Countersink Dimensions for Flat Head Screws
Countersink diameters for 82 degree inch flat head machine screws (#2 to 3/8) and 90 degree metric socket countersunk screws to ISO 10642 (M3 to M20), with the tool plunge depth and the clearance hole. Jump to inch or metric.
Need panels, covers or machined parts with countersunk holes? .
Inch 82 degree flat head machine screws
| Screw | Head dia (max, sharp) | Countersink dia, flush | Tool depth, sharp 82° | Clearance hole, close / free |
|---|---|---|---|---|
| #2 | 0.1724.37 mm | 0.1724.37 mm | 0.0992.51 mm | #43 (0.0890)#41 (0.0960) |
| #3 | 0.1995.05 mm | 0.1995.05 mm | 0.1142.91 mm | #37 (0.1040)#35 (0.1100) |
| #4 | 0.2255.72 mm | 0.2255.72 mm | 0.1293.29 mm | #32 (0.1160)#30 (0.1285) |
| #5 | 0.2526.40 mm | 0.2526.40 mm | 0.1453.68 mm | #30 (0.1285)#29 (0.1360) |
| #6 | 0.2797.09 mm | 0.2797.09 mm | 0.1604.08 mm | #27 (0.1440)#25 (0.1495) |
| #8 | 0.3328.43 mm | 0.3328.43 mm | 0.1914.85 mm | #18 (0.1695)#16 (0.1770) |
| #10 | 0.3859.78 mm | 0.3859.78 mm | 0.2215.62 mm | #9 (0.1960)#7 (0.2010) |
| #12 | 0.43811.13 mm | 0.43811.13 mm | 0.2526.40 mm | #2 (0.2210)#1 (0.2280) |
| 1/4 | 0.50712.88 mm | 0.50712.88 mm | 0.2927.41 mm | F (0.2570)H (0.2660) |
| 5/16 | 0.63516.13 mm | 0.63516.13 mm | 0.3659.28 mm | P (0.3230)Q (0.3320) |
| 3/8 | 0.76219.35 mm | 0.76219.35 mm | 0.43811.13 mm | W (0.3860)X (0.3970) |
Head diameters are ASME B18.6.3 maximum theoretical sharp values for 82 degree flat head machine screws. Socket flat head cap screws (ASME B18.3) have larger heads; size those from the screw maker's data. Sizes over 3/8 are not listed.
Metric 90 degree socket countersunk screws (ISO 10642)
| Screw | Head dia dk (max, theoretical) | Countersink dia, flush | Tool depth, sharp 90° | Clearance hole, fine / medium |
|---|---|---|---|---|
| M3 | 6.720.265 in | 6.720.265 in | 3.360.132 in | 3.2 / 3.4 |
| M4 | 8.960.353 in | 8.960.353 in | 4.480.176 in | 4.3 / 4.5 |
| M5 | 11.200.441 in | 11.200.441 in | 5.600.220 in | 5.3 / 5.5 |
| M6 | 13.440.529 in | 13.440.529 in | 6.720.265 in | 6.4 / 6.6 |
| M8 | 17.920.706 in | 17.920.706 in | 8.960.353 in | 8.4 / 9.0 |
| M10 | 22.400.882 in | 22.400.882 in | 11.200.441 in | 10.5 / 11.0 |
| M12 | 26.881.058 in | 26.881.058 in | 13.440.529 in | 13.0 / 13.5 |
| M16 | 33.601.323 in | 33.601.323 in | 16.800.661 in | 17.0 / 17.5 |
| M20 | 40.321.587 in | 40.321.587 in | 20.160.794 in | 21.0 / 22.0 |
ISO 10642 maximum theoretical head diameters. Clearance holes are ISO 273, from the clearance hole chart.
How the countersink diameter is set
Flat head screws are specified by a theoretical sharp head diameter: where the head's cone would meet its top face if the edge were not flatted or rounded. Real heads are smaller. Cutting the countersink to that sharp diameter at the surface puts the largest legal head exactly flush and any smaller head slightly below. To guarantee every head sits below the surface, add a few thousandths of an inch (or about 0.1 to 0.3 mm) to the diameter.
The tool depth column is the plunge of a sharp-pointed countersink from its tip: depth = D / (2 tan(angle / 2)), which is 0.5752 x D at 82 degrees and 0.5 x D at 90 degrees. In thin sheet the cone can reach through the material before the diameter is reached, leaving a knife edge that tears out. Keep a straight land under the countersink, or dimple or use a thicker sheet.
Frequently asked questions
- What countersink angle do flat head screws use?
- Inch flat head screws use 82 degrees. Metric countersunk screws (ISO 10642, ISO 7046 and similar) use 90 degrees. Aerospace screws and rivets (NAS and MS parts) mostly use 100 degrees. Match the countersink to the screw: a mismatched cone leaves the head bearing on a thin ring.
- What countersink diameter for a #10 flat head screw?
- 0.385 in at the surface puts a maximum-size #10 82 degree machine screw head flush. Drawings often call 0.390 to 0.400 in so every head sits just below the surface. A sharp 82 degree countersink reaches 0.385 in about 0.221 in below the tool tip.
- How deep do I plunge a countersink tool?
- For a sharp-pointed tool, depth from the tip = diameter / (2 x tan(half the included angle)). That is 0.5 x D for 90 degrees and 0.5752 x D for 82 degrees. Subtract the flat or the pilot if the tool is not sharp, or touch off on the edge of the hole and plunge by the difference between the hole and the countersink diameters.
- Why do my metric countersunk screws sit proud?
- Check the head standard. ISO 10642 socket countersunk heads are larger than the older DIN 7991 heads of the same thread size, so a countersink cut for DIN 7991 leaves an ISO 10642 head standing up. Also check that the countersink is 90 degrees, not 82.
