What do the two numbers in a metric class like 8.8 mean?

The first number times 100 is the nominal tensile strength in MPa (8 means 800 MPa). The second number is the ratio of yield to tensile strength times 10 (8 means yield is 80 percent of tensile, so 640 MPa). Class 8.8 therefore has 800 MPa tensile and 640 MPa yield.

How do SAE grades compare to metric classes?

SAE Grade 5 is roughly equivalent to metric class 8.8, and SAE Grade 8 is close to class 10.9. Grade 2 is unhardened low-carbon steel similar to class 4.6. The match is approximate because the standards define slightly different proof and yield points.

Proof load is the maximum tension a bolt can carry without taking a permanent set. It sits just below yield, typically 85 to 90 percent of yield strength, and is the figure used to set safe preload targets.

How do I turn a stress into a force?

Multiply the stress in MPa by the thread tensile stress area in square millimetres to get force in newtons. An M10 coarse thread has a stress area of about 58 mm², so at 600 MPa proof stress it carries roughly 34.8 kN.

Why is yield lower than tensile strength?

Tensile strength is the stress at fracture, while yield is where permanent deformation begins. A bolt loaded past yield but below tensile will stretch and lose preload even though it has not broken, which is why designs stay below proof load.

Bolt Grade & Strength Reference

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Bolt grade markings encode the steel’s strength. This reference decodes metric property classes and SAE grades into tensile, yield, and proof figures, and lets you turn a proof stress into an actual proof load using the thread’s tensile stress area.

How it works

A metric class such as 8.8 is two pieces of information packed together. The first digit gives nominal tensile strength: 8 × 100 = 800 MPa. The second digit gives the yield-to-tensile ratio: 0.8 × 800 = 640 MPa yield. So:

tensile (MPa) = firstDigit * 100
yield   (MPa) = (secondDigit / 10) * tensile

SAE grades use head markings (radial lines) rather than numbers but map to similar strengths, with Grade 5 ≈ class 8.8 and Grade 8 ≈ class 10.9.

To convert a proof stress into a force the tool multiplies by the thread tensile stress area:

proof load (N) = proofStress(MPa) * stressArea(mm^2)

reported in kilonewtons for convenience.

Tips and notes

Proof load is the working ceiling — keep preload below it so the joint never yields under tightening plus service load.
A bolt’s strength is set by its grade, not its size; a larger bolt of the same grade simply has more stress area and therefore more load capacity.
Plated and stainless fasteners can have lower or different strength ratings; always check the actual marking, not just the diameter.
Stress area is smaller than the nominal shank area because it is taken at the thread root; using nominal area overstates capacity.