Clearance Hole Calculator
Select your bolt or screw size choose a fit class and add optional extra clearance to compute the recommended hole diameter instantly. Supports metric and imperial units with clear results plus downloads for shop use.
Developed by: Nohman Habib
Example Data
| System | Fastener | Fit | Extra | Recommended Hole |
|---|---|---|---|---|
| Metric | M10 | Normal | 0.0 mm | 11.0 mm |
| Metric | M8 | Loose | 0.2 mm | 10.2 mm |
| Imperial | 1/4" | Close | 0.0000 in | 0.2656 in (approx) |
Formula
- Metric: Hole diameter = Clearance table value (Close/Normal/Loose) + Extra clearance.
- Imperial: Hole diameter = Nominal fastener diameter + Fit allowance + Extra clearance.
- Imperial allowances: Close = +1/64", Normal = +1/32", Loose = +1/16".
- Unit conversion: 1 in = 25.4 mm.
How to Use This Calculator
- Select the system (Metric or Imperial).
- Choose your fastener size from the dropdown.
- Select the fit: Close for tight assembly, Normal for typical work, Loose for easier alignment.
- Add optional extra clearance if paint, coating, or cutting process requires it.
- Press Calculate to view the recommended clearance hole and drill hint.
- Use Download CSV or Download PDF to save the result for your job file.
FAQs
1) What is a clearance hole?
A clearance hole is drilled larger than the bolt or screw diameter so the fastener passes through freely without cutting threads in the part.
2) Which fit should I use: close, normal, or loose?
Close is for precise alignment, normal is best for most fabrication, and loose helps when parts may be slightly misaligned or when assembly needs more tolerance.
3) When should I add extra clearance?
Add extra clearance for coatings like paint or powder, for thicker galvanizing, or for processes like laser/plasma cutting where holes may need finishing.
4) Does a larger clearance reduce strength?
The hole size can affect bearing area and joint slip. Keep clearance as small as practical for alignment, and rely on correct washer and bolt preload for joint performance.
5) Can I use this for tapped holes?
No. Tapped holes need a tap drill size and thread specification. This calculator is for through-holes that let the fastener pass without threading.
6) Why do metric and imperial methods look different?
Metric uses common close/normal/loose table values in millimeters, while imperial uses a practical allowance rule based on fractions of an inch plus any extra you add.