Sources and scope for this page: ASME B1.1-2019, covering the nominal diameters, the UNC and UNF thread series and the pitch values; and ASME B18.6.3-2013 (R2018), covering the machine screw product dimensions (one more source is tabulated below). No independent source check is recorded for this page: the content rests on the sources listed and on the working shown on the page. Full sources, values and limits on this page.
Page last changed 2026-10-19 · based on ASME B1.1-2019
Quick Answer
Machine Screw Sizes — Diameter, Pitch and Tap Drill
| Size[1] | Diameter in[2] | Diameter mm[2] | UNC TPI[1] | UNF TPI[1] | Tap drill — UNC[3] | Tap drill — UNF[3] | Nearest metric[1] |
|---|---|---|---|---|---|---|---|
| #0 # | 0.0600 | 1.5240 | — | 80 | 3/64 | 3/64 | M1.6 |
| #1 # | 0.0730 | 1.8542 | 64 | 72 | 1/16 | 1/16 | M1.8 |
| #2 # | 0.0860 | 2.1844 | 56 | 64 | 5/64 | 5/64 | M2 |
| #3 # | 0.0990 | 2.5146 | 48 | 56 | 5/64 | 5/64 | M2.5 |
| #4 # | 0.1120 | 2.8448 | 40 | 48 | 3/32 | 3/32 | M3 |
| #5 # | 0.1250 | 3.1750 | 40 | 44 | #39 | #38 | M3 |
| #6 # | 0.1380 | 3.5052 | 32 | 40 | #36 | #33 | M3.5 |
| #8 # | 0.1640 | 4.1656 | 32 | 36 | #29 | #29 | M4 |
| #10 # | 0.1900 | 4.8260 | 24 | 32 | #25 | #21 | M5 |
| #12 # | 0.2160 | 5.4864 | 24 | 28 | #17 | #15 | M5.5 |
| 1/4 # | 0.2500 | 6.3500 | 20 | 28 | #7 | #3 | M6 |
| 5/16 # | 0.3125 | 7.9375 | 18 | 24 | F | I | M8 |
| 3/8 # | 0.3750 | 9.5250 | 16 | 24 | 5/16 | Q | M10 |
| 7/16 # | 0.4375 | 11.1125 | 14 | 20 | U | 25/64 | M11 |
| 1/2 # | 0.5000 | 12.7000 | 13 | 20 | 27/64 | 29/64 | M12 |
Diameters are basic (nominal) sizes — a finished screw is slightly smaller because the thread is rolled or cut into a blank at or under nominal. The tap drill columns match the full tap drill chart on this site; they are sized for approximately 75% thread engagement in steel. The "nearest metric" column is a substitution guide only: no metric size matches an inch size exactly, and the thread angles and flank forms differ, so metric and inch fasteners are never interchangeable even when the diameters are close.
Where the Numbers Come From
Numbered screw sizes are a nineteenth-century wire-gauge convention that hardened into a standard. The rule is simple and worth memorising: diameter in inches = 0.060 + 0.013 × the screw number. A #0 screw is 0.060 in, each number adds 0.013 in, and #12 comes out at 0.216 in — the point at which the series stops and fractional sizes take over.
Two consequences follow. The numbers are not a size in any unit, so a #10 screw is 0.190 in rather than 10/anything. And the series skips #7, #9 and #11 in normal trade — they exist in the standard, but the odd numbers between #6 and #12 are rarely manufactured, which is why an assortment box jumps from #6 to #8 to #10.
Above #12 the series becomes fractional: 1/4, 5/16, 3/8, 7/16, 1/2 and upward. The diameters are then exact fractions of an inch, which is why the decimal column becomes a clean 0.2500, 0.3125, 0.3750 rather than the irregular values in the numbered series.
The awkward gap is between #12 at 0.216 in and 1/4 at 0.250 in: there is no standard size covering that 0.034 in band, which is why a part designed around a #12 screw cannot always be upsized to 1/4 without redrilling.
UNC and UNF — Which to Specify
Almost every small screw size exists in two thread series. UNC (Unified National Coarse) is the default; UNF (Unified National Fine) has more threads per inch at the same diameter. For #10 that is 24 TPI against 32, and for 1/4 in it is 20 against 28.
Fine threads are stronger in tension because the shallower thread leaves a larger minor diameter — the core of the screw is thicker. They also resist vibration loosening better and allow finer adjustment. Against that they cross-thread more easily, are more sensitive to plating thickness, and are harder to tap in deep holes because the chip is thinner and packs more readily.
