Counterbore Calculator
Size a counterbore so a socket head cap screw sits flush or below the surface, with clearance on the diameter.
Common SHCS counterbore sizes (close fit, approx.)
| Screw size | CB diameter | Min. CB depth |
|---|---|---|
| #4-40 | 0.164" | 0.120" |
| #6-32 | 0.192" | 0.149" |
| #8-32 | 0.219" | 0.170" |
| #10-24 | 0.246" | 0.195" |
| 1/4-20 | 0.312" | 0.250" |
| 5/16-18 | 0.375" | 0.312" |
| 3/8-16 | 0.438" | 0.375" |
| 1/2-13 | 0.562" | 0.500" |
Why the diameter needs clearance, not a snug fit
A counterbore sized exactly to the screw head diameter will bind or gall as the screw seats — a close fit (about 0.016" over) is the standard choice when centering matters, like a dowel-pinned assembly, while a free fit (about 0.031" over) is easier to drop a screw into and more forgiving of hole location.
Depth is head height plus a margin
Cutting the counterbore exactly as deep as the head height leaves the screw flush with the surface at best, with no room for stack-up tolerance to still land flush. A small additional margin (around 0.02" beyond head height) reliably gets the head sitting at or slightly below the surface instead of proud of it.
This isn't the same as a spot face
A spot face just cleans up a rough casting or weldment surface around a hole to give a bolt head or washer flat, uniform bearing area — it doesn't need to fully bury the fastener head the way a counterbore does. Don't confuse the two callouts on a print.