Engineering · Reference
Countersink & counterbore chart.
Two recesses, two jobs. A countersink is a cone that lets a flat-head screw sit flush; a counterbore is a flat-bottom pocket that hides a socket-head cap screw below the surface. This chart gives countersink major diameters at 82° (inch), 90° (metric), and 100° (aerospace flush-head), plus counterbore diameter and depth for SHCS — all cross-referenced to ASME B18.3 and ISO 4762.
01 · The two recesses
Cone for flat heads, pocket for socket heads.
A countersink is an angled conical recess that matches the underside of a flat-head (countersunk) screw so the head finishes flush with — or just below — the surface. The number that matters is the included angle: it must match the screw head exactly, and the cone is cut to a major diameter at the surface large enough to swallow the whole head.
A counterbore is a flat-bottomed cylindrical pocket — essentially a larger-diameter coaxial hole — that recesses the cylindrical head of a socket-head cap screw (SHCS) below the surface, with clearance around the head and a flat seat for it to bear on. The counterbore diameter must clear the head with running clearance; the depth is set so the screw sits flush or sub-flush.
Both ride on top of a through clearance hole sized to the fastener — see the clearance hole chart for the pilot diameter, and the tap drill calculator if the hole is threaded instead.
Countersink major diameter — 82° (machine-screw & socket FHCS) & 100° aerospace
82° is the standard inch flat-head angle, but two different fastener families share it with the same nominal sizes — and they do not share head diameters. Flat-head machine screws (ANSI B18.6.3) have the smaller head; socket flat countersunk head cap screws (FHCS, ASME B18.3) carry a noticeably larger head, so they need a wider countersink. Both 82° columns below are listed so you can pick the one matching your actual screw. The 100° column is the aerospace flush-head family (NAS/MS, e.g. MS24693) — different fasteners again, typically with wider, thinner heads; its values here are nominal planning references, so verify against the MS/NAS standard on your drawing. For any flush head, the countersink major diameter at the surface equals that screw's head diameter — the angle sets the cone depth, not the surface diameter. Cut so the listed major diameter falls at the part surface and the head seats flush.
| Screw size | Mach-screw head Ø (max, in) | 82° C'sink Ø — mach-screw (B18.6.3) | 82° C'sink Ø — socket flat-head (FHCS, B18.3) | 100° C'sink Ø (in) |
|---|---|---|---|---|
| #4 | 0.225 | 0.225 | 0.255 | 0.243 |
| #6 | 0.279 | 0.279 | 0.307 | 0.301 |
| #8 | 0.332 | 0.332 | 0.359 | 0.347 |
| #10 | 0.385 | 0.385 | 0.411 | 0.402 |
| 1/4" | 0.507 | 0.507 | 0.531 | 0.514 |
| 5/16" | 0.635 | 0.635 | 0.656 | 0.611 |
| 3/8" | 0.762 | 0.762 | 0.781 | 0.735 |
| 1/2" | 0.875 | 0.875 | 0.938 | 0.963 |
Two 82° conventions, side by side. Machine-screw (ANSI B18.6.3): the "Mach-screw head Ø" column is the flat-head machine-screw head at its theoretical-sharp max diameter (e.g. #10 ≈ 0.385″, 1/4″ ≈ 0.507″, 1/2″ ≈ 0.875″, per Machinery's Handbook), and its 82° countersink major Ø is sized to bring that head flush. Vendor tables that list the rounded-edge actual max (e.g. #10 ≈ 0.362″) read smaller — countersinks are cut to the sharp diameter so the head seats fully flush. Socket flat-head (FHCS, ASME B18.3): the second 82° column uses the larger theoretical-sharp head diameter (max) of the hex/spline socket flat countersunk head cap screw — e.g. 1/4″ ≈ 0.531″, 3/8″ ≈ 0.781″, 1/2″ ≈ 0.938″ — so it produces a wider countersink. Confirm which family your screw is before cutting; a machine-screw-sized countersink will leave an FHCS head proud. FHCS values per ASME B18.3 (AFT Fasteners / TorqBolt dimension tables).