The practical rule is the same as for metric threads: use coarse as the default, and reach for fine where the joint vibrates, where wall thickness is limited, or where the screw is adjusted frequently. In North American practice UNF is also common in aerospace and in instrumentation, where 1/4-28 and #10-32 appear far more often than their coarse equivalents.
Frequently Asked Questions
- What diameter is a #10 screw?
- 0.190 in, or 4.826 mm. The number is a gauge, not a measurement: numbered screw diameters follow the formula 0.060 + 0.013 × n inches, so #10 is 0.060 + 0.130 = 0.190 in. A #8 is 0.164 in and a #12 is 0.216 in. Above #12 the sizes become fractions of an inch — 1/4 in is the next standard size up.
- What is the difference between a machine screw and a wood screw?
- A machine screw has a uniform cylindrical thread and is used with a nut or into a tapped hole; a wood screw has a tapered shank, a coarser thread and a sharp point, and forms its own thread in the wood. They are specified differently — machine screws by diameter and pitch (#10-32, 1/4-20), wood screws by gauge number and length — and their diameters do not correspond, so a #10 wood screw is not 0.190 in.
- Why is there no #7 or #9 screw?
- They exist in ASME B1.1 and can be made, but they are not stocked in normal trade. The numbered series runs continuously from #0 to #12 in the standard, and the odd numbers between #6 and #12 are effectively obsolete — an assortment will jump #6, #8, #10, #12. If a drawing calls for a #7 screw it is a special order, and substituting #6 or #8 is a 0.013 in diameter change.
- What size hole do I drill for a #10-32 screw?
- A #21 drill, which is 0.159 in, for approximately 75% thread engagement in steel. For #10-24 UNC the correct drill is #25 at 0.1495 in. The two differ because the coarser thread needs a larger pilot to reach the same engagement. In softer materials you can use one size smaller for stronger threads; in harder materials one size larger to avoid breaking the tap.
- Is a #10 screw the same as an M5?
- No. A #10 screw is 0.190 in or 4.826 mm in diameter, and M5 is 5 mm — the metric screw is 3.6% larger. M5 is the nearest metric size but not an equivalent: the thread angles differ (60° for both, but the flank forms and tolerances differ), so an M5 screw will not fit a #10-32 tapped hole. The closest true match to M5 in the inch series is #10, and the mismatch is why metric and inch fastener systems are kept separate.
Related
Value Sources
References and calculation methods used for this page are listed below. Numbered markers connect table columns where present. A reference listing does not itself establish that every value has been independently checked.
| # | Source | Type | Revision / method |
|---|---|---|---|
| [1] | ASME B1.1 — Unified Inch Screw Threads (UN and UNR Thread Form) | reference | ASME B1.1-2019 |
| [2] | Value computed from the standard's defining relationship | derived | Computed at build time from the defining formula stated on the page and verified against every row, then checked against known standard values held as anchors. Nothing in this column was transcribed. |
| [3] | Tap drill selection sized for approximately 75% thread engagement in steel | reference | n/a — shop sizing convention, not a specification; matches the tap drill chart published on this site |
Data Sources
| Standard | Revision | What it covers on this page |
|---|---|---|
| ASME B1.1 — Unified Inch Screw Threads (UN and UNR Thread Form) | ASME B1.1-2019 | the nominal diameters, the UNC and UNF thread series and the pitch values |
| ASME B18.6.3 — Machine Screws, Tapping Screws and Metallic Drive Screws | ASME B18.6.3-2013 (R2018) | the machine screw product dimensions |
| ASME B1.2 — Gages and Gaging for Unified Inch Screw Threads | ASME B1.2-1983 (R2020) | the thread classes the diameters are measured against |
Verification scope
No page-specific independent source review is recorded here. The listed references and formulas explain the basis of the compilation; automated consistency checks do not verify the original sources or establish product conformity.
Derived values — the following values on this page are calculated, not taken directly from the standard:
| Value | How it is derived |
|---|---|
| Diameter in inches and millimetres | Numbered sizes from the ASME B1.1 series formula d = 0.060 + 0.013n; fractional sizes from the fraction itself. Millimetres = inches × 25.4. Anchored by known-value checks at #0, #6, #8, #10 and 1/4 in. |
Diameters are basic sizes; a manufactured screw is at or slightly below nominal, and the applicable tolerance class depends on the product standard. Tap drills are sized for roughly 75% engagement in steel and should be adjusted for the material being tapped. The nearest-metric column is a substitution guide, not an equivalence — metric and inch threads are not interchangeable.
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