How do I calculate countersink depth for a flat head screw?

Countersink depth is calculated from the head diameter and the countersink half-angle. For a 90° countersink: depth = head diameter / 2 × tan(45°) = head diameter / 2. For an 82° countersink: depth = head diameter / 2 × tan(41°) ≈ head diameter / 2 × 0.869.

How thick does my material need to be for a flat head cap screw to sit flush?

Your material must be at least as thick as the countersink depth plus the minimum thread engagement length. The calculator's material thickness checker will warn you if your material is too thin for the head to sit fully flush.

Metric Flat Head Cap Screw Calculator – Countersink Size Chart M3–M24 | CalcEngines

Mechanical Fastening Tools · CalcEngines

Flat Head Cap Screw Calculator

Full ISO 10642 / DIN 7991 dimensions — countersink diameter, depth, hex key, tap drill, clearance holes, and material thickness check for M3 to M24.

ISO 10642 / DIN 7991 M3 – M24 82° & 90° angles Thickness checker mm / inch toggle

Parameters

Countersink Angle

Nominal Size

Thread Pitch (mm) Coarse pitch shown first

Countersink Margin (mm) Added to head Ø for machining

Material Thickness (mm) For flush-fit thickness check

inch

Screw Dimensions

Select a size above and click Calculate.

Countersink Dimensions

⚠ Angle matters: Using the wrong countersink angle will cause the head to either sit proud of the surface (wrong shallow angle) or cut too deep leaving a gap around the head (wrong steep angle). Always match your countersink bit to the screw standard.

Drilling & Clearance

Material Thickness Checker

Material thickness: mm

Enter material thickness above.

Recommended Thread Engagement Length

Thread engagement rule: Steel into steel ≥ 1×d · Steel into aluminium ≥ 1.5×d · Steel into cast iron / plastic ≥ 2×d. For flat head screws, subtract the countersink depth from your material thickness to get the available thread engagement.

Full Reference Chart (ISO 10642 · 90° · coarse pitch)

Size	Pitch	Head Ø dk	Head H k	Hex Key s	Tap Drill	CS Ø (+ margin)	CS Depth	Clr Close	Clr Normal	Clr Large

Common Questions

What countersink angle should I use for metric flat head cap screws? +

Metric flat head cap screws per ISO 10642 and DIN 7991 use a 90° countersink angle. The 82° angle is used in North American practice under ASME B18.3. If you mix angles — using an 82° bit for a 90° screw — the head will sit proud of the surface. Always match the bit to the standard.

What is the countersink diameter for an M6 flat head cap screw? +

An M6 flat head cap screw (ISO 10642) has a head diameter of 11.0 mm. Your countersink diameter should be at least 11.0 mm plus a small machining margin (typically 0.2 mm), giving 11.2 mm. The calculator adjusts this for your specified margin.

How do I calculate countersink depth? +

Countersink depth is calculated from the head diameter and the countersink half-angle. For 90°: depth = (CS diameter) ÷ 2 × tan(45°) = CS diameter ÷ 2. For 82°: depth = (CS diameter) ÷ 2 × tan(41°) ≈ CS diameter ÷ 2 × 0.8693. The calculator does this automatically for both angles.

What is the difference between ISO 10642 and DIN 7991? +

ISO 10642 is the international standard and DIN 7991 is the German standard for hexagon socket countersunk head cap screws. They are dimensionally equivalent — a DIN 7991 screw meets ISO 10642. Both specify a 90° head angle.

How thick does my material need to be? +

Your material must be thick enough to contain the full countersink depth plus the minimum thread engagement needed. For example, an M8 flat head at 90° has a countersink depth of ~7.6 mm. If your material is only 8 mm thick, you have less than 0.5 mm for thread engagement — far too little. The Material Thickness Checker in the calculator shows this clearly with a green/amber/red indicator.