Countersink Depth Calculator (82, 90 and 100 Degree)

A flat-head screw sits flush only when the countersink is cut to the right diameter, and on a drill press you set that diameter by depth. This calculator turns the diameter you want at the surface into a depth, for the three common angles: 82 degrees for inch flat-head screws, 90 degrees for ISO metric countersunk screws and 100 degrees for aviation fasteners and British screws. It gives the depth below the surface and the depth from the point of the countersink, so you can set a depth stop whichever way you measure.

Countersink depth calculator

How it works

A countersink is a cone. Its half-angle is half the included angle, so each unit of depth widens the cone by 2 × tan(angle ÷ 2):

depth below the surface = (countersink diameter − hole diameter) ÷ (2 × tan(angle ÷ 2))
depth from the cutter’s point = countersink diameter ÷ (2 × tan(angle ÷ 2))

For 82 degrees the factor 1 ÷ (2 tan 41°) is 0.575, for 90 degrees it is exactly 0.5 and for 100 degrees 0.4195. The first depth is what a depth gauge reads from the work surface once the cutter touches the edge of the hole; the second is what you set when you zero the quill with the sharp point of the cutter at the surface, as you would on solid material or with a cutter whose point is intact.

Worked example

An 82 degree countersink 1/2 in across at the surface, around a 0.257 in (F drill) clearance hole for a 1/4 in flat-head screw:

Units: Inches
Countersink angle: 82 degrees (inch flat-head screws)
Countersink diameter at the surface: 0.5 in
Hole diameter: 0.257 in

The calculator gives:

Countersink depth below the surface: 0.1398 in (3.55 mm)
Depth from the cutter’s point: 0.2876 in (7.3 mm)
Depth per unit of diameter: 0.5752 x diameter
Each 0.1 in of extra depth widens the top by: 0.1739 in

The cone has to go 0.140 in below the surface. Measured from the point of the countersink it is 0.288 in deep, because the point would sit inside the hole. If the screw head should end up slightly below flush, add a few thousandths to the diameter: each 0.01 in of diameter is about 0.006 in of depth at 82 degrees. Measure the actual head with calipers rather than trusting a nominal figure; heads vary between makers.

Countersink depth from the point (inch)

Diameter at surface	82°	90°	100°
1/8 in	0.0719 in	0.0625 in	0.0524 in
3/16 in	0.1078 in	0.0938 in	0.0787 in
1/4 in	0.1438 in	0.125 in	0.1049 in
5/16 in	0.1797 in	0.1563 in	0.1311 in
3/8 in	0.2157 in	0.1875 in	0.1573 in
7/16 in	0.2516 in	0.2188 in	0.1836 in
1/2 in	0.2876 in	0.25 in	0.2098 in
5/8 in	0.3595 in	0.3125 in	0.2622 in
3/4 in	0.4314 in	0.375 in	0.3147 in
7/8 in	0.5033 in	0.4375 in	0.3671 in
1 in	0.5752 in	0.5 in	0.4195 in

Depth from the sharp point of the cutter to the surface for a cone of this top diameter.

Countersink depth from the point (metric)

Diameter at surface	82°	90°	100°
4 mm	2.3 mm	2 mm	1.68 mm
5 mm	2.88 mm	2.5 mm	2.1 mm
6 mm	3.45 mm	3 mm	2.52 mm
8 mm	4.6 mm	4 mm	3.36 mm
10 mm	5.75 mm	5 mm	4.2 mm
12 mm	6.9 mm	6 mm	5.03 mm
14 mm	8.05 mm	7 mm	5.87 mm
16 mm	9.2 mm	8 mm	6.71 mm
20 mm	11.5 mm	10 mm	8.39 mm
24 mm	13.8 mm	12 mm	10.07 mm
30 mm	17.26 mm	15 mm	12.59 mm

For depth below the surface around a hole, subtract the same calculation for the hole diameter.

Notes and limits

Match the angle to the screw. An 82 degree inch screw in a 90 degree countersink bears only on its outer rim and can pull through soft material; a metric 90 degree screw in an 82 degree seat bears only near the shank.
In wood the fibres spring back and the head often wants to sit a little deeper. A trial hole in scrap is quicker than a calculation.
Run a countersink slowly in metal, well below the speed of a twist drill of the same diameter, with light pressure; too much speed makes it chatter and leaves a rippled cone.
Clearance holes for the shank are in the clearance hole calculator; screw lengths and pilot holes for wood screws in the screw and nail size calculator.
The 100 degree angle is also used for aircraft rivets; for structural or aircraft work follow the drawing and the approved data, not a general calculator.

Frequently asked questions

What angle is a countersink for wood screws?

Inch flat-head wood and machine screws use 82 degrees. Screws made to ISO metric standards use 90 degrees, so if a screw is sold in metric sizes, check its head angle before choosing the cutter.

How deep should a countersink be?

Deep enough that the cone at the surface equals the head diameter. For an 82 degree cone that is 0.575 × (head diameter − hole diameter) below the surface; for 90 degrees, half the difference.

How deep is a 90 degree countersink 10 mm across?

5 mm from the point. Around a 5.5 mm clearance hole for an M5 screw it is 2.25 mm below the surface.

Can I use a 90 degree countersink for 82 degree screws?

It will work in wood for light duty, but the head bears only at its rim. In metal, or where the screw is loaded, use the matching angle.

Sources

Last reviewed: 25 October 2026. The calculation is covered by automated tests with known answers taken from the sources above. Found an error? Please let us know.