Drill Point & Blind-Hole Depth Calculator

How to use it: enter your drill's diameter, its point angle (the included angle at the tip - commonly 118° for standard jobber drills or 135° for split-point drills), and how deep you need the full-diameter (cylindrical) part of a blind hole to be. The tool tells you how deep to actually drive the drill.

Formula. A twist drill's point is a cone. Its axial length - the "tip length" - is set by the diameter and the point angle:

Tip length = (diameter ÷ 2) ÷ tan(point angle ÷ 2)

Because that cone has to fully form before the hole reaches full diameter, a blind hole is always deeper at the drill's centerline than it is at full width. To get a given depth of usable, full-diameter hole, you have to drive the drill that depth plus the tip length further:

Total drill depth = desired full-diameter depth + tip length

Worked example. With the defaults (10 mm diameter, 118° point, 15 mm desired full-diameter depth): tip length = (10 ÷ 2) ÷ tan(59°) = 5 ÷ 1.6643 ≈ 3.00 mm. Total drill depth = 15 + 3.00 = 18.00 mm. If you only drilled to 15 mm, the last ~3 mm of the hole would still be tapering to a point rather than full diameter - a common cause of a blind hole coming up just short of a bolt or dowel's seated depth.

The result above uses these exact values. This snapshot is included when the page is printed so the output can be checked against the original measurements.

Drill diameter

10 mm

Point angle

118 deg

Desired full-diameter depth

15 mm

Primary result: Total drill depth: 18.00 mm.

• Measure from the datum or reference edge described by this tool, and do not mix inside, outside and centerline dimensions.
• Keep inputs in the displayed units and preserve more precision than the final cutting or purchasing tolerance requires.
• When the result is close to a limit, verify it with a test piece, field measurement, manufacturer drawing or qualified project professional.

This assumes a standard conical drill point with a straight included angle, which covers the great majority of twist drills. It does not model a split-point's small secondary facets (their effect on tip length is minor), a brad-point/spur bit's centering spur, a Forstner bit's flat-bottom geometry, or a step drill.

Real drilling depth also depends on how you're measuring - from the drill flutes, a depth stop, a drill press quill scale, or a piece of tape on the bit - and on bit runout and workpiece flatness. Treat the result as the geometric target, and sneak up on a critical depth with test holes.