Countersink vs Counterbore
Both a countersink and a counterbore recess a screw head so it doesn’t stick up above the surface — but they have different shapes and suit different screws. A countersink is a cone that matches a flat-head screw. A counterbore is a flat-bottomed cylindrical pocket that swallows a socket-head cap screw and its washer. Pick the wrong one and the head won’t seat. Here is the difference, when to use each, and the sizes to model.
The difference at a glance
Countersink (cone)
A conical recess, usually 82° (imperial) or 90° (metric). Made for flat-head screws, whose head is a matching cone. The head ends up flush with — or below — the surface.
Counterbore (flat pocket)
A flat-bottomed cylindrical pocket. Made for socket-head cap screws (and fillister/round heads), which have a flat underside. Hides the whole head below the surface.
Underneath both sits an ordinary clearance hole for the screw shaft — the diameters are in the metric clearance hole size chart. The countersink or counterbore is just the recess on top.
Counterbore size chart (socket-head cap screws, mm)
For a socket-head cap screw, the counterbore diameter is the head diameter plus a little clearance, and the depth is the head height (or a touch more to sink it below flush).
| Screw | Clearance hole (mm) | Counterbore Ø (mm) | Counterbore depth (mm) |
|---|---|---|---|
| M3 | 3.4 | 6.5 | 3.0 |
| M4 | 4.5 | 8.0 | 4.0 |
| M5 | 5.5 | 10.0 | 5.0 |
| M6 | 6.6 | 11.0 | 6.0 |
| M8 | 9.0 | 15.0 | 8.0 |
| M10 | 11.0 | 18.0 | 10.0 |
Sinking it below flush
To recess the head below the surface (so a cap or another part can sit on top), add the gap you want to the depth — e.g. an M4 head is ~4 mm tall, so a 5 mm-deep counterbore leaves the head ~1 mm under the surface.
Countersink size chart (flat-head screws, mm)
For a flat-head screw, the countersink is a 90° cone (metric standard). Model the top diameter of the cone roughly equal to the screw head diameter so the head sits flush.
| Screw | Clearance hole (mm) | Countersink Ø (mm) | Angle |
|---|---|---|---|
| M3 | 3.4 | 6.3 | 90° |
| M4 | 4.5 | 8.4 | 90° |
| M5 | 5.5 | 10.4 | 90° |
| M6 | 6.6 | 12.6 | 90° |
| M8 | 9.0 | 17.3 | 90° |
Imperial flat-head screws use an 82° cone instead — match the angle to your screw or the head won’t seat flush.
Which should you use?
- Need a flush surface and you have flat-head screws — countersink. Common on panels, covers, anywhere a hand or another part slides across.
- Using socket-head cap screws and want a strong, clean recess — counterbore. The default for machined parts and structural joints.
- 3D printing — both work, but a counterbore prints more reliably (a shallow cone can look ragged on the first layers). Add the same printed-hole clearance to the recess diameter.
- Just need the head out of the way, no flush requirement — a plain clearance hole and a low-profile button-head screw is simplest.
Add a recessed hole to your part
Describe it to PartWork.ai — “add an M4 counterbored hole for a socket-head cap screw” or “countersink for an M5 flat-head” — and it builds the recess and clearance hole together. Check the head seats correctly in the 3D viewer, then export STEP for machining or STL/3MF for printing. See modifying parts to adjust an existing hole.
Free to upload & view — sign up to download
Open the studio, describe your part and its screw recesses, and export a file ready to make. More credits: 100 for $4.99 (~5¢ each).