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Drill Point Length Calculation
Guide · Updated October 28, 2026
A twist drill does not cut a flat-bottomed hole. The cone on the end means the full diameter starts some distance behind the tip, so for a blind hole you have to add the point length to the full-diameter depth on the drawing. The math is simple trigonometry: point length L = (D ÷ 2) ÷ tan(point angle ÷ 2).
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Step by step
- Read the depth callout: Check whether the drawing gives full-diameter depth or depth to the drill tip.
- Find the point angle: 118° is common on general-purpose HSS jobber drills; 135° split points are common on cobalt and carbide.
- Calculate the point length: L = (D ÷ 2) ÷ tan(angle ÷ 2).
- Add it to the depth: Tip depth = full-diameter depth + L.
- Verify: Measure the first hole with a depth mic or pin and adjust.
Point length factors
L = D × factor, where factor = 0.5 ÷ tan(angle ÷ 2):
- 90° → 0.500 × D
- 118° → 0.300 × D
- 120° → 0.289 × D
- 135° → 0.207 × D
- 140° → 0.182 × D
The familiar shop rule of 0.3 × D for a 118° drill comes straight from 0.5 ÷ tan 59° = 0.5 ÷ 1.6643 = 0.3004.
Worked example: 1/2 in drill, 118°
A blind hole needs 0.750 in of full-diameter depth with a 1/2 in drill. Point length = 0.250 ÷ tan 59° = 0.250 ÷ 1.6643 ≈ 0.150 in. Program or set the stop for a tip depth of 0.750 + 0.150 = 0.900 in.
With a 135° split-point drill of the same size, L = 0.250 ÷ tan 67.5° = 0.250 ÷ 2.4142 ≈ 0.104 in, so the tip depth is 0.854 in. Using the 118° number with a 135° drill would leave the hole about 0.046 in deeper than needed at full diameter, which can break through a thin floor.
Worked example: tap drill for a blind 1/4-20 hole
A #7 drill (0.201 in) at 118° has a point length of 0.201 × 0.3004 ≈ 0.060 in. If you need 0.500 in of full thread with a bottoming tap that has about 1.5 incomplete threads (1.5 × 0.050 = 0.075 in), drill to at least 0.500 + 0.075 + 0.060 = 0.635 in at the tip. Many shops add another thread or two of clearance for chips.
For a 90° spot drill or countersink, L = 0.5 × D, so a 0.500 in diameter chamfer made with a 90° spot drill sits 0.250 in below the surface at the tip.
Quick shop check
Common problems: mixing tip depth and full-diameter depth, forgetting that a re-sharpened drill may not hold its original angle, and using the 0.3 × D rule on a 135° drill. Check the angle with a drill point gage if the drill has been hand ground.
FAQ
What is drill point length?
The axial length of the cone on the drill tip: the distance from the tip to where the full diameter begins.
Why is it 0.3 × D for 118°?
Because 0.5 ÷ tan 59° ≈ 0.300.
Does CAM add it automatically?
Many CAM systems have a tip-or-shoulder depth option. Check which one is selected.
How do I check the result?
Measure the hole depth with a depth mic and a pin, or section a test hole. Gage blocks are handy for setting depth stops; build a stack free at gageblockcalc.com.
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