Calculator Input
Use the responsive calculator grid below. Large screens show three columns, smaller screens show two, and mobile shows one.
Formula Used
The calculator uses standard countersink geometry for a conical seat.
1. Major countersink diameter:
D = d + 2 × t × tan(A / 2)
2. Required depth for a target seat diameter:
t = (D - d) / (2 × tan(A / 2))
3. Radial bevel width:
w = (D - d) / 2
Where: D is major countersink diameter, d is pilot hole diameter, t is countersink depth, and A is included angle.
How to Use This Calculator
- Select your working unit, either millimeters or inches.
- Enter the pilot hole diameter already drilled or planned.
- Enter the countersink depth you want to test.
- Provide the included countersink angle used by your tool.
- Optionally enter the fastener head diameter and fit allowance.
- Add material thickness to check the remaining section below the seat.
- Press the calculate button to show the result above the form.
- Review the graph, fit assessment, and export the result if needed.
Example Data Table
Sample construction-focused countersink cases for quick reference and export.
| Case | Hole Diameter (mm) | Angle (°) | Depth (mm) | Major Diameter (mm) | Bevel Width (mm) |
|---|---|---|---|---|---|
| Example 1 | 6 | 82 | 1.3 | 8.2601 | 1.1301 |
| Example 2 | 8 | 90 | 1.8 | 11.6 | 1.8 |
| Example 3 | 10 | 100 | 2.2 | 15.2437 | 2.6219 |
| Example 4 | 12 | 110 | 2.7 | 19.712 | 3.856 |
Frequently Asked Questions
1. What does a countersink size calculator measure?
It estimates the mouth diameter, bevel width, and depth relationship for a conical fastener seat. This helps builders size holes for flush hardware and cleaner finishes.
2. Why is the countersink angle important?
The angle controls how quickly the countersink widens with depth. A different angle changes the major diameter, seating contact, and how the fastener head fits the surface.
3. What is the major countersink diameter?
It is the top opening diameter at the surface after the countersink is cut. It is usually the key size used to match a screw or anchor head.
4. Why add fit allowance?
Fit allowance gives extra clearance beyond the nominal head size. It helps account for coating, finish buildup, field tolerance, and easier seating during installation.
5. Can I use this calculator for inch measurements?
Yes. Change the unit selector to inches and enter all values consistently in inches. The formulas remain the same because the geometry is scale independent.
6. What does remaining thickness tell me?
Remaining thickness shows how much material stays below the countersink. Low values can indicate breakout risk, weaker support, or a need to reduce depth.
7. How can I reduce chatter while countersinking?
Use a sharp tool, stable clamping, correct speed, and controlled feed. Pilot accuracy and clean material support also improve surface finish and seating consistency.
8. What is the difference between a countersink and a counterbore?
A countersink creates a conical seat for flat-head fasteners. A counterbore creates a flat-bottom cylindrical recess, usually for socket heads or bolts with washers.