How to Measure a Screw Size (Inch and Metric)
A screw size is three separate measurements: the major diameter (across the crests of the threads), the thread spacing (threads per inch, or pitch in millimetres) and the length. Measure the diameter with calipers, check the spacing with a pitch gauge or a ruler, measure the length the way the head style demands, and match the result to a standard size such as #8-32, 1/4-20 or M6 x 1. If you only want the answer, the screw size lookup does it in one step.
Key takeaways
- Size is diameter plus thread spacing plus length. Diameter alone cannot identify a screw.
- A good thread reads slightly under its nominal size on calipers. Round to the nearest standard size instead of expecting 0.1640 in or 8.00 mm.
- For numbered inch sizes, diameter = 0.060 + 0.013 × the number.
- Length starts under the head for most heads, but at the top of the head for flat (countersunk) screws.
What a screw size is made of
People say “an 8 screw” or “a quarter-inch screw” and mean different things. A complete callout carries all three numbers. In #8-32 x 1/2, the screw is size #8, has 32 threads per inch and is 1/2 in long. In M6 x 1 x 20, it is 6 mm across, has a 1.0 mm thread pitch and is 20 mm long. Two screws can share a diameter and still not fit the same hole: 1/4-20 and 1/4-28 are both 0.2500 in across.
Step 1: measure the major diameter
Close the caliper jaws across the thread crests, square to the screw’s axis, on a clean stretch of thread away from the tip and the head. Read it twice, turning the screw a quarter turn between readings, because a screw is rarely perfectly round.
Expect a reading slightly under nominal. An external thread is made to a tolerance band that sits at or below the nominal size. For a standard M8 bolt (class 6g, from the ISO 965-1 tolerance data) the major diameter may fall anywhere from 7.760 mm to 7.972 mm, so a perfectly good M8 will not read 8.00 mm. Inch threads behave the same way. Measure, then round to the nearest standard size rather than hunting for an exact match.
Check a numbered size with the formula
Inch screws below 1/4 in are sold by number, and the number follows a formula: D = 0.060 + 0.013 × n, where D is the nominal major diameter in inches. A #8 gives 0.060 + 0.013 × 8 = 0.164 in (4.166 mm). To work backwards, subtract 0.060 from your reading and divide by 0.013.
Say the calipers read 0.163 in: (0.163 − 0.060) ÷ 0.013 = 7.9, so it is a #8. A reading of 0.188 in gives 9.8, so a #10 (0.1900 in). If the result lands near a half number, measure again; real sizes come out close to whole numbers. The formula covers the numbered sizes only. From 1/4 in upward the size is simply the fraction.
Step 2: find the thread spacing
Diameter alone cannot name a screw. The spacing finishes the job.
Inch: threads per inch (TPI)
Count the crests along 1 in of thread, or count over a shorter length and scale up. A pitch gauge is faster: try leaves until one meshes with no light showing between the teeth. A 1/4 in screw with 20 crests per inch is 1/4-20 (UNC); with 28 it is 1/4-28 (UNF).
Metric: pitch in millimetres
Metric sizes quote the distance between neighbouring crests. Measure across ten crests and divide by ten; the reading error shrinks as the count grows. An 8 mm screw with a 1.25 mm pitch is M8 x 1.25, the coarse thread. At 1.0 mm it is the fine thread, M8 x 1.
Why this step is not optional. An M8 x 1.25 screw is 8.00 mm across with about 20.3 threads per inch. A 5/16-18 screw is 0.3125 in, or 7.94 mm, with 18. The diameters differ by 0.06 mm, far less than the tolerance band above. Only the spacing tells them apart, and they will not thread together.
Step 3: measure the length
Length is the part of the screw that goes into the work, so where you start depends on the head.
- Flat (countersunk) heads: from the top of the head to the tip, because the head sits flush.
- Pan, round, hex and socket heads: from under the head to the tip, because the head sits on the surface.
- Oval heads: like flat heads, but to the edge of the head, not to the top of the raised crown. ASME B18.6.3 defines the nominal length that way and treats the overall length including the crown as a separate, longer figure.
Hold the screw against a rule, or use the caliper’s depth rod, with the starting point at zero and read to the tip. Then round to the nearest standard length.
Worked examples
| Caliper reading | Thread spacing | Size | Check |
|---|---|---|---|
| 0.163 in | 32 TPI | #8-32 | 0.060 + 0.013 × 8 = 0.164 in |
| 0.163 in | 36 TPI | #8-36 (UNF) | same diameter, finer spacing |
| 0.188 in | 24 TPI | #10-24 | 0.060 + 0.013 × 10 = 0.190 in |
| 7.9 mm | 1.25 mm pitch | M8 x 1.25 | coarse pitch for M8 |
| 7.9 mm | 1.0 mm pitch | M8 x 1 | fine pitch for M8 |
Once you have the size, you need the practical numbers: tap drill, pilot hole and clearance hole. The match-my-screw tool in the drill-bit finder turns a measured diameter into the nearest standard sizes, and the screw size chart hub has the full inch, metric and wood tables.
The dimensions in this guide come from ASME B1.1 (unified inch threads), ASME B18.6.3 (machine screws) and ISO 261 with ISO 965-1 (metric threads). Sources and method explains how the site checks its numbers.
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