A clearance hole is the hole a screw or bolt passes through, as opposed to the smaller tap drill hole it threads into. Pick the fastener and the fit, and this calculator gives the drill for a close, normal or loose fit, with the decimal size, the millimetre size and the gap left around the shank. It covers numbered machine screws from #0 to #12, fractional bolts from 1/4 in to 1 in, and metric sizes from M1.6 to M30 to ISO 273.
Clearance hole size calculator
How it works
Clearance holes come from tables, not a formula. The three fits trade accuracy against ease of assembly:
- Close fit: the smallest practical hole. Parts locate accurately, but the holes in both parts must line up almost exactly.
- Normal fit: the general-purpose choice for most assemblies, and the one to use if you are not sure.
- Loose fit: room to adjust the parts or to cope with holes drilled by hand that do not quite line up.
Metric sizes follow the fine, medium and coarse series of ISO 273 (close, normal and loose here). Inch sizes use the standard number, letter and fractional drills listed for each fit. The gap figure is (hole − nominal diameter) ÷ 2, the clearance on each side when the bolt is centred.
Worked example
A 1/4 in bolt through a bracket, normal fit:
- Screw or bolt size: 1/4" (bolt)
- Fit: Normal fit (general use)
The calculator gives:
- Clearance hole, normal fit: 17/64" drill (0.2656 in, 6.75 mm)
- Close fit: F drill (0.257 in, 6.53 mm)
- Loose fit: 9/32" drill (0.2812 in, 7.14 mm)
- Gap around the shank (each side): 0.0078 in (0.2 mm)
- Nominal diameter: 0.25 in
Drill 17/64 in (0.2656 in). A close fit would be an F drill (0.257 in), and a loose fit 9/32 in (0.2812 in). At normal fit there is under 0.01 in of clearance each side, so two parts drilled separately still need to be laid out carefully; if they are drilled together, the close-fit F drill gives the most accurate location. The same bolt threaded into a tapped part needs a #7 tap drill instead.
Inch clearance hole chart
| Size | Close fit | Normal fit | Loose fit |
|---|---|---|---|
| #0 | #52 (0.0635) | #50 (0.07) | #46 (0.081) |
| #1 | #48 (0.076) | #46 (0.081) | #43 (0.089) |
| #2 | #43 (0.089) | #41 (0.096) | #37 (0.104) |
| #3 | #38 (0.1015) | #36 (0.1065) | #32 (0.116) |
| #4 | #32 (0.116) | #30 (0.1285) | #27 (0.144) |
| #5 | #30 (0.1285) | #29 (0.136) | #23 (0.154) |
| #6 | #27 (0.144) | #25 (0.1495) | #18 (0.1695) |
| #8 | #18 (0.1695) | #15 (0.18) | #10 (0.1935) |
| #10 | #9 (0.196) | #5 (0.2055) | #2 (0.221) |
| #12 | #2 (0.221) | 7/32" (0.2344) | 1/4" (0.25) |
| 1/4" | F (0.257) | 17/64" (0.2656) | 9/32" (0.2812) |
| 5/16" | 21/64" (0.3281) | 11/32" (0.3438) | 3/8" (0.375) |
| 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" (0.5) |
| 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" (0.625) |
| 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) |
Metric clearance hole chart (ISO 273)
| Size | Close (fine) | Normal (medium) | Loose (coarse) |
|---|---|---|---|
| M1.6 | 1.7 mm | 1.8 mm | 2 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 mm |
| M8 | 8.4 mm | 9 mm | 10 mm |
| M10 | 10.5 mm | 11 mm | 12 mm |
| M12 | 13 mm | 13.5 mm | 14.5 mm |
| M14 | 15 mm | 15.5 mm | 16.5 mm |
| M16 | 17 mm | 17.5 mm | 18.5 mm |
| M20 | 21 mm | 22 mm | 24 mm |
| M24 | 25 mm | 26 mm | 28 mm |
| M30 | 31 mm | 33 mm | 35 mm |
Notes and limits
- A clearance hole lets the fastener pass freely so the head clamps the part. If the screw threads into the first part as well, the two parts cannot pull together.
- For a countersunk or counterbored head, the clearance hole is only the shank part: size the countersink or counterbore for the head separately.
- In wood, drill clearance holes for wood screws to the shank size (see the screw and nail size calculator); these machine-screw tables are for metal, plastic and bolted wood joints.
- Real drills cut slightly oversize, especially in a hand drill, so a close fit drilled by hand often behaves like a normal fit.
- Published charts differ by a drill size here and there for some inch sizes. When a drawing or a supplier gives a hole size, use that one.
Frequently asked questions
What size clearance hole for a 1/4 bolt?
17/64 in (0.2656 in) for a normal fit. An F drill (0.257 in) gives a close fit and 9/32 in (0.2812 in) a loose fit.
What is the clearance hole for an M6 bolt?
6.6 mm for a normal (medium) fit under ISO 273. Close fit is 6.4 mm and loose fit is 7 mm.
What size hole for a #10 screw to pass through?
A #5 drill (0.2055 in) for a normal fit, a #9 (0.196 in) for a close fit or a #2 (0.221 in) for a loose fit.
What is the clearance hole for M8?
9 mm normal fit, 8.4 mm close fit and 10 mm loose fit (ISO 273).
What is the difference between a tap drill and a clearance hole?
A tap drill is smaller than the screw so the tap can cut threads in the wall of the hole. A clearance hole is larger than the screw so it passes through without threading.
Sources
- Albany County Fasteners, Clearance Hole Chart (PDF): close, normal and loose fit drills for #0 to 1 in screws and bolts, and ISO 273 metric clearance holes M1.6 to M30
- ISO 273:1979, Fasteners: Clearance holes for bolts and screws (fine, medium and coarse series)
- Optimas, Recommended Tapping Drill and Clearance Hole Sizes: ISO metric coarse and fine threads, tapping drill = nominal size minus pitch
- ASME B18.2.8, Clearance Holes for Bolts, Screws and Studs
Last reviewed: 16 October 2026. The calculation is covered by automated tests with known answers taken from the sources above. Found an error? Please let us know.