Clearance Hole Calculator

Engineering clearance hole calculator for structural steel fabrication, timber framing, and mechanical assemblies. Determine standard, oversized, short-slotted, close, normal, and loose drill hole sizes across US Imperial (AISC / ASME B18.2.8) and Metric (ISO 273 / Eurocode 3) standards.

AISC / ASME Fastener Clearance Geometry
Nominal Hole Diameter
0.000 in
0/0 in (0.00 mm)
Total Diametral Clearance
0.000 in
0.000 in Radial Play
Effective Design Hole (AISC Tearout)
0.000 in
AISC Table J3.3 Compliant
Min AISC Edge Distance (Le)
0.00 in
Min Spacing (3d): 0.00 in
Structural Hole Property Calculated Specification Reference Standard Fabrication Notes

Diametral Clearance vs Bolt Diameter

AISC Plate Hole & Bolt Cross-Section

ISO 273 / Eurocode 3 Metric Hole Sizing
Nominal Drill Hole Diameter
0.0 mm
0.000 in equiv
Radial Clearance (Gap)
0.0 mm
Total Play: 0.0 mm
Standard Drill Bit Size
0.0 mm Drill
ISO Metric Tooling
Eurocode 3 Min Edge Distance (e1)
0.0 mm
Pitch Spacing (p1): 0.0 mm
ISO 273 Parameter Design Value International Standard Workshop Recommendation

ISO Fit Category Comparison

Eurocode 3 Edge & Spacing Geometry

Custom Plate Hole Fabrication Results
Calculated Hole Diameter
0.000 in
Hole Cross-Section: 0.000 sq.in
Bearing Area of Bolt (d * t)
0.000 sq.in
Structural Plate Contact
Calculated Radial Gap
0.000 in
Side-to-Side Float
Custom Design Parameter Computed Engineering Output Practical Detailing Guidance

Custom Bore Tolerance Profile

Plate Bearing & Shear Tearout Model

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Engineering Guide to Structural and Mechanical Bolt Clearance Holes

In structural steel construction, architectural metalwork, and mechanical fabrication, a threaded bolt cannot function properly unless it passes smoothly through mating materials without binding or causing uncalibrated side friction. Sizing a clearance hole is a delicate balance of engineering constraints. A hole drilled too tight restricts field assembly, prevents proper joint alignment, and introduces unintended shear bite into fastener threads. Conversely, a hole drilled too large reduces the critical contact bearing area underneath the bolt head or washer, increases the risk of joint slip, and lowers the tearout rupture capacity of the connected steel plate.

This Clearance Hole Calculator is developed for structural steel detailers, civil design engineers, fabrication shop foremen, and machinists throughout the United States, Canada, and the United Kingdom. It delivers precise calculations for standard, oversized, close, normal, and loose clearance holes based on the American Institute of Steel Construction (AISC 360 Specification for Structural Steel Buildings), ASME B18.2.8, and International Organization for Standardization (ISO 273) standards.

AISC Table J3.3 Structural Steel Hole Dimensions

In structural steel connections, bolt holes are not arbitrary drilled openings. AISC 360 Table J3.3 dictates strict maximum nominal dimensions for bolt holes based on fastener shank diameter:

  • Standard Holes (STD): Standard holes are sized exactly 1/16 inch (1.6 mm) larger than the nominal bolt diameter for fasteners up to 7/8 inch, and 1/8 inch larger for fasteners 1 inch in diameter and above. Standard holes can be utilized in all bearing, snug-tight, and slip-critical connections without reducing design shear capacities.
  • Oversized Holes (OVS): Oversized holes are typically 3/16 inch to 5/16 inch larger than the fastener diameter. They are permitted exclusively in slip-critical joints to accommodate field erection tolerances and column base alignments. Hardened structural washers (ASTM F436) are mandatory over oversized holes to bridge the widened aperture.
  • Short-Slotted Holes (SSL): Sized with a standard width perpendicular to the slot and an elongated length parallel to the slot (e.g., standard width by diameter plus 1/4 inch). These holes accommodate thermal expansion or manufacturing tolerances without requiring full structural redesign.
  • Long-Slotted Holes (LSL): Designed with an elongated dimension roughly 2.5 times the fastener diameter. Long slots are primarily specified in expansion joints or sliding architectural facade connections.

The "+1/16 Inch" Design Rule vs Nominal Drill Size

When computing structural net section fracture or tearout strength across a bolt line, AISC 360 Chapter B mandates an essential distinction between nominal hole diameter and the effective design hole diameter. During thermal plasma cutting or mechanical hole punching, a 1/32-inch ring of steel around the perimeter is damaged by micro-cracking and work hardening. To account for this metallurgical damage, structural engineers must add an extra 1/16 inch (1.6 mm) to the nominal hole diameter when calculating the net tensile area (An) and block shear capacity.

Mechanical Clearance Fit Categories: ASME B18.2.8 & ISO 273

In mechanical engineering, machinery mounting, and equipment base detailing, ASME B18.2.8 (Imperial) and ISO 273 (Metric) define three standardized clearance grades:

  • Close Fit (Fine): Intended for precision assemblies where minimal positional play is permitted. Close fit holes prevent relative movement between components before clamping torque is applied, but they require accurate CNC drilling or jig reaming.
  • Normal Fit (Medium): The default industrial engineering standard for general machinery, framing brackets, and structural plates. It provides sufficient diametral float (typically 0.5 mm to 1.5 mm depending on diameter) to accommodate minor center-to-center hole pitch tolerances.
  • Loose Fit (Coarse): Engineered for hot-dip galvanized steel, cast iron components, dirty outdoor field construction, or long multi-bolt patterns where cumulative manufacturing tolerances make tighter fits impractical.

