Clearance Hole Calculator

A clearance hole is deliberately oversized so the fastener passes through the joined part without engaging any threads, letting the head and nut do all of the clamping. The three standard series in ISO 273 are named fine, medium, and coarse, and are commonly called close, normal, and loose. For an M10 bolt the tabulated holes are 10.5, 11, and 12 mm, corresponding to multipliers of roughly 1.05, 1.10, and 1.20 on nominal diameter. The simple proportional model used here reproduces that behaviour with factors of about 1.06, 1.12, and 1.25, though real standard tables are stepped to convenient drill sizes rather than following a smooth ratio, and the proportional oversize generally shrinks on larger diameters. Clearance buys assembly tolerance, and that is its entire purpose: it absorbs hole position error, thermal movement, and weld distortion between mating parts. The costs are specific and worth understanding. In a shear loaded joint, radial clearance is slip that must be taken up before the bolt bears against the hole wall, which is why structural steel connections designed in bearing accept the movement while slip-critical joints rely on friction from preload instead. Excess clearance can also leave the head or nut face partly unsupported, which is why hardened washers are required over oversized and slotted holes. Bearing stress on the hole wall rises as the contact arc shrinks, so choosing a loose fit in a heavily loaded plate is not free.

Clearance holes only make sense in a world of interchangeable parts, and that world was built during the nineteenth century. Joseph Whitworth proposed a uniform screw thread system to the Institution of Civil Engineers in 1841, and William Sellers presented the American counterpart to the Franklin Institute in 1864. Standard threads meant standard bolt diameters, and standard bolt diameters meant the hole a bolt passes through could be tabulated once rather than negotiated drawing by drawing. Formal hole tables arrived with the twentieth century standards bodies. Britain, the United States, and Canada unified inch threads in the Unified Thread Standard of 1948, and ISO metric threads followed through ISO 68 and ISO 261. ISO 273, covering clearance holes for bolts and screws, was published in 1979 and set the fine, medium, and coarse series still in use, keyed directly to nominal fastener size rather than computed from any formula. ASME B18.2.8 provides the parallel inch series for bolts, screws, and studs. Structural steel practice diverged deliberately: American steel specifications define their own categories of standard, oversized, short slotted, and long slotted holes, with a standard hole generally 1/16 inch larger than the bolt for common sizes, because erection tolerances achievable in the field are far coarser than those of a machine shop.