Clearance Hole Calculator
Use this clearance hole calculator to estimate a hole diameter from the fastener thread diameter and head diameter. It follows the simple Omni relation that averages the two dimensions into a practical clearance-hole size. That gives you a quick starting point before you compare the result with shop standards or manufacturer recommendations.
Clearance-Hole Result
--
Quick Answer: The Omni equation is a simple average: clearance hole = (diameter + head diameter) / 2. It gives you a hole size that sits between the thread diameter and the larger head diameter, which can be useful as a quick planning value.
How to Calculate
- Enter the fastener diameter: Use the nominal thread or shank diameter for the bolt or screw.
- Enter the head diameter: Use the dimension that controls the required opening around the fastener head or feature.
- Calculate the average: The calculator averages the two dimensions into an estimated clearance-hole diameter.
- Compare with practice: Use the result as a starting point, then confirm it against your preferred drill chart or fabrication standard.
Formula
clearance hole = (thread diameter + head diameter) / 2
| Variable | Meaning | Unit |
|---|---|---|
| thread diameter | Nominal fastener diameter | length |
| head diameter | Diameter across the fastener head or relevant clearance feature | length |
| clearance hole | Estimated hole size | length |
Worked Examples
Fastener detailing - Basic screw clearance check
- Thread diameter: 8 mm
- Head diameter: 14 mm
Result: Clearance hole = 11 mm
This gives a simple midpoint between the smaller shank and the larger head dimension.
Interpretation Table
| Range | Meaning | Action |
|---|---|---|
| Smaller clearance hole | Closer to the thread diameter | Good when you want a tighter fit, but check assembly tolerance carefully. |
| Larger clearance hole | Closer to the head diameter | Useful where easier assembly or more positional tolerance is needed. |
| Estimated only | A planning dimension rather than a standard table | Always verify against the fastener system and fabrication standard you are using. |
Frequently Asked Questions
A clearance hole is an opening large enough for a fastener to pass through without the threads engaging the material. It is commonly used where a bolt or screw is tightened into another part or into a nut.
The Omni setup uses both the fastener diameter and head diameter to create a quick average-based estimate that reflects the larger feature around the fastener.
No. Use it as a fast first pass, but final fabrication should still follow the relevant fastener standard, shop chart, or manufacturer recommendation.
Note: This is an estimating tool, not a substitute for a fastener standard or a manufacturer's drill-size recommendation.
References
Last reviewed: March 14, 2026