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Clearance Hole Chart — Close, Normal & Loose Fit

Clearance hole sizes for metric bolts and screws from ISO 273 in fine, medium and coarse series, with a short list of common inch shop sizes.

Ref CT-0054ASME B18.2.8ISO 273

Draft, open for review. Check every value against the governing standard and the manufacturer’s data before use.

What it is

A clearance hole lets a bolt or screw pass through a part without threading into it. The hole is larger than the fastener's nominal diameter so parts line up despite small position errors. Three fits are common:

Close fit (ISO "fine"): smallest gap, for accurate location and where hole spacing is tightly held.
Normal fit (ISO "medium"): the general-purpose choice.
Loose fit (ISO "coarse"): for parts with looser hole position tolerance or field assembly.

ISO 273 gives metric values. ASME B18.2.8 gives inch and metric clearance holes for bolts, screws and studs.

How to use it

Find the fastener size, choose the fit the design needs, and drill to that size. Check that the head or washer still bears fully on the surface; a loose-fit hole under a small head may need a washer.

Metric clearance holes (ISO 273)

Thread	Close (fine)	Normal (medium)	Loose (coarse)
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
M16	17.0 mm	17.5 mm	18.5 mm
M20	21.0 mm	22.0 mm	24.0 mm
M24	25.0 mm	26.0 mm	28.0 mm

Inch sizes

For inch fasteners, a common shop practice is close fit about 1/64 in over nominal and normal fit about 1/32 in over nominal: for example 17/64 and 9/32 for 1/4 in, 25/64 and 13/32 for 3/8 in, and 33/64 and 17/32 for 1/2 in. ASME B18.2.8 values differ in places, especially for larger and loose-fit holes, so use its tables when a drawing references it.

Key points

The drawing governs. Many drawings call out a clearance hole by size and tolerance; follow it.
Countersunk and counterbored holes have their own sizes for the head; the clearance hole is only the shank portion.
Larger holes reduce the bearing area under the head. Washers restore it.
Slotted or oversized holes in structural steel connections are governed by the structural specification, not this chart.

Common mistakes

Drilling the tap drill size when a clearance hole was needed.
Using a loose-fit hole where parts must locate on the fasteners.
Mixing metric and inch charts: an M8 hole is not interchangeable with a 5/16 in hole.

Governing standards: ISO 273 (clearance holes for bolts and screws); ASME B18.2.8 (clearance holes for bolts, screws and studs). Project drawings govern.

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.

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