Countersink Depth Calculator — Flush-Head Screw Depth
Enter your screw head diameter, pilot hole diameter and countersink angle to find the exact axial depth you need to drill so the head sits perfectly flush with the surface. Works for 60°, 82°, 90°, 100° and 120° countersinks in millimetres or inches.
Unit
mm
mm
Countersink angle
Axial depth of the conical recess — drill this deep for a flush fit
2.588 mm
α = 82°- 1
Half angle
82° ÷ 2 = 41° - 2
tan(half-angle)
tan(41°) = 0.869287Tangent of the half-angle links the radial step to the axial depth. - 3
Diameter step
8 − 3.5 = 4.5 - 4
Countersink depth
4.5 ÷ (2 × 0.869287) = 2.588
How does this calculator work?
Countersink depth = (D − d) / (2 × tan(α/2)), where D is screw head diameter, d is pilot hole diameter, and α is the full included angle (82° for imperial, 90° for metric ISO). Enter these three values to get the exact drill depth for a flush fit.
Formula
How this is calculated
A countersink is a conical recess cut at the top of a drilled hole so that a flat- or oval-head screw can sit flush with (or below) the surface. The included angle of the cone (α) depends on the screw standard: 82° is used for imperial flat-head screws in North America, 90° is the ISO metric standard, 60° suits some metric fine-thread and rivet applications, and 100°/120° appear on specific machine and sheet-metal fasteners.
The depth formula is derived from the geometry of a right triangle: half the difference in diameters (the radius step from the pilot hole to the screw head) divided by the tangent of the half-angle. The half-angle is α/2 because the included angle spans both sides of the cone. A narrower angle (e.g. 60°) requires more depth for the same head diameter; a wider angle (e.g. 120°) requires less.
Assumptions: the head is a perfect cone (flat-head or oval-head), the screw shank fits the pilot hole exactly, and the screw head diameter is the maximum outer diameter at the top of the head. Real screws vary slightly — drill 0.1–0.2 mm shallower and test-fit before going to full depth.
Frequently asked questions
Most flat-head wood screws sold in North America use an 82° angle. European and metric wood screws typically use 90°. Check the screw packaging or datasheet if uncertain — using the wrong angle leaves the head proud or sunken.
The pilot hole is the smaller straight hole for the screw shank. Its diameter is typically the shank diameter of the screw. Enter 0 if you are countersinking all the way to a point (no straight pilot hole).
A narrow cone is tall relative to its base. To accommodate the same head diameter (the cone's base), the cone must be deeper than a shallower wide-angle cone. Mathematically, depth = (D−d) / (2·tan(α/2)) — as α decreases, tan(α/2) decreases and depth increases.
Also known as
TG we-Calculate Editorial Team. (2026). Countersink Depth Calculator — Flush-Head Screw Depth [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/countersink-depth-calculator
TG we-Calculate Editorial Team. "Countersink Depth Calculator — Flush-Head Screw Depth." TG we-Calculate. 2026. https://we-calculate.com/calculator/countersink-depth-calculator.
TG we-Calculate Editorial Team, "Countersink Depth Calculator — Flush-Head Screw Depth," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/countersink-depth-calculator
@misc{wecalculate_countersink_depth_calculator, title = {Countersink Depth Calculator — Flush-Head Screw Depth}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/countersink-depth-calculator}}, year = {2026}, note = {TG we-Calculate} }
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