Clearance Hole Size Chart
A clearance hole lets a bolt or screw pass through the first part without biting into it — assembly, alignment and disassembly all depend on picking the right size. This chart covers every common inch screw and bolt with the close and free fit values, plus the ISO 273 metric series in fine, medium and coarse.
Inch clearance hole chart (UNC / UNF screws and bolts)
| Screw / bolt | Screw Ø | Close fit | Free fit |
|---|---|---|---|
| #2 | 0.086 in | #38 (0.1015 in) | #29 (0.136 in) |
| #4 | 0.112 in | #30 (0.1285 in) | #21 (0.159 in) |
| #6 | 0.138 in | #23 (0.154 in) | #13 (0.185 in) |
| #8 | 0.164 in | #15 (0.180 in) | #3 (0.213 in) |
| #10 | 0.190 in | #5 (0.2055 in) | B (0.238 in) |
| 1/4" | 0.250 in | 17/64" (0.266 in) | 19/64" (0.297 in) |
| 5/16" | 0.313 in | 21/64" (0.328 in) | 23/64" (0.359 in) |
| 3/8" | 0.375 in | 25/64" (0.391 in) | 27/64" (0.422 in) |
| 7/16" | 0.438 in | 29/64" (0.453 in) | 31/64" (0.484 in) |
| 1/2" | 0.500 in | 17/32" (0.531 in) | 39/64" (0.609 in) |
| 5/8" | 0.625 in | 21/32" (0.656 in) | 47/64" (0.734 in) |
| 3/4" | 0.750 in | 25/32" (0.781 in) | 29/32" (0.906 in) |
Metric clearance hole chart (ISO 273)
Three hole series are standardised for each metric thread — fine (close), medium (normal) and coarse (free). Medium is the default for most assemblies:
| Thread | Screw Ø | Fine series | Medium series | Coarse series |
|---|---|---|---|---|
| M2 | 2.0 mm | 2.2 mm | 2.4 mm | 2.6 mm |
| M2.5 | 2.5 mm | 2.7 mm | 2.9 mm | 3.1 mm |
| M3 | 3.0 mm | 3.2 mm | 3.4 mm | 3.6 mm |
| M4 | 4.0 mm | 4.3 mm | 4.5 mm | 4.8 mm |
| M5 | 5.0 mm | 5.3 mm | 5.5 mm | 5.8 mm |
| M6 | 6.0 mm | 6.4 mm | 6.6 mm | 7.0 mm |
| M8 | 8.0 mm | 8.4 mm | 9.0 mm | 10.0 mm |
| M10 | 10.0 mm | 10.5 mm | 11.0 mm | 12.0 mm |
| M12 | 12.0 mm | 13.0 mm | 13.5 mm | 14.5 mm |
| M14 | 14.0 mm | 15.0 mm | 15.5 mm | 16.5 mm |
Clearance vs tap drill — the one distinction that matters
A clearance hole is drilled larger than the fastener; a tap drill is drilled smaller. The tap drill leaves material for threads to be cut into; the clearance hole removes everything the bolt would touch, so the bolt passes through and clamps against a nut or a threaded hole beyond. Drill the wrong one of the two and either the bolt will not go through at all, or the joint has no threads to hold it. Every size page on this site shows both numbers side by side — the links in the tables take you there.
How to choose between close, normal and free fit
close fit: hole ≈ screw + 1 drill size — precise location normal/medium: hole ≈ screw + 1.5 drill sizes — default choice free/coarse fit: hole ≈ screw + 3 drill sizes — movement + tolerance
Use close fit where parts must locate precisely against each other (dowel-like function, fixtures). Use free fit where assembly speed, part tolerance stacking or thermal movement matter more than position. When in doubt between two sizes, the larger one makes assembly easier and costs nothing structurally — the clamp load does the work, not the hole.
Frequently asked questions
What is a clearance hole?
A clearance hole is a hole drilled large enough that a bolt or screw passes through it unthreaded — the fastener grips a nut or a threaded hole on the other side, never this hole itself.
What size clearance hole for a 1/4-20 bolt?
Close fit is a 17/64 in bit (0.266 in) and free fit a 19/64 in bit (0.297 in). Close fit keeps parts aligned; free fit allows adjustment and easier assembly.
What size clearance hole for an M6 bolt?
ISO 273 gives 6.4 mm (fine series), 6.6 mm (medium series) and 7.0 mm (coarse series). The medium 6.6 mm is the everyday default.
Is a clearance hole the same as a tap drill?
No — and confusing them ruins the part. A tap drill is smaller than the screw so threads can be cut into it; a clearance hole is larger so the fastener passes through freely.
What is the difference between close fit and free fit?
Close fit is roughly one drill size over the screw — precise location, alignment-critical assemblies. Free fit is roughly three drill sizes over — faster assembly, tolerance absorption, movement allowed.