ENGINEERING REFERENCE CHART
Countersink Size Chart
A countersink size chart lists the diameter, depth, and pilot hole for the cone-shaped recess that lets a flat-head screw sit flush, across three standard included angles: 82° (US), 90° (ISO/metric), and 100° (aerospace). Match the angle to the fastener to get a flush fit.
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82° / 90° / 100° Included Angles
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Diameter, Depth & Pilot Hole Data
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US / ISO / Aerospace Standards
Countersink Standards
Three angles, one reference
82°
US / Imperial
90°
ISO / Metric
100°
Aerospace
Pilot
Clearance hole
Countersink diameter & depth
D = Ø + 2 × depth × tan(angle/2)
REFERENCE GUIDE
What Is a Countersink Size Chart?
A countersink size chart is a lookup table that pairs a screw size with the countersink that seats it flush: the included angle, the head (major) diameter at the surface, the pilot hole drilled first, and the resulting depth.
Engineers use it to specify hole callouts on drawings, machinists use it to pick the countersink tool, and sheet-metal buyers use it to make sure a formed part will still sit flush after finishing. The pilot hole matters: drill the pilot first, then open its mouth into a cone.
Angle
The included cone angle determines how the screw sits. The three common standards are 82° (US/imperial flat-head and machine screws), 90° (ISO/metric flat-head screws), and 100° (aerospace fasteners). Rivets use 120°.
Head (Major) Diameter
The width of the cone opening at the surface, set by the screw size. This is the “Ø” a drawing prints next to the symbol. It determines how much surface contact the screw head has for a flush finish.
Depth & Pilot Hole
Depth is how far the cone extends into the pilot hole, set by the head diameter and the pilot diameter. If the pilot is too big, the head has nothing to bite; if too small, the screw forces it wider and the material cracks.
COUNTERSINK REFERENCE DATA
Countersink Size Chart: 82°, 90°, and 100° in One Table
The table below lists common flat-head screw sizes for the three standard angles, each row showing the pilot hole, the countersink (head) diameter at the surface, and the resulting depth. Read down the angle block you need, or find your screw size and read across.
Representative rows shown. The full #2–1/2″ 82° series, complete 90° inch series, and aerospace 100° sizes ship in the page’s data file, spot-checked against ASME B18.6.3, ISO 7721, and NAS/AN/MS aerospace drawings. Pilot Ø equals the screw body diameter; depth computed via D = (OD − ID)/(2·tan(θ/2)).
| Screw Size | Angle | Pilot Hole Ø (in) | Countersink Ø (in) | Depth (in) |
|---|---|---|---|---|
| #2 | 82° | 0.089 | 0.197 | 0.062 a |
| #6 | 82° | 0.138 | 0.279 | 0.081 a |
| #8 | 82° | 0.164 | 0.322 | 0.091 a |
| #10 | 82° | 0.190 | 0.373 | 0.105 a |
| 1/4 | 82° | 0.250 | 0.477 | 0.131 a |
| 5/16 | 82° | 0.3125 | 0.567 | 0.146 a |
| 3/8 | 82° | 0.375 | 0.682 | 0.177 a |
| 1/2 | 82° | 0.500 | 0.894 | 0.227 a |
| #2 | 90° | 0.089 | 0.185 | 0.048 a |
| 1/4 | 90° | 0.250 | 0.452 | 0.101 a |
| 1/2 | 90° | 0.500 | 0.851 | 0.176 a |
| #4 | 100° | 0.112 | 0.368 | 0.107 a |
| #6 | 100° | 0.138 | 0.414 | 0.116 a |
| #8 | 100° | 0.164 | 0.460 | 0.124 a |
| #10 | 100° | 0.190 | 0.506 | 0.133 a |
| 1/4 | 100° | 0.250 | 0.644 | 0.165 a |
| 3/8 | 100° | 0.375 | 0.828 | 0.190 a |
ASME B18.6.3 & ISO 7721
Head diameters follow ASME B18.6.3 for 82°, ISO 7721 for 90°, and NAS/AN/MS aerospace drawings for 100°. Pilot hole equals the screw body diameter per standard drill tables.
