How to use this calculator
- Enter drill diameter. Use the drill body diameter, not the hole tolerance.
- Enter point angle. Use the included point angle, such as 118 degrees or 135 degrees.
- Enter full-depth requirement. Enter the depth that must be full diameter, or 0 if you only need the point length.
- Read total depth. Add the point length to the full-diameter depth for blind-hole programming.
How it works
A drill point is a cone. If the drill diameter is D and the included
point angle is A, the right triangle through the drill centreline has
opposite side D/2 and angle A/2, so:
L_p = (D / 2) / tan(A / 2)
This is the axial length from the drill tip to the point where the drill reaches full
diameter.
For a blind hole with a required cylindrical depth H, program or inspect
to H + L_p if the conical bottom can extend beyond the full-diameter
requirement. A larger point angle makes the cone flatter and shorter; a smaller point
angle makes it longer.
Worked example
Verified against the live calculator
A 10 mm drill with a standard 118° point has half angle
59°. The point length is
L_p = 5 / tan(59°) = 3.00 mm. If the drawing needs
20 mm of full-diameter depth, drill to about
20 + 3.00 = 23.00 mm from the surface.
With a 135° point on the same 10 mm drill, the allowance drops to about
2.07 mm. That difference matters on blind holes with tight depth callouts.
Frequently asked questions
How do I calculate drill point length?
For a drill with diameter D and included point angle A, the axial point length is Lp = (D/2)/tan(A/2). A 118 degree drill has a point length of about 0.300D, while a 135 degree drill is shorter at about 0.207D.
Why do I need drill point length?
For a blind hole, the bottom is conical. If the drawing calls for a full-diameter depth, you must drill deeper by the point length so the cylindrical part of the drill reaches that depth.
What point angle should I use?
Use the actual included point angle of the drill. General-purpose twist drills are often 118 degrees; many cobalt, split-point and harder-material drills use 135 degrees. Spot drills and center drills can be different.
Is this the same as countersink depth?
The geometry is similar, but the use is different. Drill point length is the conical tip allowance below a drilled hole. Countersink depth controls a conical chamfer from a top diameter and existing hole diameter.
Does this account for split points or tip flats?
No. It assumes a sharp conical point. A split point, web thinning, chisel edge, tip flat or regrind changes the real bottom geometry slightly, so use the actual tool print for tight depths.
Does this work in inches?
Yes. Toggle to imperial and enter drill diameter and full-diameter depth in inches. The angle remains degrees.
Method & assumptions
- The drill point is modeled as a sharp, centered cone of the entered included angle.
- Point length is measured axially from the tip to full diameter.
- Split points, chisel edges, tip flats, drill wear, regrinds, wander and breakthrough burrs are not included.
- For controlled hole bottoms or flat-bottom holes, use a boring bar, end mill, flat-bottom drill or vendor tool print rather than this geometry alone.