Countersink Depth Calculator
Find how deep to run a countersink so a flat-head screw sits flush. Enter the included angle (82° is the US flat-head standard, plus 90°, 100° and 120°), the countersink diameter and the pilot/clearance hole — or pick a US flat-head screw size to fill them in — and get the countersink depth, the full cone depth, the half angle, the chamfer width and the cone slant, with by-angle and by-screw tables, a chart, and a downloadable PDF. Works in inches or millimeters.
| Angle | Depth | Full cone depth | Slant |
|---|---|---|---|
| Enter a countersink to compare | |||
| Screw | Head dia | Pilot | Depth |
|---|---|---|---|
| Enter a countersink to compare | |||
How to use the countersink depth calculator
Four quick steps turn a flat-head screw and a countersink angle into the depth to run — plus the full cone depth, the half angle, the chamfer width and the cone slant, with US flat-head presets.
Pick the angle
82° for US flat-head, 90° metric, 100° or 120° for aircraft.
Choose a screw
A US flat-head preset, or enter your own head diameter.
Set the pilot hole
The clearance hole the screw passes through, at the cone’s bottom.
Read results & save a PDF
Get the countersink depth and cone geometry — then download a report.
The formulas behind the depth
The depth comes from the diameter the cone spans and the included angle; the other figures are the geometry of the same cone.
▶ Worked example
A #10 flat-head screw (head 0.385", pilot 0.196") at the US 82° angle.
Half angle = 41°; depth = (0.385 − 0.196) ÷ (2 × tan 41°) = 0.109".
Full cone depth = 0.385 ÷ (2 × tan 41°) = 0.221"; chamfer = (0.385 − 0.196) ÷ 2 = 0.094"; slant = 0.144".
Depth 0.109″ · full cone 0.221″ · half angle 41° · chamfer 0.094″
Run the 82° countersink to about 0.109 inch deep and the #10 head sits flush.
Countersink quick reference
The cone formulas, the depth by included angle, and the depth by common US flat-head screw.
| Quantity | Formula |
|---|---|
| Countersink depth | (D − d) ÷ (2 × tan(angle ÷ 2)) |
| Full cone depth | D ÷ (2 × tan(angle ÷ 2)) |
| Half angle | angle ÷ 2 |
| Chamfer width (radial) | (D − d) ÷ 2 |
| Cone wall (slant) | (D − d) ÷ (2 × sin(angle ÷ 2)) |
| Included angle | Depth | Full cone depth | Cone slant |
|---|---|---|---|
| 60° | 0.164″ | 0.333″ | 0.189″ |
| 82° (US flat) | 0.109″ | 0.221″ | 0.144″ |
| 90° (metric) | 0.094″ | 0.193″ | 0.134″ |
| 100° | 0.079″ | 0.162″ | 0.123″ |
| 110° | 0.066″ | 0.135″ | 0.115″ |
| 120° (rivet) | 0.055″ | 0.111″ | 0.109″ |
| Screw | Head dia | Pilot | Depth | Full cone |
|---|---|---|---|---|
| #4 | 0.225″ | 0.116″ | 0.063″ | 0.129″ |
| #6 | 0.279″ | 0.144″ | 0.078″ | 0.161″ |
| #8 | 0.332″ | 0.170″ | 0.093″ | 0.191″ |
| #10 | 0.385″ | 0.196″ | 0.109″ | 0.221″ |
| #12 | 0.438″ | 0.221″ | 0.125″ | 0.252″ |
| 1/4″ | 0.507″ | 0.266″ | 0.139″ | 0.292″ |
| 3/8″ | 0.762″ | 0.397″ | 0.210″ | 0.438″ |
A wider cone angle is shallower for the same diameter, so match the angle to the screw head — 82° for US flat-head. Screw head and pilot diameters are nominal; verify against your actual fasteners.
Everything the calculator returns
One calculation gives the full cone picture — the depth to run and the geometry around it.
The countersink at a glance
The standard angle, and the default result.
Built for the way you work
Whether you machine metal, build in wood, or fasten aircraft panels, the countersink depth starts from the same angle and diameters.
Setting a quill stop or a depth so a flat-head sits flush, and need the exact depth and the half angle for the setup.
- Match the angle to the screw
- Set the stop to the depth
- Use the half angle on a lathe
Want flat-head screws flush or just below the surface for plugs, and need the depth for a common US screw size.
- Use the 82° US angle
- Pick your screw from the presets
- Go a hair deeper for plugs
Countersinking for 100° flush rivets or fasteners in sheet, where the angle and depth must be exact for a flush skin.
- Use 100° or 120° as specified
- Mind thin-sheet knife-edge
- Set a microstop countersink
Countersink flush, every time
Seven things that get a flat-head sitting right.
Match the angle to the screw
82° for US flat-head, 90° metric. A mismatched angle touches only at one edge and never seats flush.
Diameter to the head
Size the surface diameter to the screw head. Deeper or wider sinks the head; shallower leaves it proud.
Cut to the countersink depth
Not the full cone depth — the pilot hole cuts off the cone’s tip, so the real depth is shallower.
Use a depth stop
A quill stop, a microstop countersink or a stop collar gives a repeatable depth across many holes.
Go slow for a clean chamfer
A slower feed and a sharp tool avoid chatter and a torn chamfer, especially in soft metal or end grain.