Depth Computation & Verification
Depth values are computed via D = (OD − ID)/(2·tan(θ/2)) and marked with (a) pending verification against standard texts. If your drawing specifies a different clearance fit, consult the pilot-hole section.
READING GUIDE
How to Read a Countersink Size Chart
Reading a countersink size chart takes three steps: find the screw size, read the pilot hole and countersink diameter, and check the depth. Here’s a worked example.
01
Find the Screw Size
Suppose your drawing calls out a #8 flat-head screw, 82°. Find the #8 row in the 82° block above.
02
Read Pilot & Countersink Ø
The row shows a 5/32″-class pilot (0.164″, the screw body) and a 0.322″ countersink Ø at the surface. The countersink tool you’d program cuts an 82° cone that opens to 0.322″.
03
Check the Depth
The row shows about 0.091″ deep into the pilot. That’s the dimension that matters in sheet metal: a 0.091″ deep cone in a 0.080″ sheet breaks through the back.
Example: Reading a Drawing Callout
On a print, a countersink is written as Ø × angle CSK Ø. The callout ⌀5 × 90° CSK ⌀10 means: the pilot (through) hole is 5 mm, the countersink has a 90° included angle and opens to 10 mm at the surface. The symbol is ⌵ (per ASME Y14.5).
⌀5
Pilot hole (mm)
90°
Included angle
⌀10
Countersink Ø at surface
METRIC REFERENCE
Metric Countersink Dimensions (90°)
Metric flat-head screws use a 90° countersink per ISO 7721. The table below follows the same layout, with countersink Ø ranges reflecting ISO head-diameter tolerances.
| Screw Size | Pilot hole Ø (mm) | Countersink Ø (mm) | Depth (mm) |
|---|---|---|---|
| M3 | 3.4 | 6.72–6.94 | 1.7 a |
| M4 | 4.5 | 8.96–9.25 | 2.2 a |
| M5 | 5.5 | 11.20–11.45 | 2.9 a |
| M6 | 6.6 | 13.44–13.71 | 3.4 a |
| M8 | 9.0 | 17.92–18.25 | 4.5 a |
| M10 | 11.0 | 22.40–22.73 | 5.7 a |
| M12 | 13.5 | 26.88–27.21 | 6.7 a |
| M16 | 17.5 | 33.99 | 8.2 a |
| M20 | 22.0 | 40.71 | 9.4 a |
Note: Countersink Ø ranges per ISO 10642 and ISO 15065:2005. Depth values (a) are approximate, computed from the nominal head Ø and pilot Ø. Pilot hole equals the screw body diameter per ISO standard drill tables.
82° vs 90° vs 100°, Which Angle Should You Use?
The angle is the one decision people get wrong most often, and it’s exactly what the name of the fastener family tells you:
| Angle | Fastener family | Standard | Where it’s used |
|---|---|---|---|
| 82° | US/imperial flat-head & machine screws | ASME B18.6.3, ASME B18.3 | US hardware, general machining |
| 90° | ISO/metric flat-head screws, socket flat head | ISO 7721, ISO 15065:2005, ISO 10642 | Global / ISO designs |
| 100° | Aerospace flat-head fasteners | AN/MS/NAS drawings | Aerospace, defense, aviation |
| 120° | Rivet heads | ASME B18.13 series | Sheet-metal riveting |
The angle-matching trap: an 82° screw in a 90° countersink touches only at the rim and never seats flush. A 90° screw in an 82° countersink seats too deep, and the head sits below the surface. Match the fastener’s angle, not the tool that’s convenient, and verify the callout before you cut.
For a clean flush fit, many shops add a small allowance to the head Ø: for an 82° or 90° flat head, countersink about 0.005–0.010″ (0.13–0.25 mm) larger than the head Ø so the head sits just below flush after finishing or paint. On a production drawing, state the allowance or leave a note; silent assumptions cause fit problems in the field.
CNC MACHINING APPLICATION
Countersinks in CNC Machining and Sheet Metal
Countersunk holes are everyday features in machined and sheet-metal parts. How you cut them changes the result.
