Clearance Hole Size Calculator
Pick the screw, pick the fit, and get the clearance hole diameter and the drill that makes it. Inch sizes follow ASME B18.2.8 and metric sizes follow ISO 273, so the numbers match the charts on the shop wall.
How Clearance Holes Are Sized.
A clearance hole lets the screw pass through without threading into the part. The allowance is fixed by standard, not by formula: ASME B18.2.8 sets close, normal, and loose holes for inch screws, and ISO 273 sets fine, medium, and coarse holes for metric. This calculator uses those tables and shows the standard drill that produces each hole.
Normal fit covers most bolted joints. Close fit is for parts that must locate off the bolt, and it demands accurate hole positions. Loose fit absorbs stack-up across a long bolt pattern or between parts made in different shops. The badge shows the total diametral clearance, which is how much the part can shift before the bolt touches the hole wall.
Clearance Hole Chart
Inch (ASME B18.2.8)
| Size | Close | Normal | Loose |
|---|---|---|---|
| #0 | #51 (0.067) | #48 (0.076) | 3/32 (0.094) |
| #1 | #46 (0.081) | #43 (0.089) | #37 (0.104) |
| #2 | 3/32 (0.094) | #38 (0.102) | #32 (0.116) |
| #3 | #36 (0.106) | #32 (0.116) | #30 (0.128) |
| #4 | #31 (0.120) | #30 (0.128) | #27 (0.144) |
| #5 | 9/64 (0.141) | 5/32 (0.156) | 11/64 (0.172) |
| #6 | #23 (0.154) | #18 (0.170) | #13 (0.185) |
| #8 | #15 (0.180) | #9 (0.196) | #3 (0.213) |
| #10 | #7 (0.201) | #2 (0.221) | B (0.238) |
| #12 | #1 (0.228) | 1/4 (0.250) | 17/64 (0.266) |
| 1/4 | 17/64 (0.266) | 9/32 (0.281) | 19/64 (0.297) |
| 5/16 | 21/64 (0.328) | 11/32 (0.344) | 23/64 (0.359) |
| 3/8 | 25/64 (0.391) | 13/32 (0.406) | 27/64 (0.422) |
| 7/16 | 29/64 (0.453) | 15/32 (0.469) | 31/64 (0.484) |
| 1/2 | 33/64 (0.516) | 17/32 (0.531) | 39/64 (0.609) |
| 9/16 | 37/64 (0.578) | 19/32 (0.594) | 11/16 (0.688) |
| 5/8 | 41/64 (0.641) | 21/32 (0.656) | 47/64 (0.734) |
| 3/4 | 49/64 (0.766) | 13/16 (0.812) | 29/32 (0.906) |
| 7/8 | 57/64 (0.891) | 15/16 (0.938) | 1-1/32 (1.031) |
| 1 | 1-1/32 (1.031) | 1-3/32 (1.094) | 1-5/32 (1.156) |
Metric (ISO 273)
| Size | Close | Normal | Loose |
|---|---|---|---|
| M1.6 | 1.7 mm | 1.8 mm | 2.0 mm |
| M2 | 2.2 mm | 2.4 mm | 2.6 mm |
| M2.5 | 2.7 mm | 2.9 mm | 3.1 mm |
| M3 | 3.2 mm | 3.4 mm | 3.6 mm |
| M4 | 4.3 mm | 4.5 mm | 4.8 mm |
| M5 | 5.3 mm | 5.5 mm | 5.8 mm |
| M6 | 6.4 mm | 6.6 mm | 7.0 mm |
| M8 | 8.4 mm | 9.0 mm | 10.0 mm |
| M10 | 10.5 mm | 11.0 mm | 12.0 mm |
| M12 | 13.0 mm | 13.5 mm | 14.5 mm |
| M14 | 15.0 mm | 15.5 mm | 16.5 mm |
| M16 | 17.0 mm | 17.5 mm | 18.5 mm |
| M18 | 19.0 mm | 20.0 mm | 21.0 mm |
| M20 | 21.0 mm | 22.0 mm | 24.0 mm |
| M24 | 25.0 mm | 26.0 mm | 28.0 mm |
Inch holes list the standard drill with the decimal hole size in parentheses. ISO 273 calls the three metric fits fine, medium, and coarse; they are shown here as close, normal, and loose to match the inch columns.
FAQs
What size hole do I drill for a 1/4-20 bolt?
A 9/32 drill (0.281 in) for a normal clearance fit. Use 17/64 (0.266 in) for a close fit or 19/64 (0.297 in) for a loose fit.
What size hole do I drill for an M6 bolt?
6.6 mm for a medium (normal) fit. Use 6.4 mm for a fine (close) fit or 7.0 mm for a coarse (loose) fit. The nearest inch drill to 6.6 mm is 17/64 (6.75 mm).
What size hole do I drill for a 3/8-16 bolt?
A 13/32 drill (0.406 in) for a normal fit, 25/64 (0.391 in) for close, or 27/64 (0.422 in) for loose.
What size hole do I drill for a #10 screw?
A #2 drill (0.221 in) for a normal fit, #7 (0.201 in) for close, or a B drill (0.238 in) for loose.
What is the difference between close, normal, and loose fit?
They differ only in how much bigger the hole is than the screw. Close fit leaves the least room and locates the part on the bolt, so hole positions need to be accurate. Normal fit is the general-purpose choice. Loose fit gives the most room and forgives position error across long bolt patterns or between parts made separately.
What is the difference between a clearance hole and a tap drill?
A clearance hole is larger than the screw so the screw passes through. A tap drill is smaller than the screw so the tap can cut threads into it. For the same screw, the clearance hole is always the bigger of the two.
Can I use a metric drill for an inch clearance hole?
Yes. Clearance holes are not precise features, so the closest drill in either system works. Switch the drill set to metric and the calculator shows the nearest millimeter drill and the fit it gives.
Do socket head cap screws need a different clearance hole?
No. The through-hole is the same. A socket head cap screw that sits flush needs a counterbore on top of the clearance hole, which is a separate dimension.
How do I identify the screw I am drilling for?
Measure the major diameter with calipers and count threads per inch or measure the pitch in millimeters, then match it to the size list above. An Identibolt does this in one step with no measuring.
