Bolting
Bolt hole diameter: normal and oversize holes (BS EN 1090-2 Table 11)
- where
- normal round hole diameter (mm)
- nominal bolt diameter (mm)
- nominal clearance for a normal hole (mm): 1 up to M14, 2 for M16 to M24, 3 above
- oversize hole diameter (mm)
- oversize clearance (mm): 3 up to M12, 4 for M14 to M22, 6 for M24, 8 above
Explain this equation
Gives the standard hole sizes for structural bolts, normal and oversize, to BS EN 1090-2. Use it when drilling or punching connections.
Holes need clearance so bolts fit with normal fabrication tolerances. The clearance steps up with bolt size as set out in the standard: for normal holes 1 mm up to M14, 2 mm for M16 to M24, 3 mm for M27 and above. Oversize holes allow more adjustment on site but need hardened washers and may reduce the design slip and bearing resistance.
A lookup of clearances added to the bolt diameter.
Sense check: M20 bolts go in 22 mm holes, or 24 mm oversize; M12 in 13 mm or 15 mm.
Watch out: preloaded (friction grip) connections in oversize or slotted holes need the reduction factors in the design. Check the drawing before going oversize.
Standard drilled or punched hole size for a metric bolt in steelwork, and the oversize hole that may be used where specified.
Enter your values to see the result.
Good to know
BS EN 1090-2 Table 11 nominal clearances for normal round holes are 1 mm for M12 and M14, 2 mm for M16 to M24, and 3 mm for M27 and larger, giving the familiar 13, 18, 22 and 26 mm holes for M12, M16, M20 and M24. Oversize holes are 3 mm larger for M12, 4 mm for M14 to M22, 6 mm for M24 and 8 mm for M27 and above. Oversize and slotted holes reduce bearing resistance and are only used where the designer specifies them. The standard allows the 1 mm clearance on M12/M14 to be increased by the coating thickness for coated fasteners.
Common mistake
Drilling everything 2 mm over: an M12 bolt in a 14 mm hole is an oversize hole and the Eurocode requires its shear resistance to be reduced (factor 0.85).
Limits
Round holes for bolts only (not fit bolts or holding-down bolts, which have their own rules). For towers and masts the standard reduces normal clearances by 0.5 mm. Check the execution specification.
Sources
- BS EN 1090-2:2018 Execution of steel structures – Technical requirements (Table 11: nominal hole clearances for bolts)
- Eurocode Applied: Table of design properties for metric bolts (As per ISO 898-1; d0 = d + EN 1090-2 Table 11 clearance; M12 d0 13, M16 18, M20 22, M24 26 mm)
- New Steel Construction AD 413: Shear resistances of M12 bolts (13 mm clearance hole for M12 to BS EN 1090)
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow the welding procedure specification (WPS), BS EN ISO 15614/9606, BS EN 1090, BS EN 1993 and manufacturers’ data. Structural welds need qualified welders and engineered designs.