From tooling selection to depth control, edge distance, and thin‑sheet limits, every decision affects whether the fastener sits flush. Here’s what our engineers check on every drawing.
Tooling & Angle
Choose the Right Cutter
A 3‑flute or multi‑flute countersink tool produces a smoother, less chatty cut and is the standard choice for CNC drilling. For thin materials and hard alloys, a single‑flute tool with lower feed avoids chatter. The tool’s included angle must match the callout exactly — running an 82° tool on a 90° callout is a reject.
Depth & Edge Distance
Program, Measure, and Position
In CNC, depth is a programmed Z stop, not a hand feel. Program the countersink depth from the head Ø you measured, and inspect with a countersink gauge or optical comparator, not by eye. Keep the hole center at least one countersink Ø from the part edge; closer than that and the cone opens into the edge, losing the flush land.
Thin Sheet & Deburring
Check Break‑Through and Sharp Edges
If the countersink depth exceeds about 60% of material thickness, the cone breaks through the back. A 0.091″ deep 82° countersink needs sheet at least 0.110–0.125″ thick. Deburr the rim, especially where the head seats, to keep the fastener from gouging and to ensure a true flush seat.
Our CNC Machining & Sheet‑Metal Capabilities
Our teams cut countersunk holes every day, from single prototypes to production runs. When a drawing has a countersink callout, we confirm the angle matches the fastener, verify the depth against the material thickness, and flag anything that won’t seat flush before we quote.
±0.001″
Tolerance on depth & Ø
CMM
Inspection & gauge verification
Deburred
Sharp edge removal standard
COUNTERSINK QUESTIONS
Countersink Size FAQ
A countersink is a cone-shaped recess cut at the opening of a pilot hole so a flat-head screw sits flush with the surface. It’s defined by three values: the included angle (82°, 90°, or 100°), the head diameter at the surface, and the depth of the cone. The drafting symbol is ⌵.
Match the angle to the fastener: 82° for US/imperial flat-head screws (ASME B18.6.3), 90° for ISO/metric flat-head screws (ISO 7721, ISO 15065), and 100° for aerospace fasteners (AN/MS/NAS). An 82° screw in a 90° countersink sits proud; a 90° screw in an 82° countersink seats too deep.
Use D = (OD − ID) / (2 × tan(θ/2)), where OD is the countersink diameter at the surface, ID is the pilot hole diameter, and θ is the included angle. For a 1/4″ 82° countersink over a 1/4″ pilot: (0.477 − 0.250) / (2 × tan 41°) = 0.131″. This countersink size chart lists depth as lookup data for standard sizes.
For a #8 82° flat head: pilot 0.164″, countersink Ø 0.322″, depth about 0.091″. For an M6 90° flat head: pilot 6.6 mm, countersink Ø 13.44–13.71 mm, depth about 3.4 mm. Depth values are approximate until verified against the standard text.
The ⌵ symbol marks a countersink. The callout gives the pilot hole Ø, the included angle, and the countersink Ø at the surface, for example ⌀5 × 90° CSK ⌀10 means a 5 mm pilot with a 90° countersink opening to 10 mm. It’s defined in the ASME Y14.5 drafting standard.
A countersink is a tapered cone that seats flat-head screws flush; a counterbore is a cylindrical, flat-bottomed recess that seats socket-head cap screws. The countersink symbol is ⌵ and the counterbore symbol is ⌴. Counterbores carry higher, more uniform clamp load; countersinks are more depth‑sensitive.
Get an Instant Quote
Need a part to match a size on this countersink size chart? Upload your CAD file and get an instant quote in 60 seconds, with live pricing, lead time, and free DFM feedback. Our engineers will confirm your countersink angle matches the fastener, check the depth against the material thickness, and machine the part to print at ±0.001 inches, then inspect it on CMM equipment before it ships from our ISO 9001 and AS9100 certified facility in Qingdao. No minimum order quantity, and parts ship in as little as 3–7 days. If you’d rather talk it through first, contact our engineering team before you upload.