Metric countersink major diameter — 90°
Metric flat-head / countersunk socket screws (ISO 10642) use a 90° included angle. Don't substitute an 82° tool on a metric head or vice-versa — a mismatched angle either leaves the head proud or bears on the rim and crushes the surface.
| Screw size | Head Ø (mm) | 90° C'sink Ø (mm) |
|---|---|---|
| M3 | 6.72 | 6.94 |
| M4 | 8.96 | 9.18 |
| M5 | 11.20 | 11.47 |
| M6 | 13.44 | 13.71 |
| M8 | 17.92 | 18.25 |
| M10 | 22.40 | 22.73 |
SHCS counterbore diameter & depth — ASME B18.3
Counterbore Ø clears the cap-screw head with running clearance; counterbore depth equals the max head height for a flush top. Go deeper than the listed depth to recess the head below the surface (sub-flush). These are the customary Machinery's Handbook counterbore sizes paired with ASME B18.3 head dimensions.
| Screw size | Head Ø (max, in) | Head height (max, in) | C'bore Ø (in) | C'bore depth (in) |
|---|---|---|---|---|
| #4 | 0.183 | 0.112 | 7/32 (0.219) | 0.112 |
| #6 | 0.226 | 0.138 | 9/32 (0.281) | 0.138 |
| #8 | 0.270 | 0.164 | 5/16 (0.313) | 0.164 |
| #10 | 0.312 | 0.190 | 3/8 (0.375) | 0.190 |
| 1/4" | 0.375 | 0.250 | 7/16 (0.438) | 0.250 |
| 5/16" | 0.469 | 0.312 | 17/32 (0.531) | 0.312 |
| 3/8" | 0.562 | 0.375 | 5/8 (0.625) | 0.375 |
| 1/2" | 0.750 | 0.500 | 13/16 (0.813) | 0.500 |
Metric SHCS counterbore diameter & depth — ISO 4762
Metric socket-head cap screws (ISO 4762 / DIN 912). Counterbore depth equals the max head height for flush; counterbore diameter gives running clearance around the head.
| Screw size | Head Ø (max) | Head height (max) | C'bore Ø | C'bore depth |
|---|---|---|---|---|
| M3 | 5.5 mm | 3.0 mm | 6.5 mm | 3.0 mm |
| M4 | 7.0 mm | 4.0 mm | 8.25 mm | 4.0 mm |
| M5 | 8.5 mm | 5.0 mm | 9.75 mm | 5.0 mm |
| M6 | 10.0 mm | 6.0 mm | 11.25 mm | 6.0 mm |
| M8 | 13.0 mm | 8.0 mm | 14.25 mm | 8.0 mm |
| M10 | 16.0 mm | 10.0 mm | 17.25 mm | 10.0 mm |
02 · How to read it
82 vs 90 vs 100, and flush vs sub-flush.
- 82° — inch standard: the default for inch flat-head machine screws and ANSI/ASME flat-head cap screws. Match the countersink tool angle to the screw or the head won't seat correctly.
- 90° — metric: metric countersunk (flat-head) screws per ISO 10642 use a 90° included angle. A 90° head in an 82° hole sits proud; an 82° head in a 90° hole bears only on its rim.
- 100° — aerospace flush-head: NAS/MS flush-head fasteners (e.g. MS24693) use a 100° angle and a flatter, wider head. The shallower cone spreads bearing load over more skin and gives a more reliable flush fit on thin aerospace sheet — but the wider head needs a larger surface diameter, so confirm you have material to swallow it. Size the countersink to that fastener's own head diameter, not the 82° value.
- Flush vs sub-flush: cut the countersink so the head face lands level with the surface (flush), or slightly deeper so the head is recessed (sub-flush) where nothing can snag it. Aerospace drawings often call a flushness tolerance (e.g. +0.000 / −0.005").
- Why SHCS counterbores carry clearance: a socket-head cap screw bears on the flat under its head, not its sides, so the counterbore is cut larger than the head — running clearance lets the head drop in freely, lets the socket key access the screw, and keeps the bore wall from interfering with the fillet under the head.
- Tool reach & spotface: a counterbore needs a pilot to stay concentric with the clearance hole; on cast or uneven surfaces a shallow version of the same cut (a spotface) just flattens a bearing seat for the head.
Size the rest of the hole.
Close / normal / loose through-hole diameters that sit under every countersink and counterbore.
Pre-tap drill sizes for UNC, UNF, and metric threads at ~75% engagement.
Major/minor diameters, pitch, and thread data for the fasteners you're recessing.
Number, letter, fractional, and metric drills cross-referenced by decimal inch.
Tightening torque for the cap screws once they're seated in the counterbore.
Callout symbols and modifiers for dimensioning recessed and located holes.
Keep exploring
Related tools & references
Through-hole sizes that pair with these head recesses.
Decimal equivalents for the pilot and counterbore drills.
Pitches and tap drills for the mating tapped holes.
Drill size for any thread at a target engagement.
Torque for the socket- and flat-head screws seated in these recesses.
Technically reviewed by the True Precision Machining engineering team · Last reviewed June 2026
Standards referenced: ASME B18.3, ASME B18.6.3, ISO 4762, ISO 10642, DIN 912, DIN 7991
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