Inch Clearance Hole Size Chart

Metric Clearance Hole Size Chart

A clearance hole is an unthreaded hole intentionally larger than the fastener so the bolt, screw, or stud can pass through freely. Its purpose is to allow assembly, positional tolerance, alignment, and installation clearance. It is not intended to engage the external fastener thread.

For a nominal fastener diameter d, the clearance-hole diameter D_h must be greater than d. The difference, C = D_h minus d, represents total diametral clearance. Clearance is deliberately added; the hole should not normally equal nominal fastener diameter.

Clearance affects assembly speed, alignment tolerance, positioning accuracy, movement or play, bearing area, washer requirements, edge distance, and fabrication tolerance. Too small a hole causes difficult assembly, binding, or thread damage. Too large a hole causes unnecessary movement, lower positional accuracy, and potential bearing or detailing concerns.

Use a close fit when alignment needs to be tighter, positional play should be minimized, or parts are accurately manufactured and aligned. Close fit still means clearance, not an interference or press fit.

Use normal fit where ordinary assembly is required, manufacturing tolerances need accommodation, and no unusually tight or loose fit is necessary. This is typically the most common default, though it is not automatically correct for every application.

Loose clearing is useful when alignment is less controlled or assembly ease is more important, and where fabrication variation is larger. Larger holes can have consequences for positioning, washer bearing, and load-bearing assemblies.

ASME states that B18.2.8 covers recommended clearance holes for bolts, screws, and studs, inch fasteners from #0 through 1.5 in., and metric fasteners from M1.6 through M100, in three classes: close, normal, and loose. The standard is under Stabilized Maintenance, but ASME explicitly states the version remains in effect.

For machine screw thread designations, see the Machine Screw Size Chart.

ISO 273 specifically recognizes fine, medium, and coarse series.

Medium is a recognized standard series, not automatically the correct choice for every assembly.

ISO 273:1979 remains current after confirmation in 2024. It covers general-purpose clearance holes, uses fine, medium, and coarse series, and covers thread diameters from 1 mm to 150 mm. Importantly, ISO 273 also states that holes for special applications should be selected based on design requirements, meaning these tabulated series are general-purpose starting points, not a mandatory value for every situation.

Do not assume close equals fine, normal equals medium, or loose equals coarse as a strict one-to-one dimensional equivalence unless values are checked size by size; they are conceptually similar categories, but the standards remain separate.

An M8 coarse tap drill measures approximately 6.8 mm, while a general M8 clearance hole ranges from about 8.4 to 10 mm depending on fit. That large difference makes the concept clear. See the Tap Drill Chart for complete tapped-hole sizing.

Pilot-hole diameter depends on material, screw type, and thread geometry. See the Pilot Hole Size Chart for material-specific pilot-hole data.

AISC explicitly distinguishes these structural categories and states that holes exceeding a category limit must be treated as the next larger category. See the Bolt Hole Size Chart for the complete structural-steel hole reference.

Structural-hole geometry affects slip-critical design, bearing, washer requirements, net-section behavior, edge distance, and installation. AISC’s current standard family includes ANSI/AISC 360-22, while RCSC’s 2025 Specification for Structural Joints Using High-Strength Bolts is the current governing document for bolt installation and joint requirements. Keep only a brief structural cross-reference here and send detailed intent to the Bolt Hole Size Chart.

Washer selection may depend on hole size, bolt diameter, material, and whether the application is structural or nonstructural. See the Washer Size Chart for dimensional data; structural washer requirements are covered specifically on the Bolt Hole Size Chart rather than duplicated here.

Beyond the shank clearance itself, check head bearing diameter, washer fit, and countersink or counterbore geometry if applicable. See the Hex Bolt Size Chart for head dimensional data.

Larger clearance can compensate for hole-location variation, dimensional stackup, part misalignment, and multi-fastener assemblies. Excessive clearance, however, can reduce positional control, which is why fit selection should match the actual assembly requirement rather than defaulting to the largest available option.

In thin sheet metal, consider bearing around the hole, deformation, washer use, burrs, and hole quality; do not prescribe a universal enlargement, but pay attention to edge distance, local bearing, and larger washers or flanges where needed. In thick plate, consider drill wander, reaming, hole straightness, and alignment between stacked parts; for precision assemblies, achieving a close fit through thick stacked material can be harder without controlled machining.

Structural-specific fabrication details, such as reaming limits for AISC/RCSC connections, are covered on the Bolt Hole Size Chart.

Practical fabrication also involves actual drilled diameter, tool wear, hole roundness, positional tolerance, and burrs. A clearance-hole chart gives the nominal target size; drawing tolerances still need to be specified separately when fit is critical.

Zinc plating and hot-dip galvanizing can affect actual external diameter, assembly clearance, and friction. Do not automatically oversize holes without the applicable specification. AISC specifically states that galvanized structural bolts or members are not permitted to use holes larger than the specified structural-hole limits merely because they are galvanized, a useful caution even on this general clearance page.

ISO 273 states that holes for special applications should be selected based on design requirements rather than the general fine, medium, or coarse series. This means unusual assembly conditions, high-precision alignment, or specific project specifications may call for a clearance value outside these general tables entirely.