Spot Drill Depth Calculator

The formula

Depth = (target spot diameter ÷ 2) ÷ tan(included angle ÷ 2). A spot drill cuts a cone at its included angle, so the depth needed to reach a given diameter is exactly the geometry of that cone — a wider included angle (like 142°) reaches the same target diameter at a shallower depth than a narrower one (like 90°), because it flares out faster.

Why the angle you pick isn't arbitrary

A 90° spot is often chosen to match a countersink or chamfer callout directly. A 120° spot is a common general-purpose choice, wide enough to guide a subsequent twist drill without wandering. A 142° spot is chosen specifically to match a 135° split-point twist drill's own point angle, so the spot's cone and the drill's point seat together cleanly rather than the drill's point sitting inside a mismatched cone.

Spotting depth vs. drilling depth

The spot's job is usually just to give the following drill an accurate, centered starting point and prevent it from walking — it doesn't need to reach full diameter engagement if you're spotting ahead of a twist drill, only enough of a starting cone to guide the drill point. If the goal is actually a finished chamfer or countersink at a specific diameter, this depth is exactly what you want; if it's just a center-starting spot, a shallower depth than the full target diameter calculation is often all that's needed.