Engineering · Tool 01

Bolt Grades

Axial Strength · Shear Strength · Torque

Pick a diameter, a property class and a friction condition, and the calculator works out the proof load, breaking load, shear strength and the torque needed to reach a sensible preload. The table below it lines every grade up side by side for the diameter you chose.

Reference only. Figures use standard ISO 898-1 property-class values and common design approximations for shear and torque — they're a starting point for estimation, not a substitute for the fastener manufacturer's datasheet or the governing design standard on anything load-bearing or safety-critical.

Calculator

Nominal diameter

Property class (grade)

Friction condition

Target preload (% of proof load) — 75%

Tensile stress area

—

mm²

Proof load

—

Ultimate axial (breaking) load

—

Shear strength (single shear, approx.)

—

Recommended tightening torque

—

N·m

Torque, imperial

—

lb·ft

Preload achieved

—

kN, at target %

Nut factor (K) used

—

friction coefficient

Torque is a proxy for preload, not a guarantee of it. Friction at the threads and under the head accounts for roughly 90% of the torque applied — only about 10% becomes clamp force. Real preload from a given torque can vary ±25–30% around this estimate depending on surface finish, lubrication and thread condition.

All grades at —, —, 75% target preload
Grade	Proof load	Ultimate axial	Shear (approx.)	Torque

Explanatory Notes

What the grade number means

A metric property class such as 8.8 or 10.9 is two numbers, not a name. Multiply the first by 100 to get the nominal ultimate tensile strength in MPa — 8.8 is 800 MPa, 10.9 is 1040 MPa. Multiply that result by a tenth of the second number to get the approximate yield strength — for 8.8, 800 × 0.8 = 640 MPa. Higher numbers mean a harder, stronger, more brittle bolt — not simply "better" for every joint.

Axial (tensile) strength

This is the load pulling straight along the bolt's axis. Two figures matter: the proof load, below which the bolt returns to its original length when the load is removed, and the ultimate load, beyond which it breaks. Design loads should stay under the proof load with a safety margin — treating the ultimate load as a target invites permanent stretch or failure.

Shear strength

Shear loads act across the bolt, trying to slide the clamped parts past one another. As a working approximation, single-shear strength is often taken as roughly 60% of the ultimate tensile load — this calculator uses that factor. It assumes the shear plane falls through the shank; if it falls through the threads instead, effective strength drops further, and double-shear joints roughly double the load capacity for the same bolt.

Torque and preload

Tightening torque exists to create clamp force (preload), not to directly measure it. The short-form relationship is T = K · D · F — torque equals a friction factor (K) times the nominal diameter (D) times the target preload (F). Because K depends heavily on lubrication, plating and surface condition, torque alone is an imprecise way to control preload; where it matters, torque-angle methods, bolt tension gauges or ultrasonic measurement do better.

Basis: mechanical properties per ISO 898-1 (property classes 4.6–12.9, coarse pitch); tensile stress areas per the standard ISO metric coarse-thread table; shear approximated at 60% of ultimate tensile; torque from the short-form T = K·D·F relationship with a 75% default target preload. For structural steel connections, use ASTM A325/A490 (or ISO equivalents) directly rather than this general tool.