Clearance Hole Calculator - Calculate Bolt Hole Sizes
Determine the correct clearance hole diameter for various bolt sizes and fit classes according to ISO 273 standards.
Clearance Hole Inputs
Results
What is a Clearance Hole?
A Clearance Hole is a hole drilled through a component that is slightly larger than the fastener (bolt or screw) intended to pass through it. This allows the fastener to slide through without threading into the material, ensuring that the clamping force is applied correctly.
This calculator determines hole sizes based on ISO 273 for:
- Close Fit - For precision assembly requiring accurate alignment.
- Medium Fit - For general engineering and construction applications.
- Free Fit - For ease of assembly or where thermal expansion or corrosion is a concern.
To ensure your structure is sound after drilling, consider checking our Beam Load Calculator to analyze load-bearing capacities.
For projects involving extensive drilling in metal, you might find our Metal Weight Calculator useful for material planning.
If you are fastening wood components, use our Lumber Calculator to estimate your material needs.
For outdoor deck projects requiring fasteners, try our Decking Calculator.
How It Works
The calculator uses standard ISO 273 tables for clearance holes:
Where:
- Bolt Diameter is the nominal size (e.g., M6 = 6mm).
- Clearance depends on the fit class (e.g., +0.6mm for M6 Medium Fit).
Key Concepts
Close Fit (Fine)
Used when alignment is critical. Smallest clearance, requiring precise hole positioning.
Medium Fit
The standard for most applications. Balances ease of assembly with alignment accuracy.
Free Fit (Loose)
Used for quick assembly, rough alignment, or to accommodate thermal expansion.
Tolerance
The allowable variation in hole size. Free fit has larger tolerances.
How to Use
Select Bolt Size
Choose the nominal metric bolt size (e.g., M8).
Choose Fit Class
Select Close, Medium, or Free fit depending on your needs.
Calculate
View the required drill bit or hole diameter.
Benefits
- • Proper Alignment: Ensures parts fit together without forcing.
- • Reduced Binding: Prevents threads from catching on the top plate.
- • Standard Compliance: Follows ISO 273 for professional results.
- • Easy Assembly: Saves time by ensuring holes are the right size.
Factors Affecting Results
Positional Tolerance
If hole locations are not precise, a larger (Free) fit may be needed.
Material Coating
Thick plating or galvanizing on bolts requires larger clearance holes.
Frequently Asked Questions (FAQ)
Q: What are the three classes of clearance holes?
A: The three classes are Close Fit (for precise alignment), Medium Fit (general purpose), and Free Fit (for easy assembly or when corrosion is a factor).
Q: Why do I need a clearance hole?
A: A clearance hole allows a bolt or screw to pass through a part freely, ensuring that the clamping force is generated between the head and the nut (or threaded hole) without binding in the top plate.
Q: How much larger should a clearance hole be?
A: It depends on the fit. Typically, for metric bolts, a Close fit is +0.2mm to +0.5mm, Medium is +0.4mm to +1.0mm, and Free is +1.0mm to +2.0mm larger than the bolt diameter, varying by size.
Q: Can I use a standard drill bit for clearance holes?
A: Yes, standard drill bits are often sized to provide clearance for standard bolt sizes. However, for precision work, you may need specific metric or fractional bits to achieve the exact class of fit.