Calculator inputs
Example thread data
| Designation | System | Nominal diameter (mm) | Pitch (mm) | Pitch diameter (mm) | Minor diameter (mm) | Comment |
|---|---|---|---|---|---|---|
| M10 × 1.5 | Metric | 10.000 | 1.500 | 9.026 | 8.160 | Common bolt for structural brackets. |
| M12 × 1.75 | Metric | 12.000 | 1.750 | 10.863 | 9.853 | Frequently used for machinery mounting. |
| 1/2–13 UNC | Unified | 12.700 | 1.954 | 11.431 | 10.303 | Coarse pitch for general engineering use. |
| 1/2–20 UNF | Unified | 12.700 | 1.270 | 11.874 | 11.048 | Fine pitch for precision, thinner nuts. |
Values above are approximate, generated using the same equations as the calculator.
Formula used
The calculator assumes symmetrical 60° external threads similar to ISO metric and unified screw threads. All results are expressed in millimetres for direct comparison.
- Nominal (major) diameter, d: basic outer diameter of the thread profile.
- Pitch, P: axial distance from one thread crest to the next.
- Pitch diameter, d₂ (approx.): d₂ = d − 0.64952 × P
- Minor diameter, d₃ (approx.): d₃ = d − 1.22687 × P
- Tensile stress area, As (approx.): As = π/4 × (d − 0.9382 × P)²
- Tensile capacity, Ft (approx.): Ft = As × Rm / SF
Rm is the ultimate tensile strength in megapascals and SF is the chosen safety factor. Use this estimate only for preliminary sizing of threaded fasteners.
How to use this calculator
- Select the thread standard: metric (ISO) or unified (UNC/UNF).
- Choose the thread type: external fastener or internal tapped hole.
- Optionally click a preset to fill nominal size and pitch automatically.
- Enter the nominal diameter and pitch or threads per inch for custom dimensions.
- Optionally specify material strength and safety factor to estimate tensile capacity.
- Enable inch output if you want both units displayed in the results.
- Click Calculate dimensions to view all geometric and strength values.
- Use CSV or PDF export buttons for reporting, traceability, or design documentation.
Always compare calculated values with the relevant fastener standard for critical joints or safety-related connections.
Threaded fasteners in structural connections
Bolts and studs in baseplates, splice joints, and bearing connections rely on accurate thread dimensions for clamp force and slip resistance. Use this tool alongside plate and beam design checks for complete connection verification.
For purely metric work you can cross-check values with the Metric Thread Dimensions Calculator when you need standard coarse or fine thread data.
Coordinating clearance holes and bolt sizes
Thread geometry needs compatible clearance holes in plates, ribs, and brackets. After finding nominal and minor diameters here, use the Clearance Hole Calculator to choose appropriate hole sizes for close, normal, or loose fit.
Steelwork detailing and fabrication
Fabricators need clear instructions for drilling, punching, and tapping. Combine thread dimensions from this page with a Steel Clearance Hole Calculator to ensure shop drawings specify realistic hole tolerances and finishing allowances.
Anchor bolts, baseplates, and holding-down systems
For anchors cast into concrete, thread length, minor diameter, and tensile area influence design tension capacity. The capacity estimate here gives a quick sense of demand on anchor groups before detailed concrete code checks.
Serviceability, vibration, and preload loss
Under cyclic loading, bolts can relax or lose preload. Knowing the tensile stress area helps when estimating tightening torque, preload level, and residual stress after long-term service, especially in vibrating machinery supports or crane beams.
Exporting thread schedules for site teams
CSV export lets you build bolt schedules including designation, pitch, and diameters. Attach these schedules to general arrangement drawings or method statements so site supervisors and inspectors can quickly confirm the correct fasteners are used at each connection.
Frequently asked questions
1. Which unit system should I select first?
Choose metric when drawings show millimetres and ISO thread designations like M16 × 2.0. Use unified when specifications use inch-based bolts such as 1/2–13 UNC or 3/4–10 UNF.
2. Can I design safety-critical bolts using this tool alone?
No. The calculator provides approximate geometry and basic tensile capacity only. Always perform full code checks for shear, combined actions, edge distances, and slip resistance using applicable structural and material design standards.
3. How are internal threads handled in the results?
When you select an internal thread, the minor diameter is shown as an approximate tap drill size. You should still confirm the final drill according to manufacturer data or relevant tapping tables.
4. Why do I sometimes see very small tensile capacities?
Low capacities usually mean a small diameter, large pitch, low material strength, or high safety factor. Check that you entered the correct units and values before relying on preliminary capacity estimates from this calculator.
5. How does this differ from the metric-only thread tool?
This page supports both metric and unified threads with optional strength checks. The dedicated metric-only tool focuses on ISO sizes, offering more preset designations and options specific to metric fasteners.
6. Can I show both millimetres and inches together?
Yes. Enable the inch display switch before calculating. The table will then show each key diameter in millimetres and inches, which is helpful for mixed-unit projects and international suppliers.
7. What is the best way to document results for the site?
Use CSV export for bolt schedules and PDF export for simple design notes. Combine these with connection details, torque requirements, and installation instructions in your project documentation pack.