Countersink Depth Calculator

Countersink Depth Calculator finds how deep to cut a countersink from its diameter, hole size and angle. It also gives the Z plunge for a sharp or flat-tipped tool in inches or mm.

Two Depths in the Countersink Depth Calculator

People mean two different things by countersink depth, and the Countersink Depth Calculator reports both. The headline figure is the cone depth, measured from the surface down to where the cone meets the through-hole. The first card is the Z plunge, which is how far the tool travels after its tip touches the surface.

The two are far apart. A 0.531 in countersink over a 0.25 in hole at 82° has a cone depth of 0.1616 in. A sharp tool has to plunge 0.3054 in to cut it, because its point drops 0.1438 in into the hole before the cone reaches full width.

Use the cone depth to check a drawing, a head height or the material thickness. Use the Z plunge for a CNC program or a drill press stop. Formulas that leave out the hole diameter return the Z plunge of a sharp tool, not the cone depth.

Countersink Depth Formula With Angle

The cone depth in the Countersink Depth Calculator comes from the gap between the countersink diameter and the hole diameter, and from the included angle of the tool. Half the gap is divided by the tangent of half the angle.

$$Depth=\frac{D-d}{2\tan (A/2)}$$

D is the countersink diameter at the surface, d is the through-hole diameter and A is the included angle. The Z plunge uses the same shape, with the tool's tip flat in place of the hole. For a sharp point, T is zero.

$$Z=\frac{D-T}{2\tan (A/2)}$$

Diameter from Depth turns the Countersink Depth Calculator around. Enter how deep the cone runs and it returns the diameter it opens to at the surface. A cone 0.15 in deep over a 0.25 in hole at 82° opens to 0.5108 in.

$$D=d+2h\tan (A/2)$$

The angle changes the answer more than most people expect. The table below keeps the same 0.531 in diameter and 0.25 in hole and changes only the tool angle. The depth factor is the number that multiplies the radius difference.

Countersink Depth Chart for Inch Flat Heads

The screw list beside Countersink Diameter loads socket flat head cap screws from ASME B18.3. Each entry sets the head diameter, the 82° angle and a hole equal to the basic screw diameter. The chart shows what the Countersink Depth Calculator returns for each size.

These head diameters are the theoretical sharp maximum in the standard, as listed in the ASME B18.3 flat socket head tables. ASME describes that value as the countersink diameter in which a screw with the largest allowed head sits flush. Real heads are smaller.

The standard also gives 0.480 in as the smallest 1/4 in head. In a countersink cut to 0.531 in, that head sits 0.029 in below the surface. So a countersink cut to the chart never leaves a head proud, but it can leave one slightly low.

A real clearance hole is larger than the basic diameter. With a 9/32 in drill for the 1/4 in screw, the cone depth falls to 0.1437 in while the Z plunge stays at 0.3054 in. The Clearance Hole Calculator gives the drill size to enter.

Metric Countersink Depth at 90°

Switch Units to millimeters and the list changes to ISO 10642 socket countersunk screws, which have a 90° head. At 90° the depth factor is exactly 1, so the cone depth is simply half the difference between the two diameters.

Cutting a metric head with an 82° tool, or an inch head with a 90° tool, is the most common mismatch. The Countersink Depth Calculator flags it when a listed screw is loaded with the wrong angle. The head then bears on a thin ring instead of the full cone face.

100 Degree Countersink Depth

Aircraft flush rivets and screws use a 100° head. The wider angle makes a shallower cone, with a depth factor of 0.8391 against 1.1504 at 82°. The same 0.531 in diameter over a 0.25 in hole is only 0.1179 in deep at 100°.

To use the tool as a 100 degree Countersink Depth Calculator, pick 100° from the Included Angle list and type the fastener's head diameter as a Custom value. For the rivet itself, the Solid Rivet Length Calculator works out the shank length from the grip.

Tip Flat, Thickness and Head Recess

Most countersinks do not come to a perfect point. Open Tool Tip and Material in the Countersink Depth Calculator and enter the flat on the tip. A 1/16 in flat shortens the Z plunge for the 1/4 in example from 0.3054 in to 0.2695 in. Programming the sharp-point figure with that tool would cut the countersink too large.

Material Thickness checks that the cone stays inside the part. The 0.1616 in cone uses 43.1% of a 0.375 in plate and leaves 0.2134 in of straight hole. In 0.125 in sheet the same cone breaks through the back by 0.0366 in and leaves a knife edge.

The Breakthrough Diameter row is the largest countersink that thickness allows. For a 0.25 in hole in 0.125 in sheet at 82° it is 0.4673 in. Any head wider than that needs thicker stock or a different fastener.

Head Below Surface sets the head under flush on purpose. A 0.01 in recess on the 1/4 in screw raises the surface diameter to 0.5484 in and the cone depth to 0.1716 in. Flat head screw length is measured overall, head included, so check it in the Screw Length Calculator after adding a recess.

Setting a Countersink Depth Stop or Gauge

A microstop cage sets depth in steps of 0.001 in, as Cutting Tool Engineering describes. The last card in the Countersink Depth Calculator shows what one step does. At 82°, each 0.001 in of depth changes the diameter by 0.0017 in. At 100° it is 0.0024 in.

The same card shows the effect of the tool angle. On the 1/4 in example, a tool that is 1° off moves the depth by 0.0028 in. That matters because the screw head angle itself carries a tolerance.

A countersink gauge reads the diameter at the surface, not the depth. Measure the diameter, then use the Countersink Depth Calculator in either direction to convert between the two. Cut a test piece first, since a worn or flat-tipped cutter cuts wider than a sharp one at the same Z.

Countersink Depth FAQ

How deep should I countersink for a 1/4-20 flat head screw?

For a socket flat head, cut the countersink to 0.531 in diameter at 82°. That is a cone depth of 0.1616 in over a 0.25 in hole, or 0.1437 in over a 9/32 in clearance hole. The Z plunge for a sharp tool is 0.3054 in either way.

Slotted and Phillips flat head machine screws follow a different standard and can have a different head diameter. Measure the head across its widest point and enter it as a Custom diameter in the Countersink Depth Calculator.

Can I get the countersink depth as a PDF?

Yes. Print Results opens the browser's print dialog with only the calculator on the page, showing your inputs and results. Choose Save as PDF as the printer to keep a copy. The printout includes the four cards and the Depth by Countersink Angle table for your inputs.