Math & Engineering
Clearance Hole Calculator
Calculate the correct clearance hole size for bolts and screws to ensure proper fit and assembly.
Reviewed by the calculator.uk.com Team · Last reviewed 13 June 2026 · Our methodology
Select bolt size to calculate clearance hole diameter
Related to Clearance Hole Calculator
The Clearance Hole Calculator determines the appropriate hole size needed for a bolt or screw to pass through freely without threading. A clearance hole is slightly larger than the nominal bolt diameter to allow for easy assembly and to accommodate manufacturing tolerances. This calculator provides standard clearance hole sizes for both metric and imperial bolt systems based on established engineering standards.
Metric System (mm)
For metric bolts, the clearance hole is typically 0.4mm to 3mm larger than the nominal bolt diameter, depending on the bolt size. The increase is proportional to the bolt diameter, with larger bolts requiring more clearance to account for greater manufacturing tolerances.
Imperial System (inches)
For imperial bolts, the clearance hole is usually 0.016" to 0.016" larger than the nominal bolt diameter. The calculator includes standard sizes for numbered bolts (#4 through #10) and fractional sizes (1/4" through 1"), following common engineering practices.
The calculator provides the recommended clearance hole diameter based on your selected bolt size. This diameter ensures proper fit and assembly while maintaining structural integrity. The results include both the nominal bolt size and the corresponding clearance hole diameter in the chosen measurement system.
Close Fit vs. Standard Clearance
The provided clearance hole sizes represent standard clearance for general applications. For applications requiring tighter tolerances, you may need to reduce the clearance hole size slightly. Conversely, for applications where easy assembly is prioritized over precision, you might choose a slightly larger clearance.
Practical Considerations
When drilling clearance holes, it's recommended to use the exact drill size closest to the calculated clearance diameter. The hole should allow the bolt to pass through freely without excessive play. Consider factors such as material thickness, bolt length, and assembly requirements when implementing the results.
1. What is a clearance hole?
A clearance hole is an unthreaded hole that allows a bolt or screw to pass through freely. It's slightly larger than the fastener's nominal diameter to ensure easy assembly and accommodate manufacturing tolerances.
2. Why is the clearance hole larger than the bolt size?
The clearance hole needs to be larger to account for manufacturing tolerances, allow for easy assembly, and accommodate slight misalignments between mating parts. This prevents binding and ensures proper fit.
3. How do I choose between metric and imperial bolt systems?
The choice between metric and imperial systems typically depends on your location, industry standards, and the existing equipment or components you're working with. Metric is common in Europe and Asia, while imperial is prevalent in the United States.
4. Can I use a smaller clearance hole than recommended?
While it's possible to use a smaller clearance hole for tighter fits, it's generally not recommended as it can make assembly difficult and may cause binding. The recommended sizes are based on established engineering standards for optimal performance.
5. What is the scientific source for this calculator?
The metric hole sizes in this calculator follow the medium-fit series from ISO 273, the international standard for clearance holes for bolts and screws, while the imperial sizes follow the equivalent conventions used in ASME B18.2.8 for fastener clearance holes. Both standards size the hole slightly larger than the nominal fastener diameter to absorb manufacturing tolerances and allow for minor misalignment during assembly. These are dimensional standards rather than physics - they codify decades of accumulated fastening practice into a single reference table.