Machining and Fastener Charts
Bolt clearance hole size.
Pick a metric bolt or an inch screw and a fit. You get the clearance hole diameter and, for inch screws, the drill size. The full metric and inch charts are below. Runs in your browser; nothing is sent or stored.
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Answer first. For an M6 bolt the clearance hole is 6.4 mm (close), 6.6 mm (normal) or 7.0 mm (loose). For an M8 it is 8.4, 9.0 or 10.0 mm. For a 1/4 inch screw the normal fit is a 17/64 inch drill.
Worked example
An M12 bolt through a plate with a normal fit: 13.5 mm, which is 1.5 mm bigger than the bolt (0.75 mm of play per side if centred). The close fit is 13 mm and the loose fit 14.5 mm.
How it works
Three fits. ISO 273 gives clearance holes in fine, medium and coarse series; the source chart labels them close (H12), normal (H13) and loose (H14). A close fit keeps parts aligned. A normal fit is the general default. A loose fit allows adjustment, slotted holes, thermal movement and misaligned hole patterns.
Clearance is not a tap drill. A clearance hole lets the bolt pass through. A tap drill leaves material for the thread to cut into and is much smaller. Use the tap drill calculator for that.
Note. This chart is for general reference. Material thickness, fastener grade, joint design and load can call for different holes, and a drawing or standard that governs your job overrides the chart.
Metric clearance hole chart (ISO 273)
| Bolt | Close (mm) | Normal (mm) | Loose (mm) |
|---|---|---|---|
| M1.6 | 1.7 | 1.8 | 2.0 |
| M2 | 2.2 | 2.4 | 2.6 |
| M2.5 | 2.7 | 2.9 | 3.1 |
| M3 | 3.2 | 3.4 | 3.6 |
| M4 | 4.3 | 4.5 | 4.8 |
| M5 | 5.3 | 5.5 | 5.8 |
| M6 | 6.4 | 6.6 | 7.0 |
| M8 | 8.4 | 9.0 | 10.0 |
| M10 | 10.5 | 11.0 | 12.0 |
| M12 | 13.0 | 13.5 | 14.5 |
| M14 | 15.0 | 15.5 | 16.5 |
| M16 | 17.0 | 17.5 | 18.5 |
| M20 | 21.0 | 22.0 | 24.0 |
| M24 | 25.0 | 26.0 | 28.0 |
| M30 | 31.0 | 33.0 | 35.0 |
Inch clearance hole chart
| Screw | Close drill | Normal drill | Loose drill |
|---|---|---|---|
| #0 | #52 (0.0635) | #50 (0.0700) | #46 (0.0810) |
| #1 | #48 (0.0760) | #46 (0.0810) | #43 (0.0890) |
| #2 | #43 (0.0890) | #41 (0.0960) | #37 (0.1040) |
| #3 | #38 (0.1015) | #36 (0.1065) | #32 (0.1160) |
| #4 | #32 (0.1160) | #30 (0.1285) | #27 (0.1440) |
| #5 | #30 (0.1285) | #29 (0.1360) | #23 (0.1540) |
| #6 | #27 (0.1440) | #25 (0.1495) | #18 (0.1695) |
| #8 | #18 (0.1695) | #15 (0.1800) | #10 (0.1935) |
| #10 | #9 (0.1960) | #5 (0.2055) | #2 (0.2210) |
| #12 | #2 (0.2210) | 7/32" (0.2344) | 1/4" (0.2500) |
| 1/4" | F (0.2570) | 17/64" (0.2656) | 9/32" (0.2812) |
| 5/16" | 21/64" (0.3281) | 11/32" (0.3438) | 3/8" (0.3750) |
| 3/8" | 25/64" (0.3906) | 13/32" (0.4062) | 7/16" (0.4375) |
| 7/16" | 29/64" (0.4531) | 15/32" (0.4688) | 1/2" (0.5000) |
| 1/2" | 33/64" (0.5156) | 17/32" (0.5312) | 9/16" (0.5625) |
| 9/16" | 37/64" (0.5781) | 19/32" (0.5938) | 5/8" (0.6250) |
| 5/8" | 41/64" (0.6406) | 21/32" (0.6562) | 11/16" (0.6875) |
| 3/4" | 49/64" (0.7656) | 25/32" (0.7812) | 13/16" (0.8125) |
| 7/8" | 57/64" (0.8906) | 29/32" (0.9062) | 15/16" (0.9375) |
| 1" | 1-1/64" (1.0156) | 1-1/32" (1.0312) | 1-1/16" (1.0625) |
Sources
- Clearance Hole Chart (PDF) - Albany County Fasteners (metric ISO 273 and inch charts) (opened 2026-10-16)
- ISO 273:1979 - Fasteners, clearance holes for bolts and screws - ISO (scope of the standard) (opened 2026-10-16)
All hole sizes in the tables and the tool come from the first page. The second is the standard's catalogue entry; it is paywalled and was not used for numbers.
FAQ
What size hole for an M6 bolt?
A normal-fit clearance hole for an M6 bolt is 6.6 mm. The close fit is 6.4 mm and the loose fit is 7.0 mm.
What size clearance hole for an M8 bolt?
A normal-fit clearance hole for an M8 bolt is 9.0 mm. The close fit is 8.4 mm and the loose fit is 10.0 mm.
What is the clearance drill for a 1/4-20 screw?
For a normal fit use 17/64 inch (0.2656 in). A close fit is a letter F drill (0.257 in) and a loose fit is 9/32 inch.
What is the difference between a clearance hole and a tap drill?
A clearance hole is larger than the bolt so it passes through. A tap drill is close to the minor diameter of the thread so the tap can cut a thread in it.
Which fit should I choose?
Normal is the usual default. Choose close for precise alignment and loose where you need adjustment or the hole pattern may not line up.