Allowable tension and shear for one bolt — never above the bolt's proof load.
With the diameter, calculates the tensile-stress area.
Leave empty if you entered the diameter and pitch.
Shank area πd²/4 if the shear plane is in the plain shank; leave empty to use A_t (threads in the shear plane — conservative).
Not needed when a class is chosen.
If unknown, 0.55 × S_u is assumed (conservative).
Empty = 2.0.
Example Result
Your result will appear here
The calculated result will appear here after you press Calculate.
A bolt fails in one of two ways: it stretches and breaks across the threads under tension, or it is cut across under shear. This calculator estimates the load one bolt may carry in each case. Tension acts on the tensile-stress area A_t — smaller than the shank because of the threads — while shear acts on the shank or the thread root, depending on where the shear plane falls. Pick a property class, or type the strength yourself; enter the stress area, or the diameter and pitch to calculate it; set a safety factor of at least 1.0. The calculator returns the allowable tension and shear and never lets either exceed what the bolt can carry at its proof load. It is a sizing estimate for a single fastener, not a joint design.
Choose a property class (or leave it blank and type S_u and, if known, S_p).
Enter the tensile-stress area, or the diameter and thread pitch to calculate it.
Enter the shank area if the shear plane passes through the plain shank; otherwise the threaded area is used.
Set a safety factor of at least 1.0 (2.0 if left empty) and the number of shear planes.
Optionally enter the applied tension and shear to check them together, then press Calculate.
By O.E. — Partner, chairman of the board and CEO of a metal products manufacturer for 21 years; the company exports to 45 countries.
Drafted with AI assistance; calculations checked against primary sources and tested with the site's own code. About FlowUtil
Last reviewed
Tension acts on the tensile-stress area A_t = π/4 (d − 0.9382 P)² for metric threads (π/4 (d − 0.9743 ÷ n)² for inch threads), smaller than the shank because of the threads. The allowable tension is the tensile strength × A_t ÷ the safety factor, but never more than the proof load S_p × A_t — the load the bolt must carry without permanent stretch. Shear acts on the shank area πd²/4 when the shear plane passes through the plain shank, or on A_t when it passes through the threads; the allowable shear is 0.577 × S_u × area ÷ SF per plane (0.577 is the von Mises ratio 1/√3), likewise capped at the proof stress. The safety factor must be at least 1.0.
| Symbol | Meaning |
|---|---|
| A_t | Tensile-stress area, mm² or in² |
| S_u | Minimum tensile strength of the class or grade |
| S_p | Proof strength — the allowable load never exceeds S_p × A_t |
| SF | Safety factor on tensile strength, at least 1.0 |
The two numbers on a metric bolt head encode its strength: the first × 100 is the nominal tensile strength in MPa, and the decimal × that value is the nominal yield. An 8.8 bolt is nominally 800 MPa tensile and 640 MPa yield. Minimum values from ISO 898-1:
| Class | Proof stress S_p (MPa) | Yield R_p0.2 (MPa) | Tensile R_m (MPa) |
|---|---|---|---|
| 8.8, d ≤ 16 mm | 580 | 640 | 800 |
| 8.8, d > 16 mm | 600 | 660 | 830 |
| 10.9 | 830 | 940 | 1,040 |
| 12.9 | 970 | 1,100 | 1,220 |
For inch bolts, SAE J429 Grade 5 (¼–1 in) is 85 ksi proof and 120 ksi tensile.
1M12 × 1.75, class 8.8, safety factor 2, shank in the shear plane (113.1 mm²).
A_t = π/4 (12 − 0.9382 × 1.75)² = 84.3 mm²; 800 × 84.3 ÷ 2; 0.577 × 800 × 113.1 ÷ 2
Allowable tension 33.7 kN (proof load 48.9 kN) · allowable shear 26.1 kN
The allowable tension is 69% of the proof load. With the threads in the shear plane the shear capacity falls to 19.5 kN — specify a bolt long enough that its plain shank crosses the joint.
21/2-13 UNC, SAE Grade 5 (S_u 120 ksi, S_p 85 ksi), safety factor 2, double shear on the 0.1963 in² shank.
A_t = 0.7854 (0.5 − 0.9743 ÷ 13)² = 0.1419 in²; 120,000 × 0.1419 ÷ 2
Allowable tension 8,514 lbf (proof load 12,062 lbf) · allowable shear 13,592 lbf
With a 10.9 M20 bolt (A_t = 245 mm²) and a safety factor of 1.2 on tensile strength, S_u × A_t ÷ SF is 1,040 × 245 ÷ 1.2 = 212 kN — more than the 203 kN proof load. High-strength classes have a yield close to their tensile strength, so a factor applied to tensile strength alone leaves little room: the bolt would take a permanent set before the "allowable" load. The calculator therefore limits the result to 203.4 kN and says so. If you type your own strengths without a proof value, it assumes 0.55 × S_u — below the proof/tensile ratio of every common class.
A bolt that carries both is checked with an interaction rule. A common engineering form is (F_t ÷ F_t,allow)² + (F_v ÷ F_v,allow)² ≤ 1. For the M12 above carrying 20 kN tension and 15 kN shank shear: (20 ÷ 33.7)² + (15 ÷ 26.1)² = 0.35 + 0.33 = 0.68 — acceptable. Structural codes use their own interaction formulas and partial factors.
Deepen your understanding with in-depth engineering guides, formulas, and worked examples.
Browse Engineering Guides