Countersink Depth Calculator

1. Measure the screw head diameter across its widest point with calipers or from the manufacturer data sheet.
2. Enter the pilot or clearance hole diameter you have drilled or plan to drill.
3. Enter the countersink included angle: 82 degrees for US flat-head screws, 90 degrees for ISO metric, or 100 degrees for aerospace fasteners.
4. Read the countersink depth and set your countersink bit or stop collar to that depth.

Here is what each result means:

A countersink is a cone-shaped recess cut into a material surface so that a flat-head screw or bolt sits flush with or slightly below the surface. The cone angle of the recess matches the angle of the screw head, creating full contact between the head and the material.

Countersinking is used in woodworking, metalworking, cabinetry, furniture building and sheet-metal assembly. Without a countersink, a flat-head screw would sit proud of the surface, leaving a bump that interferes with mating parts, finishes or safety.

The depth of the countersink determines whether the head sits flush, proud or below the surface. This calculator finds the exact depth for a flush seat.

The calculator models the countersink as a truncated cone. The screw head is a cone with a known diameter and included angle. The depth of the cone from the surface down to where it meets the pilot hole is the countersink depth.

Where:

• h = countersink depth (the result)
• D = screw head diameter (widest point)
• d = pilot or clearance hole diameter
• angle = included angle of the countersink (82, 90 or 100 degrees)

1. Subtract the hole diameter from the head diameter to get the radial difference.
2. Divide the included angle by two and take the tangent of that half-angle.
3. Divide the diameter difference by twice that tangent to get the depth.

For an 82-degree countersink, the factor is about 0.575, so the depth is roughly 57.5% of the diameter difference. For 90 degrees, the factor is exactly 0.5.

Find the countersink depth for a #8 flat-head screw with an 82-degree countersink.

Given: Head diameter D = 0.332 in, clearance hole d = 0.164 in, included angle = 82 degrees.

Step 1. Diameter difference = 0.332 – 0.164 = 0.168 in.

Step 2. Half-angle = 82 / 2 = 41 degrees.

Step 3. tan(41 degrees) = 0.8693.

Step 4. Depth = 0.168 / (2 x 0.8693) = 0.168 / 1.739 = 0.097 in.

Countersink to 0.097 in (about 3/32 in) for the #8 screw head to sit flush with the surface.

Check: The angle factor for 82 degrees is 1 / (2 x 0.8693) = 0.5752. Multiplying: 0.168 x 0.5752 = 0.097 in. This confirms the result.

Countersink Angle Mismatch

Using an 82-degree bit on a 90-degree screw (or vice versa) creates a ring contact instead of full cone contact. The head either sits too high or too deep. Always match the countersink angle to the screw head angle.

Material Hardness

Hardwood, steel and stainless steel require slower feed rates and sharper bits. A dull bit in hard material chatters and overcuts, leaving a ragged, oversized countersink. Softwood and sheet metal cut faster but can tear out around the rim.

Bit Wear

A worn countersink bit cuts a slightly shallower cone at the same depth setting because the cutting edge rounds off. Check the bit periodically and replace it when the edges show visible wear.

Hole Diameter Accuracy

If the pilot hole is slightly oversized, the countersink needs to go deeper for the head to reach flush. Measure the actual drilled hole with a pin gauge rather than relying on the nominal drill size.

Desired Finish

For a perfectly flush seat, use the calculated depth exactly. For a slightly recessed head (common in furniture to accept a wood plug), add 1/32 to 1/16 inch to the depth.

Three common hole preparations for fasteners serve different purposes:

If you need the screw head hidden below the surface and capped with a plug, use a counterbore instead of a countersink.

Flush-mount Woodworking

Cabinet doors, drawer fronts, furniture panels and trim where screw heads must sit flush for a clean look or to accept wood filler.

Sheet Metal Assembly

Thin sheet metal with flat-head machine screws that must not protrude past the surface.

Furniture and Cabinetry

Hinge installation, drawer slides and any joint where a protruding head would interfere with a moving part or a mating surface.

Deck and Framing Hardware

Structural screws with flat heads used in joist hangers, ledger boards and stair stringers where code requires a flush surface.

1. Using the Wrong Countersink Angle

An 82-degree bit on a 90-degree screw leaves the head sitting high because the cone is steeper than the screw. Always check whether your screws are ASME (82 degrees) or ISO (90 degrees) before selecting the bit.

2. Measuring the Head Diameter Incorrectly

The head diameter is the widest point across the flat part of the screw head, not the top or the shank. Use dial calipers for an accurate measurement. A 0.010 in error in head diameter changes the depth by about 0.005 in.

3. Countersinking Too Deep

Over-countersinking creates a gap around the head and reduces the bearing area. The head loses clamping force and the joint weakens. Use a stop collar on the bit to limit the depth.

4. Countersinking Too Shallow

If the head sits proud, it interferes with mating surfaces, finishes and safety. Test-fit a screw in a scrap piece first and adjust the depth before drilling the workpiece.

5. Ignoring the Pilot Hole Diameter

The formula requires the actual hole diameter, not just the screw shank size. A clearance hole is larger than the shank, which reduces the countersink depth compared to a tight pilot hole.

The calculator uses the exact trigonometric formula for a perfect cone. Real countersink bits may have slight profile variations, but the formula is accurate to within 0.002 in for sharp, well-made bits.

Most US flat-head wood screws have an 82-degree head angle per ASME B18.6.3. Use an 82-degree countersink bit to match. If you are using metric screws, use a 90-degree bit.

For a #8 flat-head screw (head diameter 0.332 in) with a 0.164 in clearance hole and an 82-degree countersink, the depth is about 0.097 in, or roughly 3/32 in.

A countersink is a cone-shaped recess for a flat-head screw to sit flush. A counterbore is a cylindrical flat-bottom hole for a socket-head cap screw to sit below the surface.

It will work, but the fit is poor. The shallower 90-degree cone contacts only the rim of the 82-degree head, reducing clamping area and leaving a slight gap. Match the angles for a full-contact seat.

Use dial or digital calipers across the widest point of the flat head. Do not measure the top surface or the shank. The widest diameter is where the underside cone meets the top face.

Add 1/32 to 1/16 in to the calculated depth. This recesses the head so you can fill over it with a wood plug, filler or cap.

Yes. Enter the head diameter and hole diameter in the same unit (millimeters or inches) and set the angle to 90 degrees for ISO metric flat heads. The result is in the same unit.

Use a countersink bit with an adjustable stop collar. Set the collar so the bit reaches the calculated depth, then drill each hole to the stop. This prevents over-countersinking.

Chatter happens when the bit is dull, the feed rate is too fast, or the workpiece is not clamped firmly. Slow down, sharpen or replace the bit, and clamp the work to a solid surface.

No. The calculation runs entirely in your browser and nothing you enter is stored or sent anywhere.

• OmniCalculator, Countersink Depth Calculator (countersink depth formula and worked examples).
• BaetrocCNC, Countersink Size Chart for Flat Head Screws (82, 90 and 100 degree countersink dimensions by screw size).
• EasyProTools, Countersink Depth Calculator (online countersink depth solver with angle factor table).