Countersink Depth Calculator

Countersink Depth Calculator

Use this countersink calculator to find what depth to run your countersink to get a particular countersink (chamfer) diameter, or what diameter you'll get at a given depth:

This calculator makes no assumptions about your countersink diameter (tool diameter). You must make sure the chamfer diameter is always less than the countersink's diameter.

How to Calculate Countersink Depth

A countersink is a cone, so the depth and diameter are related by the countersink's angle:

Depth = (Countersink Diameter / 2) / tan(Angle / 2)

For example, an 82 degree countersink that's 0.500" across at the surface needs the tool tip 0.250 / tan(41) = 0.2876" below the surface. That's the Z depth to program when you touch the tool's point off on the top of the part.

If the hole is already drilled, the countersink only cuts between the hole edge and the surface. For a 0.257" hole, the chamfer is (0.500 - 0.257) / 2 = 0.1215" wide on each side and 0.1215 / tan(41) = 0.1398" deep.

Countersink Angles

• 82 degrees: inch flat head screws, including flat head socket cap screws.
• 90 degrees: metric flat head screws, and the usual angle for chamfering hole edges.
• 100 degrees: flush rivets and screws in sheet metal and aircraft work.
• 60 degrees: center holes for lathe centers.
• 118 and 120 degrees: the point angles of common twist drills, handy if you're spot drilling to chamfer a hole.

Match the countersink to the screw's head angle, and look up the head diameter in the fastener maker's data so the screw sits flush.

Countersink Feeds and Speeds

Run a countersink or chamfer mill at the surface speed for its largest diameter in the cut, the same as an end mill. Our Milling Speeds and Feeds Calculator will give you a starting point. If you're tapping the hole as well, see our Tap Speeds and Feeds Calculator.