What is the tensile stress area of a bolt?

The tensile stress area (As) is the effective cross-sectional area used to calculate the tensile load capacity of a threaded fastener. It is smaller than the nominal shank area because it accounts for the thread root geometry. It is defined in ISO 898-1 and depends on the pitch diameter and minor diameter of the thread.

How is bolt tensile strength calculated?

Bolt ultimate tensile load is calculated as Ft = Rm × As, where Rm is the ultimate tensile strength of the bolt material (determined by property class) and As is the tensile stress area. For example, a class 8.8 bolt has Rm = 800 MPa.

What does bolt property class 8.8 mean?

The first number (8) represents 1/100th of the minimum ultimate tensile strength in MPa (800 MPa). The second number (8) represents 10× the ratio of yield strength to ultimate tensile strength (0.8). So yield = 800 × 0.8 = 640 MPa.

How is bolt shear strength calculated?

ISO 898-1 does not specify shear strength — it covers tensile and proof-load properties only. A common simplified estimate takes shear strength as 60% of tensile strength: Fs = 0.6 × Rm × As, based on the von Mises yield criterion. Design codes such as Eurocode 3 (EN 1993-1-8) apply their own factors (0.6 or 0.5 depending on property class) plus partial safety factors. For single shear, use one stress area; for double shear, multiply by 2.

What is the difference between proof load and tensile strength?

Proof load is the maximum force a bolt can withstand without permanent deformation (elastic limit). Ultimate tensile strength is the maximum force before fracture. The proof load stress (Sp) is approximately 90% of the yield strength for most property classes.

Free Engineering Tool

Bolt Tensile & Shear Strength Calculator

Calculate the ultimate tensile load of metric bolts from ISO 898-1 property-class data, plus a simplified shear capacity estimate. Select bolt size and property class.

ISO 898-1 M3 – M64 Class 4.6 – 12.9

Bolt Size

Property Class

Shear Planes

Quick presets

Results

Ultimate Tensile Load (Ft)

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Shear Capacity (Fs, simplified estimate)

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Tensile Stress Area (As)

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Yield Load (Fy)

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Ultimate Tensile Strength (Rm)

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Ultimate Tensile Load

The maximum tensile force a bolt can sustain before fracture:

R_m — ultimate tensile strength (MPa), from property class
A_s — tensile stress area (mm²), from ISO 898-1 / ISO 261

Shear Capacity (Simplified Estimate)

ISO 898-1 defines the tensile and proof-load properties of bolts — it does not specify shear strength. The value shown here is a widely used simplified engineering estimate that takes shear strength as approximately 60% of tensile strength (based on the von Mises criterion, τ ≈ 0.58 σ):

For double shear, multiply by 2 (two shear planes).

⚠️ Shear design note: Actual shear capacity depends on whether the shear plane passes through the threads or the unthreaded shank, on joint slip and plate bearing, and on fatigue loading. Design codes define their own values — e.g. Eurocode 3 (EN 1993-1-8) uses F_v = α_v × f_ub × A with α_v = 0.6 for classes 4.6, 5.6 and 8.8 but 0.5 for classes 4.8, 5.8, 6.8 and 10.9, together with partial safety factors. Always verify shear-loaded joints against the applicable structural or mechanical design code, not this simplified estimate alone.

Property Classes per ISO 898-1

Class	Rm (MPa)	Rp0.2 (MPa)	Typical Use
4.6	400	240	Low-strength, general purpose
5.8	520	420	Structural, non-preloaded
8.8	800	640	Automotive, machinery (most common)
10.9	1040	940	High-strength structural
12.9	1220	1100	Maximum strength (alloy steel)

Practical Example

Example — M10 × 8.8 Bolt

Given: M10 bolt, property class 8.8, single shear

A_s = 58.0 mm², R_m = 800 MPa

F_t = 800 × 58.0 = 46,400 N = 46.4 kN

F_s = 0.6 × 800 × 58.0 = 27,840 N = 27.8 kN

⚠️ Note: These are ultimate capacities without safety factors. Apply appropriate safety factors (typically 2–3 for static, 4–6 for fatigue) for design purposes.