Bolt Tensile Strength Calculator

Calculator

Example data table

Formula used

Tensile stress area (metric):

At = (π/4) × (d − 0.9382p)²

Tensile stress area (unified):

At = 0.7854 × (d − 0.9743/n)²   where n = TPI

Loads:

• Proof load = At × proof strength
• Ultimate load = At × ultimate tensile strength
• Allowable load = Proof load ÷ Safety factor

How to use this calculator

1. Select Metric or Unified and choose an output unit.
2. Enter diameter and pitch (metric) or diameter and TPI (unified).
3. Choose a material preset, or input custom proof and ultimate strengths.
4. Set an appropriate safety factor for your design approach.
5. Click Calculate to see results above the form.
6. Use Download CSV or Download PDF to save records.

Technical article

Why bolt tensile checks matter on sites

Bolted connections transfer critical forces in steelwork, formwork, plant supports, and temporary bracing. A quick tensile check helps verify that a selected bolt can carry the required axial demand without yielding threads or risking sudden fracture during loading or tightening.

Stress area drives tensile capacity

Tensile capacity is controlled by the tensile stress area (At), not the gross shank area. Threads reduce the effective section, so At is smaller than πd²/4. The calculator estimates At from diameter and pitch (metric) or diameter and TPI (unified).

Metric thread example: M16 coarse

For an M16 bolt with a 2.0 mm pitch, the approximation gives At ≈ 157 mm². With a proof strength of 600 MPa, the proof load is about 94 kN. Using a safety factor of 2.0 produces an allowable load near 47 kN for routine checks.

Unified thread example: 1/2-13

For a 1/2 in bolt with 13 TPI, At ≈ 0.142 in². With an 85 ksi proof strength, the proof load is about 12,100 lbf (≈ 54 kN). With a safety factor of 2.0, the allowable load is roughly 27 kN, depending on rounding and unit selection.

Proof strength vs ultimate strength

Proof strength is a practical limit for repeated service loading because it avoids permanent set. Ultimate tensile strength represents the minimum fracture level for the material. Comparing both values helps you judge how much reserve exists beyond the proof-based allowable. Common property classes list 8.8 at 800 MPa ultimate and 600 MPa proof, while 10.9 is 1040 MPa ultimate and 830 MPa proof. Always confirm the specified standard for your project.

Choosing a safety factor for practice

Safety factors vary with codes, uncertainty, and consequence. Higher factors are common for temporary works and unknown installation quality. Lower factors may be used when loads, materials, and tightening procedures are controlled and documented.

Accounting for washers, grip, and engagement

Capacity is only one part of performance. Ensure adequate thread engagement, compatible washers, and proper grip length so the nut seats well and the joint is clamped as intended. Avoid mixing grades and verify that corrosion protection does not change fit.

Recording results for inspection and QA

Keep inputs, units, and material grade with every calculation. Exporting a CSV or PDF supports traceability for permits, method statements, and inspections. It also makes field verification faster when crews must match bolts to drawings and schedules.

FAQs

Q1. What does the calculator output?
It reports tensile stress area, proof load, ultimate load, and an allowable load based on your safety factor. You can view results in kN, N, or lbf and export the last run to CSV or PDF.

Q2. Which strength value should I use for design?
Use proof strength for service-based checks where permanent deformation must be avoided. Ultimate strength is useful for comparing reserve to fracture, but allowable decisions are typically proof-based unless your specification states otherwise.

Q3. Can I enter a stress area from standards tables?
Yes. Enable the custom At option and enter the tabulated tensile stress area for your bolt size and thread. This is helpful when you have manufacturer data or code tables for a specific series.

Q4. Why are my allowable loads different from a handbook?
Handbooks may use tabulated At values, different proof strengths, or additional reduction factors for temperature, coating, or thread condition. Confirm that diameter, pitch/TPI, material grade, and units match exactly.

Q5. Does this replace connection design checks?
No. Tensile capacity is only one part of connection design. You still must check bearing, shear, combined loading, prying action, slip, fatigue, and installation requirements per the governing project standard.

Q6. How should I pick a safety factor?
Choose a factor consistent with your project method statement, code, and risk. Temporary works or uncertain installation often justify higher factors, while controlled fabrication and documented tightening can support lower factors.

Q7. What inputs matter most in the field?
Bolt size, thread series, and verified grade are the biggest drivers. Also confirm thread engagement, nut type, and washer use. Record the unit system and safety factor so the saved report can be audited later.