Spot Drill Depth Calculator

Inch system. Work out how deep to run a spot drill so the chamfer it leaves matches the drill that follows — from the drill diameter, the chamfer allowance you want per side, and the spot drill's tip angle.

Unit
Inch (in)
Drill the chamfer is for
The diameter of the drill that runs after the spot. Accepts decimal (0.1875) or fraction (3/16).
How far the chamfer extends past the drill edge, per side. Added to both sides, so the chamfer diameter at the surface = Ø + 2 × allowance.
Spot drill
The full included angle ground on the spot drill point. A 90° spot is the most common; 82° suits flat-head screws.
Glossary — what do these terms mean?
Spot drill (spotting / centering)
A short, stiff tool run before the real drill to make a precise cone that locates the drill point and stops it walking. Run a little deeper than the drill's own diameter and the same cone also leaves a chamfer around the finished hole — which is the case this page sizes.
Chamfer allowance (per side)
How far past the drill's edge you want the chamfer to reach, measured on one side. It's applied to both sides, so the chamfer diameter cut at the surface is Ø + 2 × allowance. With a 3/16 (.1875) drill and .005 per side, the chamfer diameter is .1875 + .010 = .1975.
Chamfer diameter (at the surface)
The diameter of the cone the spot drill cuts where it breaks the surface (Z0). It equals the drill diameter plus the allowance on each side: Ø + 2 × allowance. Its radius — half of that — is what sets the depth.
Tip angle (included angle)
The total angle ground on the spot drill point, measured between the two cutting edges. The cone the tool cuts opens at this angle, so the same depth on a wider angle leaves a wider chamfer. A 90° spot is the most common; 82° matches flat-head screws, and 120° is a common countersink angle.
Spot depth Z (the answer)
How far the spot drill tip goes below the surface to open the cone to the chamfer diameter. From the cone geometry, depth = chamfer radius / tan(angle / 2). For a 90° tip that's exactly the radius (because tan 45° = 1); a wider angle needs less depth, a narrower angle more. In a G81 line it's the negative Z: Z = −depth.
Why 90° = radius
A 90° included angle is two 45° half-angles. Going down by d opens the cone radius by d × tan 45° = d — depth and radius move one-for-one. So a .1975 chamfer (radius .09875) on a 90° spot needs exactly .09875 of depth. Any other angle scales that by 1 / tan(angle / 2).
Sample G81 spotting block
Spotting is usually a plain G81 — feed straight to the spot depth and retract, no peck. The Z on that line is exactly what this calculator sizes; every X Y after it repeats the spot at a new hole until G80 cancels. 
(--- setup ---)
G00 G90 G17 G20 G40 G49 G80 M05
T02 M06                  (load 90° spot drill)
G90 G54 S2400 M03        (spindle on)
G00 X0.0 Y0.0
G43 H02 Z1.0 M08
G00 Z.1

(--- G81 spot cycle ---)
G81 G99 Z-.0988 R.050 F9.0   (feed to spot depth)
X1.0  Y0.0              (spot repeats at hole 2)
X2.0  Y0.0             (... hole 3)
G80                     (cancel canned cycle)

(--- clean up ---)
G00 Z.1 M09
G28 G91 Z0. M05
G90
M30

Spot depth = (Ø + 2 × allowance) / 2 / tan(angle / 2). Verify against your post-processor and tooling before running.