Flat-head screws sit flush only if the countersink is the right diameter. Countersinks come in standard included angles – 82° for inch flat-head screws, 90° for metric, 100° for aerospace rivets, 118° and 120° for some other uses. Machinists set depth stops rather than measuring the diameter, so converting diameter to depth is handy.
How to use the Countersink Depth Calculator
- Countersink diameter
- Countersink angle
- Pilot hole diameter
- Every result field is editable: type into Depth to cone tip and Countersink diameter plus every other field update to match.
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter the countersink diameter at the surface, the angle and the pilot hole diameter. Type a depth into the result to find the diameter it produces.
Tips for an accurate estimate
- Measure angles with a digital angle finder or sliding bevel, not by eye.
- Make a test cut in scrap before cutting expensive stock.
- Small angle errors add up over many joints – check the first assembly before cutting the rest.
- Keep a written list of the dimensions and angles you set on the saw or machine.
Common mistakes to avoid
- Reading the saw scale as the angle from the edge rather than from square.
- Rounding intermediate results too early.
- Mixing degrees and percent grade.
Inputs explained
| Field | What to enter |
|---|---|
| Countersink diameter | Units: mm, in, cm |
| Countersink angle | Choose one: 82° (inch flat-head screws), 90° (metric screws), 100° (aircraft rivets), 110° … |
| Pilot hole diameter | Units: mm, in, cm |
What each result means
- Depth to cone tip = ([Countersink diameter]÷2)÷tan(rad([Countersink angle]÷2))
- Depth to edge of pilot hole = max([Countersink diameter] – [Pilot hole diameter], 0)÷(2×tan(rad([Countersink angle]÷2)))
Example values
How the main result changes when Countersink diameter changes and everything else stays at the example values above:
| Countersink diameter (mm) | Depth to cone tip (mm) |
|---|---|
| 6 | 3 |
| 9 | 4.5 |
| 12 | 6 |
| 15 | 7.5 |
| 18 | 9 |
| 24 | 12 |
Formula
Depth to cone tip = (D ÷ 2) ÷ tan(angle ÷ 2)Depth below surface to the hole edge = (D − d) ÷ (2 × tan(angle ÷ 2))Worked example
Inputs — Countersink diameter: 12 mm; Countersink angle: 90° (metric screws); Pilot hole diameter: 6.6 mm.
Results — Depth to cone tip = 6 mm; Depth to edge of pilot hole = 2.7 mm.
Frequently asked questions
What angle is a countersink?
82° for unified (inch) flat-head screws, 90° for ISO metric flat-head screws, 100° for many aerospace rivets.
How deep should a countersink be?
Deep enough that the head finishes flush or slightly below the surface – use the head diameter as the countersink diameter.
What formula does the Countersink Depth Calculator use?
Depth to cone tip = (D ÷ 2) ÷ tan(angle ÷ 2).