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Metric to Imperial Screw Converter
Short answer
Metric and imperial fasteners are near neighbours, not substitutes: M6 and 1/4 inch are 0.35 mm apart, M8 and 5/16 inch just 0.06 mm apart. Convert the diameter below to find the nearest size in each system, then confirm with the pitch, because that is the only difference a hand measurement can actually see.
Nearest fastener sizes
Enter a diameter in either field.
Millimetres to inches
Convert a screw diameter from millimetres to inches: the exact decimal and the nearest 1/64 fraction for every common size from 1 mm to 25 mm.
| Millimetres | Inches (decimal) | Nearest fraction | Nearest screw size |
|---|---|---|---|
| 1 mm | 0.0394 in | 3/64" | M1.6 |
| 1.5 mm | 0.0591 in | 1/16" | M1.6 |
| 2 mm | 0.0787 in | 5/64" | M2 |
| 2.5 mm | 0.0984 in | 3/32" | M2.5 |
| 3 mm | 0.1181 in | 1/8" | M3 |
| 3.5 mm | 0.1378 in | 9/64" | M3.5 |
| 4 mm | 0.1575 in | 5/32" | M4 |
| 4.5 mm | 0.1772 in | 11/64" | M4 |
| 5 mm | 0.1969 in | 13/64" | M5 |
| 5.5 mm | 0.2165 in | 7/32" | M5 |
| 6 mm | 0.2362 in | 15/64" | M6 |
| 6.5 mm | 0.2559 in | 1/4" | M6 |
| 7 mm | 0.2756 in | 9/32" | M6 |
| 7.5 mm | 0.2953 in | 19/64" | M8 |
| 8 mm | 0.3150 in | 5/16" | M8 |
| 8.5 mm | 0.3346 in | 21/64" | M8 |
| 9 mm | 0.3543 in | 23/64" | M8 |
| 9.5 mm | 0.3740 in | 3/8" | M10 |
| 10 mm | 0.3937 in | 25/64" | M10 |
| 11 mm | 0.4331 in | 7/16" | M10 |
| 12 mm | 0.4724 in | 15/32" | M12 |
| 13 mm | 0.5118 in | 33/64" | M12 |
| 14 mm | 0.5512 in | 35/64" | M14 |
| 15 mm | 0.5906 in | 19/32" | M14 |
| 16 mm | 0.6299 in | 5/8" | M16 |
| 18 mm | 0.7087 in | 45/64" | M16 |
| 20 mm | 0.7874 in | 25/32" | M20 |
| 22 mm | 0.8661 in | 55/64" | M20 |
| 24 mm | 0.9449 in | 15/16" | M20 |
| 25 mm | 0.9843 in | 63/64" | M20 |
Metric screw sizes in inches
Every metric screw size as a diameter in inches, with the closest imperial fastener — the table behind questions like "what is M6 in inches" or "is M8 the same as 5/16".
| Metric size | Diameter (mm) | Diameter (in) | Nearest imperial |
|---|---|---|---|
| M1.6 | 1.60 | 0.0630 in | #0 |
| M2 | 2.00 | 0.0787 in | #1 |
| M2.5 | 2.50 | 0.0984 in | #3 |
| M3 | 3.00 | 0.1181 in | #4 |
| M3.5 | 3.50 | 0.1378 in | #6 |
| M4 | 4.00 | 0.1575 in | #8 |
| M5 | 5.00 | 0.1969 in | #10 |
| M6 | 6.00 | 0.2362 in | 1/4" |
| M8 | 8.00 | 0.3150 in | 5/16" |
| M10 | 10.00 | 0.3937 in | 3/8" |
| M12 | 12.00 | 0.4724 in | 1/2" |
| M14 | 14.00 | 0.5512 in | 9/16" |
| M16 | 16.00 | 0.6299 in | 5/8" |
| M20 | 20.00 | 0.7874 in | 5/8" |
Metric and inch near neighbours
| Metric | Nearest inch | Gap (mm) | By diameter alone |
|---|---|---|---|
| M1.6 × 0.35 | #0 | 0.08 | Too close to call |
| M2 × 0.40 | #1-64 | 0.15 | Too close to call |
| M2.5 × 0.45 | #3-48 | 0.02 | Too close to call |
| M3 × 0.50 | #4-40 | 0.15 | Too close to call |
| M3.5 × 0.60 | #6-32 | 0.00 | Too close to call |
| M4 × 0.70 | #8-32 | 0.17 | Too close to call |
| M5 × 0.80 | #10-24 | 0.17 | Too close to call |
| M6 × 1.00 | 1/4"-20 | 0.35 | Distinguishable |
| M8 × 1.25 | 5/16"-18 | 0.06 | Too close to call |
| M10 × 1.50 | 3/8"-16 | 0.47 | Distinguishable |
| M12 × 1.75 | 1/2"-13 | 0.70 | Distinguishable |
| M14 × 2.00 | 9/16"-12 | 0.29 | Too close to call |
| M16 × 2.00 | 5/8"-11 | 0.13 | Too close to call |
| M20 × 2.50 | 5/8"-11 | 4.13 | Distinguishable |
Why the near misses are the dangerous part
There is no metric size that equals an inch size. What there is instead is a set of pairs close enough that a ruler, a caliper and often a parts drawer cannot separate them. M8 and 5/16 inch differ by 0.06 mm. M6 and 1/4 inch differ by 0.35 mm. M3.5 and #6 differ by 0.005 mm.
Because their pitches differ, a near-miss fastener threads in cleanly for a turn or two and then binds. That is the sequence that strips an aluminium casting, and it is why a cross-system match should always be confirmed on pitch rather than diameter.
What to do when both systems are plausible
Use the context. A machine built in North America before the 1980s, most plumbing fittings, and anything from an American power tool are likely inch. European and Asian appliances, bicycles, furniture fittings and almost all electronics are metric.
Then confirm with pitch. Inch pitches converted to millimetres rarely land on a round number: 1/4-20 is 1.27 mm, 5/16-18 is 1.41 mm. A measured pitch that is almost exactly 1.25 mm or 1.5 mm is telling you the fastener is metric.
Frequently asked questions
Is M6 the same as 1/4 inch?
No. M6 is 6.00 mm and 1/4 inch is 6.35 mm, a gap of 0.35 mm. A 1/4 inch bolt will not enter an M6 hole, and an M6 bolt in a 1/4-20 nut will feel loose and cross-thread.
What is #8 in metric?
A #8 machine screw is 0.164 inch, which is 4.17 mm — between M4 (4.0 mm) and M4.5. In practice #8 is replaced with M4 where the hole can be redrilled, never in an existing tapped thread.
How do I convert threads per inch to a metric pitch?
Divide 25.4 by the threads per inch. Twenty threads per inch is 25.4 / 20 = 1.27 mm. To go the other way, divide 25.4 by the pitch in millimetres.
Other tools
Would rather just take a photo?
These charts are the same tables the app matches against. Screw Identifier does the measuring for you: photograph the fastener next to a ruler, or measure it on the screen against a calibrated on-screen caliper.
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