Quick reference for clearance hole sizes by bolt size and fit type (close, normal, or loose), based on ASME B4.1 and ISO standards. Covers common sizes; consult standards for complete tables or special applications.
Last updated: March 2026
Close: tight tolerances | Normal: most common | Loose: 3-4% clearance
Scope: This calculator is a reference table for common bolt sizes based on ASME B4.1 and ISO 286. It covers only the sizes shown; consult the full standards (ASME B4.1 or ISO 286) for extended tables, metric conversions, or uncommon sizes. Always verify hole sizes against your fastener manufacturer's specifications and engineering drawings, as tolerances and requirements vary by application and material.
A clearance hole is a drilled hole that allows a fastener (bolt, screw, or rivet) to pass through without resistance. Unlike a tension hole, which is slightly tighter for a precise fit, a clearance hole provides deliberate space between the fastener and the material. This is the most common hole type in mechanical assembly.
The size of a clearance hole depends on the fastener diameter and the desired fit: close fit (precise alignment), normal fit (standard applications), or loose fit (rapid assembly, misalignment tolerance). Standards like ASME B4.1 and ISO 286 define these clearances to ensure interchangeability across industries and countries.
Finding the correct drill bit size for a standard 1/4" bolt with normal fit clearance:
Clearance holes allow fasteners to pass through freely. Tapping holes (or thread-cutting holes) are smaller and designed for screws to cut their own threads into the material.
Yes, a slightly larger bit (up to +0.015" in imperial) is often used for convenience, but too much clearance may cause misalignment or weak connections. Follow ASME standards when precision matters.
The next larger standard size is acceptable. Never use a smaller drill bit, as the fastener may not fit or could get stuck. Test-fit whenever possible.
Close fit is for precise alignment and structural work. Normal fit is standard for most applications. Loose fit allows for assembly speed, corrosion tolerance, or misaligned parts.
Yes. Metric (ISO 286) and Imperial (ASME B4.1) use different standards, though the philosophy is the same. Always check which standard your drawing specifies.
Measure the fastener and hole carefully. You can use the next larger clearance fit size if the existing hole is slightly oversized. Document the fit type used.
No. Press-fit connections require interference fits (smaller holes), not clearance fits. Use transition or interference fit calculations for press-fit applications.
Yes. Plating, anodizing, or other coatings add thickness to the fastener. Account for additional clearance when coating is applied (typically +0.001-0.003" per side in imperial).
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