Bolt Hole Sizes Reference

Reference bolt hole sizing by bolt diameter and hole type. Always verify against the governing standard.

Why hole type and size matter

Bolt hole dimensions directly affect two critical aspects of connection design: the net area available to resist tension or shear, and the degree of slip permitted at the faying surface. A larger hole means more material removed, reducing the net section capacity. It also changes the behavior of slip-critical connections, because the bolt can move further before bearing engages. Choosing the wrong hole type in a calculation -- or worse, having the calculation assume one hole type while the shop drawing shows another -- is one of the most common coordination errors in steel connection design.

The four standard hole types recognized by AISC 360, AS 4100, EN 1993-1-8, and CSA S16 are: standard (STD), oversize (OVS), short-slotted (SSL), and long-slotted (LSL). Each has a defined clearance relative to the bolt diameter, and these clearances differ slightly between codes. For imperial bolts per AISC, a standard hole for a 3/4" bolt is 13/16" diameter (1/16" clearance), while an oversize hole for the same bolt is 15/16". For metric bolts per EN 1993-1-8, the standard clearance is 1 mm for M12-M14 bolts and 2 mm for M16-M24 bolts.

Getting hole deductions wrong is particularly consequential for tension members, where net section rupture (using Fu and net area) often governs over gross section yielding (using Fy and gross area). The hole deduction is the hole diameter -- not the bolt diameter -- plus any additional allowance for damage from punching (typically an extra 1/16" or 2 mm, depending on the code and fabrication method).

Hole type selection guide

When reviewing or designing a bolted connection, verify the following:

• Standard holes (STD) are the default for most connections. They provide minimal clearance for erection while maximizing net area. Use these unless there is a specific reason to upsize.
• Oversize holes (OVS) provide extra clearance to accommodate field fit-up tolerances. They are commonly used in slip-critical connections where bearing is not the governing limit state. Most codes restrict OVS holes to slip-critical joints only.
• Short-slotted holes (SSL) allow adjustment in one direction. The slot length is typically the bolt diameter plus a small increment. They are useful where one direction of adjustment is needed during erection (for example, in beam-to-column shear connections where beam length tolerances accumulate).
• Long-slotted holes (LSL) allow significant movement in one direction and are used for connections that must accommodate thermal expansion or structural movement. They require careful attention to washer and plate-washer requirements.
• Hole deduction for net area: always use the hole diameter (not the bolt diameter) in net area calculations. For punched holes in some codes, add an additional 2 mm (or 1/16") to account for material damage around the hole.
• Drawing vs. calculation consistency: confirm that the hole type shown on the shop drawing matches what was used in the design calculation. This is one of the most frequently missed coordination items.

For the full verification and documentation workflow, see How to verify calculator results.

FAQ

What is the difference between standard and oversize holes? Standard holes have the minimum clearance needed for bolt installation -- typically 1/16" (2 mm) larger than the bolt diameter for common bolt sizes. Oversize holes are larger still, providing extra tolerance for field fit-up. The trade-off is reduced net area and, for bearing-type connections, reduced bearing capacity. Most codes only permit oversize holes in slip-critical connections.

How do I calculate the hole deduction for net section area? The hole deduction equals the hole diameter (which depends on hole type), not the bolt diameter. Some codes also require adding an allowance for damage if holes are punched rather than drilled -- typically an extra 1/16" (2 mm). For staggered bolt patterns, use the "s-squared over 4g" rule to account for the diagonal path. Always confirm which code edition you are using, as these rules have been revised over time.

When should I use slotted holes? Short-slotted holes are useful when erection tolerances require adjustment in one direction -- for example, horizontal slots in beam web connections to accommodate beam length variation. Long-slotted holes are used where the connection must accommodate ongoing movement (thermal expansion, settlement). Long slots in the direction of load generally require plate washers and may have reduced slip resistance.

Do metric and imperial standards use the same hole clearances? No. AISC 360 Table J3.3 and EN 1993-1-8 Table 3.3 define different clearances for their respective bolt series. For example, a 3/4" (≈ M20) bolt has a standard hole of 13/16" per AISC, while an M20 bolt has a standard hole of 22 mm (bolt + 2 mm) per EN 1993. Always use the clearances from the code you are designing to, not a converted value from another standard.

How does hole type affect slip-critical connection capacity? Slip resistance depends on the clamping force, the slip coefficient, and a hole-type factor. Oversize and slotted holes reduce the slip resistance factor because the bolt can travel further before bearing engages. AISC 360 Table J3.1 lists reduction factors: 1.00 for standard holes, 0.85 for oversize and short-slotted, and 0.70 for long-slotted holes loaded parallel to the slot. Similar reductions exist in other codes.

Related pages

• Bolted connections calculator
• Splice connection calculator
• Gusset plate calculator
• Steel grades reference
• Unit converter
• Tools directory
• Reference tables directory
• Guides and checklists
• How to verify calculator results
• Disclaimer (educational use only)

Disclaimer (educational use only)

This page is provided for general technical information and educational use only. It does not constitute professional engineering advice, a design service, or a substitute for an independent review by a qualified structural engineer. Any calculations, outputs, examples, and workflows discussed here are simplified descriptions intended to support understanding and preliminary estimation.

All real-world structural design depends on project-specific factors (loads, combinations, stability, detailing, fabrication, erection, tolerances, site conditions, and the governing standard and project specification). You are responsible for verifying inputs, validating results with an independent method, checking constructability and code compliance, and obtaining professional sign-off where required.

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