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Countersink holes let fasteners sit flush with a surface. They are one of the most common types of holes in engineering, used in CNC machining, sheet metal fabrication, and injection molding across aerospace, automotive, electronics, and furniture manufacturing.
This guide covers countersink hole types, standard angles, ANSI and ISO size charts, drawing callouts, machining methods, and common design mistakes.
What Is a Countersink Hole?
A countersink hole is a cone-shaped recess machined into the opening of a drilled hole. The angled walls taper outward from the hole to form a conical seat on the surface of the workpiece.
This shape matches the underside of a flat-head screw or rivet. When the fastener is driven in, it sits flush with or slightly below the surface.
Three parameters define a countersink hole:
- Diameter measures at the surface and must match the fastener head size.
- Angle must match the fastener standard. 82° for ANSI imperial fasteners, 90° for ISO metric fasteners.
- Depth determines how far the head sits below the surface.
Countersink Hole Symbol and Drawing Callout
The countersink symbol is ⌵. On an engineering drawing, it tells the machine shop to cut a conical recess at the hole opening.
A standard countersink callout looks like this:
- ⌵ Ø17.30× 90°
This means: countersink to a 17.30mm diameter at a 90° included angle.
A complete callout includes three dimensions:
- Pilot hole diameter
- Pilot hole depth
- Countersink diameter
- Countersink angle
Example: Ø9.0 ↓ 40.0 / ⌵ Ø17.3 × 90°
Drill a 5mm pilot hole, depth 40mm, then countersink to 17.3mm at 90°.
When to Use Countersink Holes
Use a countersink hole when the fastener head cannot sit above the surface.
- The surface must be flush. Mating parts that sit flat against each other cannot have protruding fastener heads between them. Any protrusion creates a gap.
- Moving parts are nearby. Raised screw heads catch on cables, seals, and adjacent components. Countersinking removes the interference.
- Aerodynamic performance matters. In aerospace, a protruding rivet head at high speed creates drag. Every fastener on an aircraft skin is countersunk.
- Safety is a concern. Raised fastener heads on panels and equipment are cut and snag hazards. Flush heads remove the risk.
- Aesthetics matter. Consumer electronics, furniture, and enclosures look unfinished with visible screw heads.
Countersink Hole Dimensions and Size Charts
The countersink diameter must match the fastener. If the countersink is too small, the screw head sits proud of the surface. If it is too large, the head loses contact with the angled walls and the joint loses clamping force.
The tables below list the standard countersink dimensions for socket flat head fasteners under ANSI and ISO specifications.
Table 1: Socket Flat Head Countersink (ANSI Inch, by Fastener Size Number)
| Fastener Size | Countersink Diameter（in） | Countersink Angle (°) |
|---|---|---|
| #0 | 5/32 | 82 |
| #1 | 3/16 | 82 |
| #2 | 7/32 | 82 |
| #3 | 1/4 | 82 |
| #4 | 9/32 | 82 |
| #5 | 5/16 | 82 |
| #6 | 11/32 | 82 |
| #8 | 3/8 | 82 |
| #10 | 7/16 | 82 |
Table 2: Socket Flat Head Countersink (ANSI Inch, by Thread Size)
| Thread Size | Countersink Diameter（in) | Countersink Angle (°) |
|---|---|---|
| 1/4″ | 9/16 | 82 |
| 5/16″ | 11/16 | 82 |
| 3/8″ | 27/32 | 82 |
| 7/16″ | 29/32 | 82 |
| 1/2″ | 1 | 82 |
| 5/8″ | 1-1/4 | 82 |
| 3/4″ | 1-1/2 | 82 |
| 7/8″ | 1-3/4 | 82 |
| 1″ | 2 | 82 |
| 1-1/8″ | 2-1/4 | 82 |
| 1-1/4″ | 2-1/2 | 82 |
Table 3: Socket Flat Head Countersink (ISO Metric, by Thread Size)
| Thread | Countersink Diameter (mm) | Countersink Angle (°) |
|---|---|---|
| M3 | 6.94 | 90 |
| M3.5 | 7.95 | 90 |
| M4 | 9.18 | 90 |
| M5 | 11.47 | 90 |
| M6 | 13.71 | 90 |
| M8 | 18.25 | 90 |
| M10 | 22.73 | 90 |
| M12 | 27.21 | 90 |
| M16 | 33.99 | 90 |
| M20 | 40.71 | 90 |
Applications Of Countersink Holes
Countersink holes are used across almost every manufacturing industry.
Aerospace
Aircraft skins use countersunk rivets on every panel. At cruising speed, even a slightly raised fastener head creates aerodynamic drag. Flush riveting is not optional in aerospace. It is a structural and performance requirement.
Automotive
Interior trim panels, brake components, and body panels use countersunk screws to keep surfaces smooth and prevent interference with adjacent parts.
Consumer Electronics
Laptops, smartphones, and enclosures use countersunk screws to keep device casings flat and gap-free. In electronics, a protruding screw head can also interfere with internal components or create short circuit risk.
Sheet Metal Fabrication
Enclosures, brackets, and frames use countersink holes to keep assembly surfaces flush. In thin sheet metal, dimpling is used instead of machining to preserve material strength.
Furniture and Woodworking
Flat-head screws are countersunk into wood surfaces and covered with plugs or filler. The result is a clean surface with no visible hardware.
Medical Devices
Surgical instruments and equipment housings use countersunk fasteners to eliminate crevices where bacteria can accumulate and to prevent sharp edges from contacting patients.
Countersink Holes vs Counterbore Holes
Countersink holes and counterbore holes both recess a fastener below the surface. The difference is the shape and the fastener type.
A countersink has angled walls that form a cone. It is used with flat-head screws and flush rivets. The fastener head seats into the cone and stops at the surface.
A counterbore has straight walls and a flat bottom. It is used with socket head cap screws. The screw head sits in a cylindrical pocket below the surface.
| Feature | Countersink Hole | Counterbore Hole |
|---|---|---|
| Shape | Cone-shaped, tapered sides | Cylindrical with a flat bottom |
| Callout Symbol | ∨ (angled V shape) | ⊔ (flat-bottomed shape) |
| Drilling Angle | Common angles: 82°, 90°, 100°, 110°, 120°, matched to the fastener head | No angle needed, sides are straight and parallel |
| Depth | Shallow, just deep enough for the tapered head to sit flush | Deeper than a countersink, sized to fully sink the head or washer |
| Used With | Flat-head screws, oval-head screws, rivets | Socket head cap screws, hex bolts, washers |
| Clamping Strength | Moderate, load spreads unevenly along the tapered face | Higher, load spreads evenly across the flat bottom |
| Manufacturing Complexity | Simple, fast to machine, lower cost | Requires a separate boring step, slightly higher cost |
| Typical Material | Plastic, wood, light metals | Steel, aluminum, heavy-duty metals |
| Common Applications | Decorative hardware, electronics housings, furniture | Machinery assembly, automotive parts, structural frames |
Not Sure Which Hole Type You Need?
Specifying countersink holes correctly takes practice. Wrong angle, missing callout, or wrong hole type on the drawing are the issues we see on incoming parts every week at Aria.
If you are designing a part and need a second opinion on your countersink specifications, or you are ready to get your parts quoted, send us your drawing. We will get back to you within 24 hours.
Custom Quality Parts By Aria
Contact Us: Sales@madearia.com
Written By
Gavin Leo
Custom Quality Parts By Aria
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