Clearance Hole Calculator

The clearance hole calculator helps you to quickly calculate the diameter of a hole through an object or substrate for a bolt or a screw using its thread and head diameters.

Normal-fit clearance hole0.28 in7.112 mm — the usual choice

Close fit0.265 infor accurate location, harder to assemble

Loose fit0.3 infor adjustment or where alignment is poor

Bolt diameter0.25 in6.35 mm

Clearance at normal fit0.03 in total0.015 in all round

Minimum edge distance0.375 ina common rule: 1.5 diameters from the edge

Minimum spacing between holes0.75 inthree diameters centre to centre

Washer outside diameter0.55 inroughly, for a standard flat washer

The formula

countersink depth = (Dcs − Dhole) ÷ (2 tan(angle ÷ 2))

Four hole problems, one page

A countersink’s depth follows from simple trigonometry: the cone narrows from the countersink diameter to the hole diameter, and how fast depends on the half angle. Note that 82° is the imperial standard for flat-head screws while metric uses 90° — mixing them leaves the head either proud or sunk.

True position is the one people get wrong most often. GD&T expresses it as a diameter, so the deviation is twice the radial error: being 0.003 out in x and 0.004 in y is a 0.005 radial error but a 0.010 true position deviation.

The clearance and rivet figures are conventions rather than laws, but they are the conventions drawings assume: 1.5 diameters to an edge, 3 diameters between fasteners.