: the closest metric size

A fractional drill set usually runs from 1/16 to 1/2 inch in 1/64 steps. Metric sets run in 0.5 mm or 0.1 mm steps. When a plan calls for one system and your bits are the other, pick the nearest size that is not smaller than you need for a clearance hole, and not larger for a pilot hole.

Close matches worth remembering: 1/8 in (3.175 mm) sits between 3.0 and 3.2 mm; 3/16 in (4.763 mm) is closest to 4.8 mm; 1/4 in (6.35 mm) is closest to 6.3 or 6.4 mm; 5/16 in (7.938 mm) is almost exactly 8 mm; 3/8 in (9.525 mm) is closest to 9.5 mm; 1/2 in (12.7 mm) is closest to 12.7 or 13 mm.

For tapping threads, always use the tap chart for the thread you are cutting rather than a converted size — a tap drill is chosen for thread engagement, not for the nearest number.

Wrenches and sockets: which metric size fits an SAE bolt?

A wrench has to be equal to or slightly larger than the hex across its flats. A metric wrench smaller than the SAE size will not go on; one much larger will round the corners under load. Here is how close the usual pairs are, worked from the exact conversion.

5/16 in = 7.94 mm → an 8 mm wrench is 0.06 mm larger: a good fit. 7/16 in = 11.11 mm → 11 mm is 0.11 mm smaller and usually will not fit; use 7/16. 1/2 in = 12.7 mm → 13 mm is 0.3 mm larger: it fits but is loose, fine for light work only.

9/16 in = 14.29 mm → 14 mm is too small, 15 mm is 0.71 mm too big. 5/8 in = 15.88 mm → 16 mm is 0.12 mm larger: a good fit. 3/4 in = 19.05 mm → 19 mm is 0.05 mm smaller; on many heads it is tight but goes on, on others it will not. 7/8 in = 22.23 mm → 22 mm is too small.

The pattern: only a handful of SAE sizes have a true metric twin (5/16 ↔ 8 mm and 5/8 ↔ 16 mm are the clean ones). For anything torqued to a specification — wheel nuts, cylinder heads, suspension — use the correct system so the tool does not slip. Our torque converter turns lb·ft specs into N·m if your wrench reads metric.

Reading a tape measure in millimeters

Most US tapes mark sixteenths of an inch: each small tick is 1/16 in, which is 1.5875 mm. Two ticks are 1/8 in (3.175 mm), four ticks 1/4 in (6.35 mm), eight ticks 1/2 in (12.7 mm).

A practical estimate: one sixteenth is about 1.6 mm, so count the ticks past the last inch and multiply by 1.6. Three ticks past 4 inches is about 4 × 25.4 + 3 × 1.6 = 101.6 + 4.8 = 106.4 mm (the exact answer is 106.36 mm).

Rounding: how many decimals do you need?

Woodworking: round to the nearest 0.5 mm; wood moves more than that with humidity. Metalwork and 3D printing: keep two decimals (0.01 mm). Machining to drawings: keep three decimals or work in thousandths of an inch and convert only at the end.

Round once, at the end. Converting 1/64 in to 0.4 mm and then multiplying by 20 gives 8 mm, but 20/64 in is really 7.9375 mm — rounding early added a 0.06 mm error that rounding once at the end avoids.