Sheet Metal Screw Size Chart — Diameter, TPI & Drill Sizes by Gauge
Sheet metal screw dimensions per ASME B18.6.4: major diameter, threads per inch by thread type, and clearance drill sizes by screw gauge.
Updated August 22, 2026
Sheet metal screws use the same gauge numbers as machine and wood screws — the familiar 0.060 + 0.013 × gauge ladder gives the nominal basis (#8 ≈ 0.164 in) — but the tapping-screw standard, ASME B18.6.4, sets its own thread envelope: the rolled spaced thread’s actual major diameter tops out slightly off the nominal ladder, a couple of thousandths over at the small gauges and a thousandth or more under from #10 up. Both values appear below: the computed gauge basis, and the transcribed B18.6.4 Type AB/B maximum the screw actually measures. The threads-per-inch column is the modern spaced-thread pitch shared by Type AB (gimlet point) and Type B (blunt point); the obsolescent Type A used coarser pitches and survives mostly in old drawings.
The clearance-drill column is selected by code, not by hand: it is the smallest standard number, letter, or fractional drill at least as large as the screw’s maximum major diameter — the hole for the sheet the screw must pass through freely while it bites the sheet behind. Deliberately absent: root/minor diameters (published reproductions disagree beyond tolerance — the same trap that killed the wood-screw pilot chart) and pilot-hole-in-steel recommendations, which honest catalogs publish per sheet thickness rather than per screw; the FAQ explains how to handle both.
Tapping Screw Threads — ASME B18.6.4 Types AB & B
| Gauge | Gauge basis, 0.060 + 0.013·g (in, computed) | Thread major Ø, max (in) — B18.6.4 | Major Ø (mm, computed) | Threads per inch (AB / B) | Clearance drill (code-selected) |
|---|---|---|---|---|---|
| #4 | 0.112 | 0.114 | 2.90 | 24 | #32 (0.1160″) |
| #6 | 0.138 | 0.139 | 3.53 | 20 | #28 (0.1405″) |
| #8 | 0.164 | 0.166 | 4.22 | 18 | #19 (0.1660″) |
| #10 | 0.190 | 0.189 | 4.80 | 16 | #12 (0.1890″) |
| #12 | 0.216 | 0.215 | 5.46 | 14 | 7/32″ (0.2188″) |
Major diameters are the B18.6.4 thread maxima for Types AB and B (which share pitches at these sizes); the rolled thread’s max deviates from the nominal gauge ladder by up to ±0.004 in. Type A (obsolescent) uses coarser pitches and is not tabulated. Clearance drill = smallest standard drill at or above the major max, selected from the ANSI B94.11M drill table in code.
Sources & Further Reading
- ASME B18.6.4 (Thread Forming and Thread Cutting Tapping Screws) as reproduced in the Electronic Fasteners Inc. “Thread Dimensions — Tapping Screws, American Standard” technical pages (Types A, B, AB, F, U — full dimensional tables)
- Optimas Solutions self-tapping screw thread dimension tables (independent digit-verification of the shipped major maxima and TPI, #4–#12); rows and columns the two reproductions do not both carry are dropped
- ANSI/ASME B94.11M standard drill diameters via the tap drill calculator’s INCH_DRILLS export (clearance-drill selection); gauge basis 0.060 + 0.013·g via the nail-screw chart’s screwMajorIn export (both computed, never hand-picked)
Frequently Asked Questions
What size hole do I drill for a #8 sheet metal screw?
Two different holes. The sheet the screw grips needs a pilot smaller than the thread — sized to the sheet thickness and material, which is why this chart does not print a single pilot number (a #8 in 26-ga steel wants a much smaller hole than in 16-ga; use the screw maker’s per-thickness chart). The sheet the screw passes through needs the clearance drill in the table: #19 (0.166″) for a #8, the smallest standard drill at or above the thread’s maximum diameter.
What is the difference between Type A, Type AB, and Type B screws?
All are spaced-thread tapping screws. Type A is the original gimlet-point “sheet metal screw” with a coarse pitch — B18.6.4 calls it obsolescent and it survives mainly on legacy drawings. Type B has a finer pitch ladder (the TPI column here) and a blunt point; Type AB combines Type B’s thread with Type A’s gimlet point, so it starts like an A and holds like a B. AB and B interchange in the same hole; substituting either for an old Type A generally works because the majors match.
Is a #10 sheet metal screw the same diameter as a #10 machine screw?
Nominally yes — both hang off the 0.060 + 0.013 × gauge ladder, so #10 means 0.190 in. But the threads differ completely: the machine screw carries a 24- or 32-TPI 60° thread to fit a tapped hole, while the tapping screw carries a 16-TPI spaced thread whose rolled major actually maxes at 0.189 in per B18.6.4. They are not interchangeable — one mates with a nut, the other forms its own thread in sheet stock.
Why does this chart skip root diameters and pilot holes in steel?
Because the published numbers do not survive cross-checking. The two B18.6.4 reproductions used here agree digit-for-digit on major maxima and TPI, but their minor-diameter columns disagree by up to 0.003 in — the same failure that got the wood-screw pilot chart cut in an earlier wave. Pilot holes in sheet steel additionally depend on sheet thickness: a proper catalog prints a pilot matrix per gauge of sheet, so a single per-screw pilot column would be false precision. Use the fastener manufacturer’s matrix for the actual sheet.
Try the Calculators
Sheet Metal Screw Size Chart — Diameter, TPI & Drill Sizes by Gauge — reuven.tools/reference/sheet-metal-screw-sizes — verified against: ASME B18.6.4 (Thread Forming and Thread Cutting Tapping Screws) as reproduced in the Electronic Fasteners Inc. “Thread Dimensions — Tapping Screws, American Standard” technical pages (Types A, B, AB, F, U — full dimensional tables); Optimas Solutions self-tapping screw thread dimension tables (independent digit-verification of the shipped major maxima and TPI, #4–#12); rows and columns the two reproductions do not both carry are dropped; ANSI/ASME B94.11M standard drill diameters via the tap drill calculator’s INCH_DRILLS export (clearance-drill selection); gauge basis 0.060 + 0.013·g via the nail-screw chart’s screwMajorIn export (both computed, never hand-picked)