When coordinating foundation anchors or heavy column splices, verify connection designs using our Column Load Calculator[cite: 1, 2]. For framing attachments and roof girders, cross-reference hole deductions against the structural capacity derived from the Beam Load Calculator[cite: 1, 2] and deflection limits from the Beam Deflection Calculator[cite: 1, 2]. For heavy flange brackets and hub mountings, verify fastener spacing using our Bolt Circle Calculator[cite: 1, 2], calculate tightening preloads with the Bolt Torque Calculator[cite: 1, 2], and evaluate welded joint alternatives with the Welding Calculator[cite: 1, 2].

AISC Minimum Edge Distance and Bolt Spacing Rules

Drilling a clearance hole too close to the edge of a structural plate or flange introduces a severe risk of tearout rupture under tensile or shear loading. AISC Table J3.4 defines mandatory minimum edge distances (Le) measured from the center of a standard hole to the edge of the connected part:

  1. Sheared Edge Distance: For a 3/4-inch structural bolt, AISC requires a minimum distance of 1-1/4 inches from a sheared edge.
  2. Rolled, Gas-Cut, or Saw-Cut Edge: The minimum edge distance for a 3/4-inch bolt is 1.0 inch when edges are finished cleanly by saw cutting or thermal slicing.
  3. Standard Center-to-Center Spacing: The preferred center-to-center pitch between adjacent bolt holes is 3 times the nominal bolt diameter (3d), with an absolute code minimum spacing of 2-2/3 times the diameter (2.67d).

How to Use This Clearance Hole Calculator

  1. Select Calculation Mode: Choose between US Structural (AISC / ASME), Metric International (ISO 273), or Custom Plate Sizing.
  2. Select Bolt Diameter: Pick your nominal fastener diameter (e.g., 3/4" UNC or M20 metric).
  3. Choose Hole Fit Specification: Select standard, oversized, short slot, close, normal, or loose clearance depending on joint requirements.
  4. Input Plate Thickness: Enter connected plate thickness to determine bolt bearing surface area and shear engagement.
  5. Click "Calculate Hole Size": The system instantly computes nominal hole diameter, fractional drill size, diametral clearance, radial gap, effective design diameter, and minimum AISC/Eurocode edge distances.
  6. Print, Reset, or Share: Generate an isolated, auto-formatted single-page A4 engineering job-site report or share calculation parameters instantly.

Frequently Asked Questions (FAQ)

1. What is the standard clearance hole size for a 3/4" structural bolt?
Per AISC 360 Table J3.3, the standard clearance hole diameter for a 3/4" nominal structural bolt is 13/16 inch (0.8125 inches or 20.64 mm), which equals nominal diameter plus 1/16 inch.
2. Why are oversized holes permitted only in slip-critical connections?
Oversized holes have significant play that would allow excessive joint slip under service loads in standard bearing-type connections. In slip-critical joints, clamping force generated by high-strength bolts (like ASTM A325 or A490) prevents the steel plies from moving through friction, making hole size secondary to clamping force.
3. What is the difference between normal fit and close fit clearance holes?
Close fit (fine) holes provide very tight clearances (typically 0.015" to 0.030" or 0.2 to 0.5 mm) to ensure accurate part location and prevent backlash. Normal fit (medium) provides standard manufacturing clearance (1/16" or 1.0 to 1.5 mm) to facilitate rapid, unforced job-site alignment.
4. How does hole punching affect structural steel compared to drilling?
Mechanical hole punching shears through steel rapidly but leaves a cold-worked, micro-fissured perimeter. In structural engineering calculations, building codes require adding an extra 1/16" damage allowance to punched hole diameters when calculating effective net tensile area, whereas drilled or reamed holes create clean cylindrical cuts without micro-cracking.
5. What is the standard clearance for an M20 metric bolt under ISO 273?
Under ISO 273, an M20 bolt has a medium (normal) clearance hole diameter of 22.0 mm (+2.0 mm clearance), a fine (close) fit of 21.0 mm (+1.0 mm), and a coarse (loose) fit of 24.0 mm (+4.0 mm).
6. Why are hardened washers required over oversized and slotted holes?
Hardened washers (ASTM F436) distribute the heavy clamping force evenly across the wider hole opening, preventing the bolt head or nut from dishing into the gap, galling the plate edge, or losing vital tension.
7. How is minimum edge distance measured in structural steel detailing?
AISC and Eurocode standards measure edge distance from the center of the bolt hole to the nearest plate edge along the line of force, guaranteeing adequate resistance against shear tearout failure.
8. What clearance is recommended for hot-dip galvanized assemblies?
Hot-dip galvanizing adds a protective zinc coating varying from 3 to 6 mils (75 to 150 microns) in thickness. Because zinc accumulates in holes, engineers specify loose fit holes (or tap nuts oversize) to prevent bolt binding during erection.
9. When should slotted holes be oriented perpendicular vs parallel to load?
In slip-critical connections, slots oriented perpendicular to the direction of applied shear provide high slip resistance. Slots oriented parallel to the load are utilized when joint expansion, structural deflection, or seismic thermal movement must be accommodated without transferring axial force.
10. Can clearance holes be created using thermal laser or plasma cutting?
Yes. Modern CNC plasma and laser systems produce high-quality structural holes. However, AWS D1.1 and AISC specifications require that thermally cut holes have a surface roughness not exceeding 1,000 micro-inches, and any gouges deeper than 1/16" must be ground smooth or repaired before fastener installation.