Watch thin sheet
On thin material a deep countersink can cut to a knife-edge or break through. Dimple instead where it’s too thin.
Verify the screw specs
Head and clearance diameters vary by standard and maker. The presets are nominal — measure your actual screws.
Frequently asked questions
Common questions about countersink depth, the included angle, cone geometry and flush screws.
Countersink depth is how far below the surface you run the cone so a flat-head screw sits flush. It comes from the difference between the countersink (surface) diameter and the pilot hole, divided by twice the tangent of half the included angle: depth = (D minus d) divided by (2 times the tangent of the angle over 2).
The surface diameter is usually the screw’s head diameter, and the pilot is the clearance hole the screw passes through. A wider head or a steeper cone means more depth.
For example, a #10 flat-head with a 0.385 inch head, a 0.196 inch pilot and an 82 degree angle needs a countersink depth of about 0.109 inch. This calculator gives the depth, the full cone depth and the cone geometry from your angle and diameters.
The US standard for flat-head machine and wood screws is an 82 degree included angle, so an 82 degree countersink matches those heads. Metric flat-head screws and much general work use 90 degrees, and aircraft fasteners and rivets use 100 or 120 degrees.
The countersink angle must match the screw head angle, or the head will only touch at the top or bottom edge and will not seat flush or pull evenly.
For example, a US #10 flat-head screw uses an 82 degree countersink. This calculator includes 82, 90, 100 and 120 degree angles, plus any custom angle, and works out the depth for each.
The countersink depth is how far down you actually run the tool — from the surface to where the cone meets the pilot hole. The full cone depth is the deeper figure the cone would reach if it carried on to a theoretical point with no pilot hole.
You cut to the countersink depth, not the full cone depth, because the pilot hole cuts off the tip of the cone. The full cone depth is a reference for the geometry.
For example, a #10 flat-head at 82 degrees has a countersink depth of about 0.109 inch but a full cone depth of about 0.221 inch to the point. This calculator reports both, so you set the stop to the countersink depth.
The countersink diameter at the surface should match the flat-head screw’s head diameter, so the head sits flush with the top. That surface diameter is the large end of the cone; the cone narrows down to the pilot hole at the bottom.
Using the head diameter gives a flush head; a slightly larger diameter sinks the head below the surface, and a smaller one leaves it standing proud.
For example, a #10 flat-head has about a 0.385 inch head, so you countersink to 0.385 inch at the surface. This calculator includes head diameters for common US flat-head screws, or you can enter your own.
For a US #10 flat-head screw at the standard 82 degree angle, with a 0.385 inch head and a 0.196 inch clearance hole, the countersink depth is about 0.109 inch. That sinks the head flush with the surface.
The depth depends on the angle: the same #10 head at 90 degrees needs about 0.095 inch, and at 100 degrees about 0.079 inch, because a wider cone is shallower for the same diameter.
For example, run the 82 degree countersink to about 0.109 inch deep for a #10 flat-head. This calculator gives the exact depth for any screw size and angle, and lists common US flat-head presets.
The half angle is simply half of the included angle — the angle of the cone wall measured from the center axis. An 82 degree countersink has a 41 degree half angle, a 90 degree one has 45 degrees, and so on.
It matters when you set a cone on a lathe or a mill by tilting the work or the tool, because that setup is referenced to the half angle, not the full included angle.
For example, an 82 degree countersink has a 41 degree half angle. This calculator reports the half angle alongside the depth, so you can set up a single-point or tilted cut.
A countersink is a cone-shaped recess that lets a flat (tapered) screw head sit flush; a counterbore is a flat-bottomed cylindrical recess that lets a socket-head or hex-head screw and its washer sit below the surface. One is angled, the other is a straight bore.
You match a countersink to a flat-head screw’s cone angle, while a counterbore is sized to the diameter and height of a cylindrical head. They solve the same flush-mounting problem for different head shapes.
This calculator covers the countersink — the angled cone for flat-head screws. For a flat-bottomed recess for a socket-head screw, you would size a counterbore by diameter and depth instead.
Usually the countersink depth or diameter is off. If the head stands proud of the surface, the countersink is too shallow or too small in diameter — run it a little deeper so the surface diameter reaches the head diameter. If the head is sunk below the surface, the countersink is too deep or too wide, and you have gone past flush.
A mismatched angle also causes it: an 82 degree head in a 90 degree countersink touches only at the rim and will not seat correctly.
For example, a #10 flat-head needs about a 0.385 inch surface diameter and 0.109 inch depth at 82 degrees to sit flush. This calculator gives the target depth and diameter, so you can dial the countersink in and test on scrap.
Disclaimer: US or metric units (inches or millimeters; angles are the same either way). Countersink depth = (D − d) ÷ (2 × tan(angle ÷ 2)); full cone depth = D ÷ (2 × tan(angle ÷ 2)); half angle = angle ÷ 2; chamfer width = (D − d) ÷ 2; cone slant = (D − d) ÷ (2 × sin(angle ÷ 2)), where D is the surface (head) diameter and d the pilot hole. US flat-head is 82°; metric 90°; aircraft 100° or 120°. Screw head and clearance diameters are nominal and vary by standard and manufacturer. A reference aid only — verify against your actual screws and test-cut on scrap before the finished work.