Clearance Hole Chart

A clearance hole lets a screw or bolt pass through one part into a tapped hole or a nut beyond it. Both standards give three sizes for each screw. Normal is the one to use unless there is a reason not to. Close is for when the parts must line up tightly or a wall is thin; the head may then need a countersink or counterbore on the entry side to seat past the fillet under it. Loose is for when the parts need room to be adjusted at assembly. ISO 273 calls the three fine, medium and coarse.

Where the numbers come from. Neither standard has a formula behind its table, so the chart holds the published sizes. Metric values are ISO 273's, as printed in four tables in Machinery's Handbook, 27th ed.: pp. 1556, 1604 and 1659 (from ANSI tables that follow ISO 273) and p. 1938 (BS 4186, an older British edition that differs at M12 to M16, where this chart follows the newer value). Inch values are ASME B18.2.8-1999's, read from reproductions of its table and pending a check against the standard itself.

The drill. Each inch hole is shown with the standard drill nearest it, reading each drill's size to three decimal places and taking a fractional drill over a letter or number drill of the same size. For every inch hole here, the rule picks the drill named beside it, as reproductions of B18.2.8 name them. Because the drills are read to three places, a drill can sit a few ten-thousandths of an inch under the minimum printed beside it (17/64, 0.2656 in, for a 0.266 in hole); that drill is the hole the table means. Metric holes are given as the size to make: BS 4186, which follows ISO 273, says its sizes were chosen to need the fewest drills (Machinery's Handbook, 27th ed., p. 1938). Above 30 mm, a round figure of ours rather than a standard's, the answer says drill or bore.

Worked example. An M6 screw at normal fit takes a 6.6 mm hole, 0.6 mm over the screw. A 1/4 in screw at close fit takes 0.266 in, drilled 17/64. Load the first with the button under the inputs.

Not covered: the plus tolerance each standard puts on these minimum sizes, counterbores and countersinks, the separate tables for socket head cap screws, tapping screws and square neck bolts, and the hole position tolerance that decides whether the parts go together at all. The tap and drill chart's close and free columns are tighter than this chart's close and normal at most sizes, and the same at a few (M10 and M18 to M24).

Reference: ISO 273, Fasteners: Clearance holes for bolts and screws; ASME B18.2.8-1999, Clearance Holes for Bolts, Screws, and Studs.