Countersink Depth Calculator

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Walk-through

How to Use This Calculator

4 steps

1

Enter the screw head diameter

Measure or look up the widest point of the flat or oval screw head and enter it, in inches, in the Screw head diameter field.

2

Choose the included angle

Select 82° (US standard flat head), 90° (metric/European standard), 100° (structural/self-tapping), or 120° (rivets/heavy duty) to match your fastener.

3

Add a below-flush offset (optional)

Leave the offset at 0 for a flush finish, or enter a positive value to recess the screw head further below the surface — useful before filling or painting over the head.

4

Read the countersink depth

The result updates instantly, showing the depth to drill or machine and the diameter the countersink reaches at that depth.

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Reference

Formula & Methodology

2 formulas

Countersink depth

Depth = (Head diameter ÷ 2) ÷ tan(Included angle ÷ 2) + Offset

A flat or oval screw head is a cone. Depth is measured straight down from the work surface to the point where the cone's diameter matches the screw head's diameter. Half the head diameter divided by the tangent of half the included angle gives the flush depth; adding a below-flush offset recesses the head further.

Diameter at depth

Diameter at depth = Head diameter + 2 × Offset × tan(Included angle ÷ 2)

When an offset is added, the countersink must be cut wider than the screw head diameter to keep the same cone angle down to the extra depth. This is the diameter of the cut at the bottom of that extra depth.

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Glossary

Key Terms Explained

6 terms

Countersink A conical recess machined or drilled into a material so a flat- or oval-head screw sits flush with (or below) the surface instead of protruding.

Included angle The full angle at the apex of the countersink cone, matching the angle of the screw head. Common values are 82° (US standard), 90° (metric), 100°, and 120°.

Flush The screw head sits exactly level with the surrounding surface once seated — no offset (offset = 0).

Head diameter The widest point of a flat- or oval-head screw, measured across the top of the head. This is the diameter the countersink must reach at flush depth.

CSK depth Shop shorthand for countersink depth — the distance from the work surface down to where the countersink reaches the screw head's diameter.

Screw head The top of the fastener that the countersink accommodates. Flat and oval heads are conical and countersink-compatible; pan, round, and truss heads are not — they sit on top of the surface instead.

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Scenarios

Real-World Examples

3 worked examples

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DIY furniture builder

1/2 in flat-head screw, 82° countersink, flush

Head diameter 0.5 in Included angle 82° Offset 0 in (flush)

Half the head diameter is 0.25 in, and half the 82° angle is 41°. Dividing 0.25 by tan(41°) ≈ 0.869 gives a countersink depth of about 0.288 in. Drilling or machining to that depth sinks the screw head exactly flush with the surface.

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Machinist comparing standard angles

Same 1/2 in head, but at 90° instead of 82°

Head diameter 0.5 in Included angle 90° Offset 0 in (flush)

At 90°, half the angle is exactly 45°, where tangent equals 1 — so depth simplifies to exactly half the head diameter divided by 2, or 0.25 in. Because 90° is a wider angle than 82°, the same head diameter needs a noticeably shallower countersink: 0.25 in versus 0.288 in.

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Cabinetmaker sinking a screw for filling

#8 screw, 82° countersink, recessed 1/16 in below flush

Head diameter 0.309 in (#8 screw) Included angle 82° Offset 0.0625 in below flush

The flush depth for a #8 screw at 82° is about 0.178 in. Adding a 0.0625 in offset to sink the head further for filling brings the total depth to about 0.240 in, and widens the countersink's diameter at that depth to about 0.418 in — wider than the screw head diameter, because the cone keeps expanding past the flush point.

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Deep Dive

Understanding the Countersink Depth Calculator

3 chapters · 3 min read

Countersinking lets a flat- or oval-head screw sit flush with, or below, a surface instead of protruding. Getting the depth right — not too shallow, not too deep — comes down to simple cone geometry: the screw head's diameter and the countersink's included angle.

How the calculation works

A countersink is a cone, and so is the underside of a flat or oval screw head. When the two match, the head sits flush once the countersink is cut deep enough that its diameter equals the head's diameter. Because the countersink is a cone, that depth is found with basic trigonometry: half the head diameter is the radius at the top of the cone, and dividing that radius by the tangent of half the included angle gives the vertical depth. A below-flush offset simply adds extra depth on top of that flush point, and because the cone keeps widening as it goes deeper, the diameter at that extra depth is larger than the screw head itself.

Choosing the right included angle

82° is the long-standing US standard for flat-head machine screws (ANSI/ASME B18.6.2) and flat-head wood screws (ANSI/ASME B18.6.1), so it's the default angle for most American hardware. 90° shows up on metric and European fasteners (ISO/DIN standards) and on some structural and aerospace applications. 100° and 120° are less common but appear on certain self-tapping screws, structural fasteners, and rivets. Using the wrong angle for a given screw leaves the head either proud of the surface (countersink too shallow or too narrow an angle) or sunk unevenly with a visible gap around the head (countersink too wide an angle).

When to add a below-flush offset

A flush finish (offset = 0) is the standard target for most fastening work. Woodworkers commonly recess screws further below flush — often 1/16 in to 1/8 in — so the head sits below the surface entirely, leaving room to fill the hole with wood putty or a plug before finishing. Because the countersink cone keeps expanding with depth, recessing further always requires cutting a wider hole than the screw head diameter alone would suggest — the Diameter at Depth stat shows exactly how much wider.

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Questions

Frequently Asked Questions

6 questions

What is the countersink depth formula?+

Depth = (Head diameter ÷ 2) ÷ tan(Included angle ÷ 2), plus any below-flush offset you add. It comes directly from the cone geometry of a screw head sinking into a matching conical countersink.

82° vs 90° countersink — which should I use?+

82° is the US standard included angle for flat-head machine screws and wood screws. 90° is common on metric and European fasteners, and on some structural and aerospace applications. Always match the angle to your screw's actual head angle — a mismatched angle leaves the head proud or unevenly sunk.

What does "flush" mean here?+

Flush means the screw head sits exactly level with the surrounding surface once seated — an offset of 0. That's this calculator's default unless you enter a below-flush offset.

How do I sink a screw below flush?+

Enter a positive value in the offset field. That amount is added directly to the flush depth, and the calculator also shows the wider diameter the countersink reaches at that extra depth.

What are common screw head diameters?+

For flat-head machine screws: #4 ≈ 0.211 in, #6 ≈ 0.26 in, #8 ≈ 0.309 in, #10 ≈ 0.359 in, #12 ≈ 0.41 in, and 1/4 in ≈ 0.492 in. The By Screw Size tab lists these with the depth pre-computed at your chosen angle, and lets you apply one with a click.

What units does this calculator use?+

Inches by default — but the formula is a pure ratio with no unit conversion baked in, so entering millimeters for the head diameter and offset returns the depth in millimeters too. Just stay consistent with one unit at a time.

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