BoltLabBolt Load Capacity Calculator
Quick answer
This calculator estimates the tensile load capacity of a single fastener from its tensile stress area, a source-verified property-class strength value, and a safety factor you supply.
It is tension-only and covers the fastener itself — it does not estimate shear, joint, or connected-material capacity.
You must enter your own safety factor; BoltLab does not provide a default one.
Select a unit system, fastener size, and property class, then enter a safety factor appropriate to your application. See tensile stress area and fastener property classes for how these values are sourced.
Calculator
Select a size, property class, and safety factor to calculate.
How this estimate works
The calculator first computes the fastener's tensile stress area — a smaller, load-bearing cross-section than the area implied by nominal diameter alone, since a threaded fastener fails through its thread root, not its full shank. Metric sizes use At = 0.7854 × (D − 0.938194 × P)²; inch sizes use At = 0.7854 × (D − 0.9743 / n)², where D is nominal diameter, P is pitch, and n is threads per inch. See tensile stress area for the full derivation and sourcing.
Tensile stress area is then multiplied by the selected property class's source-verified minimum proof strength to give the proof-strength-based tensile capacity — the primary basis for this estimate, consistent with standard fastener design practice. The same area is also multiplied by the minimum tensile (ultimate) strength as a secondary, reference-only figure; proof strength, not ultimate strength, governs the calculator's allowable-capacity result.
Dividing the proof-based capacity by your supplied safety factor gives the allowable capacity. BoltLab does not choose this number for you: the correct safety factor depends on your application, the governing design code, and consequences of failure, and reputable general engineering references (including Shigley's Mechanical Engineering Design and related sources reviewed during this tool's development) do not converge on one universal value. See fastener property classes for exactly which classes and diameter ranges this calculator currently supports and why others are not yet included.
This calculator estimates the tensile load capacity of the selected fastener based on its tensile stress area, the selected property class's strength value, and your supplied safety factor. Actual joint capacity may be lower and depends on the complete joint design — see the limitations below.
What this calculator does not cover
This is a fastener tensile-capacity estimate only. It does not account for, and must not be used as a substitute for engineering analysis of:
Nothing on this page constitutes professional engineering advice. For safety-critical, structural, or regulated applications, verify results with a qualified engineer and the applicable design code — see BoltLab's Engineering Data Methodology and Disclaimer.