About this tool
This calculator gives the clearance hole — the hole a bolt or screw passes through without threading — for metric sizes M1.6 to M42 using the ISO 273 fine, medium, and coarse series, and for inch sizes #0 to 1-1/2" using the close- and normal-fit drills in the ASME socket screw tables. It also suggests the nearest drill in the other measurement system, rounded up, and the counterbore for a socket head cap screw.
How to use
- Choose Metric (ISO 273) or Inch (ASME).
- Select the bolt or screw size, such as M6 or 1/4".
- Pick the fit. Medium (metric) or normal (inch) suits most joints; choose fine or close only when hole positions are accurate.
- Read the hole or drill size, the clearance it leaves, and the nearest drill in the other system if you do not have the exact size.
- For a socket head cap screw, read the counterbore diameter and depth, or use the chart below to compare every size.
Fine, medium, and coarse clearance holes
ISO 273 gives three clearance hole series for metric bolts and screws. Fine leaves the least play and suits parts located accurately, often by dowels. Medium is the general-purpose choice. Coarse allows the most positional error, for example in fabricated steel or when several holes must line up. For an M6 bolt the three holes are 6.4, 6.6, and 7.0 mm; for M8 they are 8.4, 9.0, and 10.0 mm. The standard assigns tolerance classes H12, H13, and H14 to the fine, medium, and coarse series.
Close and normal fit for inch screws
For inch socket screws, the ASME tables give a close-fit and a normal-fit drill. Close fit is meant for a single screw, or for holes produced together at assembly or by matched tooling. Normal fit allows for longer screws and for holes made to ordinary tolerances in assemblies with two or more screws. A 1/4" screw takes a 17/64" drill for close fit and a 9/32" drill for normal fit; a #10 screw takes a #5 or a #2 drill.
Drilling with the other system’s bits
If you do not have the exact drill, the tool suggests the smallest drill in the other system that is at least as large as the hole, so the bolt still passes. For a 6.6 mm medium hole for M6, that is a letter G drill (0.261", 6.629 mm). For a 1/4" close-fit hole (17/64", 6.747 mm), it is a 6.8 mm drill. Rounding up keeps the clearance; rounding down can leave a hole the bolt will not enter.
Counterbores for socket head cap screws
A counterbore lets a socket head cap screw sit flush or below the surface. The tool gives the counterbore diameter from the ASME socket screw tables and a minimum depth equal to the head height, which for a standard cap screw equals its nominal diameter — 6 mm for M6, 1/4" for a 1/4" screw. Add a little depth if the head must sit below the surface. Metric counterbore diameters vary between standards, so check your drawing if it specifies DIN or ISO counterbores.
Privacy
The sizes you select are processed in your browser. This tool does not send them to a server or store them after you close or reload the page.
Continue this workflow
Use the adjacent tool when the next step calls for a different input, output, or method.
Assumptions and limitations
- Metric holes cover M1.6 to M42 from ISO 273; inch holes cover #0 to 1-1/2" from the ASME socket screw drill tables.
- Hole sizes are nominal. Positional tolerance, plating thickness, washers, and slotted or oversized holes are not calculated.
- Counterbore diameters are for socket head cap screws only and are not listed for every metric size. Other head styles and other counterbore standards use different sizes.
- This tool does not give tap drill sizes. For a threaded hole, use the tap drill calculator instead.
Sources and standards
These authoritative references were used to verify the method and guidance on this page.
- ISO 273:1979 — Fasteners — Clearance holes for bolts and screws — International Organization for Standardization
- Clearance Hole Sizes (ISO 273) — Engineering Hardware
- Clearance Hole Chart: Metric & Inch Drill Sizes — ISO CNC
- UNBRAKO Engineering Guide — drill and counterbore sizes — Unbrako (copy hosted by the University of Arizona)
Frequently asked questions
What size clearance hole do I need for an M6 bolt?
6.6 mm for a normal (medium) fit. Use 6.4 mm for a close (fine) fit or 7.0 mm for a loose (coarse) fit.
What size hole should I drill for an M8 bolt?
9.0 mm for a medium fit, 8.4 mm for a fine fit, or 10.0 mm for a coarse fit. M8 jumps by more than smaller sizes because ISO 273 steps the clearance up at this size.
What drill size is a clearance hole for a 1/4" bolt?
A 17/64" drill (0.266") for a close fit, or a 9/32" drill (0.281") for a normal fit.
Which fit should I use?
Use medium (metric) or normal (inch) unless you have a reason not to. Choose fine or close only when the holes are positioned accurately, and coarse when hole positions may vary.
Is a clearance hole the same as a tap drill?
No. A clearance hole is larger than the bolt so it passes through freely. A tap drill is smaller than the bolt so threads can be cut into the hole.
How deep should a counterbore for a socket head cap screw be?
At least the head height, which for a standard cap screw equals its diameter — 6 mm for M6. Go a little deeper if the head must sit below the surface.