By Zencalcy Editorial TeamLast reviewed About us
To sink a flat-head screw flush, the countersink must remove a cone whose depth depends on head diameter, shank diameter and the angle of the bit. For an 82° bit and a #8 screw, that is only about 0.11 inch — but getting it wrong splits the wood or leaves the head proud.
Depth to sink a screw head flush.
depth = (head − shank) ÷ (2 × tan(angle ÷ 2))
Head diameter is measured across the flat head, not the drive recess. Add a small amount for a tight fit and test on scrap: the screw should sit just below the surface and the fibres should not tear.
| Symbol | Meaning |
|---|---|
| D | Head diameter. |
| d | Shank diameter. |
| α | Countersink angle. |
Depth = (0.312 − 0.164) ÷ (2 × tan 41°) = 0.085 in, plus a little for a flush fit.
Continue with the Angle Cut Calculator, Angle of Depression Calculator, Bolt Circle Calculator, or the Bowl Segment Calculator — or browse more layout & geometry and all construction calculators.
82° for imperial flat-head wood screws, 90° for metric and machine screws. Using the wrong angle leaves a ring gap.
Results use standard construction estimating arithmetic: volumes from measured area and depth, unit weights from published material densities, and code-referenced spans or clearances where the page names a code. Quantities are rounded up to whole saleable units and include the stated waste allowance. These tools are planning aids, not structural design, and do not replace a licensed engineer, a local code check, or a contractor's site measurement.
Reference material used to build and check this calculator:
Last reviewed: September 2026. See how we build and check every tool on the about page.
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Content updated: September 2026