General-Purpose Inch Bolt Clearance Hole Chart

These are general-purpose clearance-hole categories, not structural-steel bolt-hole dimensions.

Structural Steel Bolt Hole Chart

These dimensions come from the current AISC structural-hole table framework (Table J3.3). Note that the standard hole for a 1-in. bolt is 1-1/16 in., not a simple bolt-plus-1/8-in. shortcut.

Metric Bolt Clearance Hole Chart

ISO 273 remains current and covers general-purpose clearance holes for metric threads from 1 mm to 150 mm using fine, medium, and coarse hole series.

A clearance hole is intentionally larger than the nominal bolt diameter so the bolt can pass through without threading into the hole. The hole must provide enough clearance for insertion, alignment, and fabrication tolerance during assembly, without becoming unnecessarily large.

A 3/4-in. bolt does not ordinarily use an exactly 3/4-in. clearance hole. In structural steel, the standard hole is 13/16 in. for a 3/4-in. bolt. Nominal bolt diameter does not equal clearance-hole diameter. See the Bolt Diameter Chart for the complete bolt dimensional reference.

ASME B18.2.8 specifically recognizes these three classes of clearing.

ASME states B18.2.8 covers recommended clearance holes for #0 through 1-1/2-in. inch fasteners and M1.6 through M100 metric fasteners, using close, normal, and loose fits. ASME’s metric coverage does not replace ISO 273 where ISO conventions are specifically required.

Standard structural holes for these common sizes add 1/16 in. over nominal bolt diameter, but this is not a formula that extends unchanged to every larger bolt size or hole category.

Oversized holes are a recognized structural-hole category, but their use affects connection design, washers, slip behavior, and pretension requirements.

Do not imply slots may be oriented arbitrarily in every connection; slot orientation is a specific design detail.

AISC’s guidance confirms that if actual hole dimensions exceed one category’s maximum, the connection must be treated as the next larger recognized category.

ANSI/AISC 360-22 is the current AISC Specification associated with the 16th Edition Manual, and it governs structural hole dimensions through Table J3.3. The Research Council on Structural Connections (RCSC) issued a 2025 Specification for Structural Joints Using High-Strength Bolts, which is the current governing document for bolt installation, pretensioning, and related joint requirements; earlier 2020 editions should not be cited as current.

General machine and mechanical clearance holes mainly address assembly, fit, and clearance. Structural steel holes additionally affect bearing strength, net-section behavior, slip-critical performance, washer requirements, bolt installation, edge distance, and connection classification. Do not use a generic ASME clearance-hole chart as a substitute for AISC/RCSC structural hole requirements.

For sizes M36 and larger: standard = d + 3 mm, oversize = d + 8 mm, short slot = (d+3) x (d+10) mm, long slot = (d+3) x 2.5d mm, per the AISC metric structural-hole table.

ISO 273 remains current and covers fine, medium, and coarse clearance-hole series for metric threads from 1 mm to 150 mm. Keep this separate from the AISC metric structural table above.

The same M12 bolt can legitimately use different hole diameters depending on the intended fit.

Do not mix the two. Tap-drill size is not a bolt clearance-hole size. See the Tap Drill Chart for threaded-hole sizing.

Methods can include drilling, punching, reaming, thermal cutting where permitted, and CNC machining. AISC specifically notes that reamed holes still have to meet bolt-hole size and tolerance, spacing, and edge-distance provisions.

Considerations include plate thickness, edge quality, and deformation. Avoid making one process universally preferable; check the applicable specification. Reaming is not unlimited permission to enlarge a structural hole.

This is structural-steel AISC context, not a universal machine-design rule for every application.

Edge distance affects tearout, bearing, edge rupture, and fabrication clearance. It depends on bolt diameter, hole type, edge type, load direction, and structural specification. Use AISC, RCSC, or project criteria rather than a generic rule of thumb.

RCSC’s structural-joint requirements include specific washer provisions for pretensioned and slip-critical joints with oversized or slotted holes, historically requiring plate washers, bars, or extra-thick F436 washers for certain oversized or slotted-hole conditions. Because RCSC issued a 2025 specification, verify the exact current washer requirement against that edition before applying a prescriptive table. See the Washer Size Chart for washer dimensional data.

In slip-critical joints, hole type can affect permitted slip, pretension, washer requirements, design resistance, and surface conditions. Do not imply oversized or slotted holes behave exactly like standard holes; the structural-hole chart must not be used without connection context.

Bearing-type behavior relies partly on bolt bearing against the hole, plate bearing, and shear transfer. Hole type therefore has direct design consequences, though detailed equations are beyond the scope of this general reference chart.

Woodworking and wood structural requirements are not governed by AISC structural-steel hole tables. Hole size can depend on bolt type, the applicable wood connection standard, manufacturer guidance, and whether a tight or clearance fit is intended. Do not copy metal-hole rules into timber construction.

Measure the hole’s inside diameter directly with a caliper, note whether the surrounding connection is structural steel or general assembly, and compare the reading against the applicable ASME, AISC, or ISO table for the corresponding bolt size to identify the hole category.