Calculate Countersink & Counterbore Dimensions
Calculate countersink diameters, angles, depths, and counterbore dimensions for metric and imperial fasteners. Check screw head clearance, flush or recessed seating, and remaining material thickness to help prevent assembly interference and machining errors.
Input Parameters
Select the fastener family and size. All populated dimensions remain editable for your actual supplier's drawing.
Through-hole diameter, not a tap-drill diameter.
For ISO 10642, the theoretical sharp diameter is used for seating geometry.
Reference height for flat heads; maximum height for socket heads.
Zero means theoretically flush; positive means below surface.
Total diameter allowance, not clearance per side.
Editable project criterion, not a universal standard minimum.
Reference dimensions are starting values. Confirm the exact screw family, actual head tolerances, coating, under-head fillet, cutter geometry and assembly requirements before machining.
Calculation Results
Interactive Section View
Calculation Formulas & Depth Definitions
A countersink is a conical recess. Its axial depth from the top face to the point where the cone meets the existing clearance hole is not the theoretical sharp-point depth of the cutter.
Conical depth h = (D − d) / [2 × tan(θ/2)]
Top diameter D = d + 2 × h × tan(θ/2)
Theoretical sharp-point depth = D / [2 × tan(θ/2)]Here D is the countersink opening diameter, d is the clearance-hole diameter and θ is the included cutter angle. These are ideal geometric relationships. A truncated or radiused cutter tip changes the actual machine Z coordinate.
Counterbore Ø = maximum head Ø + diametral clearance
Counterbore depth = maximum head height + desired recess
Remaining land = part thickness − recess depthThe calculator compares remaining land with your own minimum requirement. It does not verify bearing stress, fastener preload, fatigue resistance or thin-sheet structural adequacy.
Worked Examples
Metric M6 Flat-Head Countersink
With the ISO 10642 theoretical 13.44 mm head diameter, a 6.6 mm clearance hole, a 90° angle and zero nominal recess, the countersink cone depth is (13.44 − 6.6)/2 = 3.42 mm. The theoretical sharp-point depth is 13.44/2 = 6.72 mm. They refer to different datum geometries.
Inch 82° Countersink
For an opening diameter of 0.500 in and a 0.257 in hole, conical depth is approximately 0.1398 in; sharp-point depth from the top face is approximately 0.2876 in. Check tool tip shape and actual screw dimensions.
Metric M6 Socket-Head Counterbore
For an ISO 4762 M6 screw with a maximum 10 mm head diameter and 6 mm head height, a selected 1.25 mm diametral clearance and 0.5 mm head recess require an 11.25 mm counterbore diameter and 6.50 mm depth. In a 10 mm part, the nominal remaining land is 3.50 mm.
Standards & Design Considerations
- ISO 10642: hexagon socket countersunk head screws, typically 90°.
- ISO 4762: cylindrical socket head cap screws for counterbored seats.
- ISO 273: metric clearance-hole fit series.
- ASME B18.3: inch socket head cap screws and flat socket head families, typically 82° for inch flat heads.
- ASME B18.6.3: separate flat-head machine-screw family with potentially different head dimensions.
Critical distinction: ISO 10642 uses a theoretical sharp head diameter to define a flush conical seat. The physical truncated head may measure smaller. Do not substitute measured head outside diameter without checking the manufacturer's standard drawing.
Scope: provided geometry is a planning estimate, not an acceptance certificate or a structural design approval. Actual cutter, fastener, fit and inspection requirements govern final dimensions.
Reference data: Lindstrom Fasteners technical manual and clearance and counterbore dimensions.
Common Questions
What is the difference between a countersink and a counterbore?
A countersink is a conical seat for a flat head; a counterbore is a flat-bottomed cylindrical recess for a cylindrical head such as a socket head cap screw.
Is a countersink 82° or 90°?
Common inch flat socket head screws use 82°, while ISO metric countersunk socket heads typically use 90°. Some aerospace fasteners use 100°. Verify the exact screw family and drawing.
Why does screw head diameter differ from the theoretical countersink diameter?
Standards such as ISO 10642 specify a theoretical sharp envelope for the cone. The real head is truncated, so its directly measured maximum diameter may be smaller.
Can I program the calculated countersink depth directly as CNC Z?
Not automatically. Conical depth to the clearance-hole edge, theoretical pointed-tool depth and a real cutter's machine Z can differ. Account for the cutter tip, tool-length offset and actual touch-off datum.
How do I make a socket head sit slightly below flush?
Choose the counterbore depth as maximum head height plus a controlled positive recess allowance, and make sure the pocket diameter clears the head and under-head radius.
Can I use the same hole diameter as a tap drill?
No. A clearance hole allows a screw shank to pass through. A tap drill is typically smaller because it leaves stock for forming an internal thread.
Does a matching countersink angle guarantee correct assembly?
No. The actual head diameter, truncation, depth tolerance, tool geometry, hole location, coating and mating-part features also affect seating and interference.
How much material must remain below a counterbore?
There is no one universal safe residual thickness. Specify a project-specific minimum land and check bearing, preload, material and application requirements when necessary.
What if the selected screw is not in the size list?
Select Custom, enter the screw head dimensions and shaft clearance from the actual supplier drawing, then check the computed recess geometry.
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