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.

What is the Bolt Grade & Strength Reference?

Reference table of bolt strength for metric property classes (4.6 to 12.9) and SAE grades (2, 5, 8) showing proof load, yield, and tensile strength, plus a proof-load estimator from thread stress area. It runs free in your browser on Gera Tools, with nothing uploaded.

Bolt Grade & Strength Reference

Name: Bolt Grade & Strength Reference
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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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.

Decoding metric property class markings

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.

Quick reference: common bolt grades

Grade	Tensile strength	Yield strength	Typical use
Metric 4.6	400 MPa	240 MPa	General structural, non-critical
Metric 8.8	800 MPa	640 MPa	Standard structural bolting (most common)
Metric 10.9	1000 MPa	900 MPa	High-strength structural, automotive
Metric 12.9	1200 MPa	1080 MPa	Very high-strength; never galvanise (hydrogen embrittlement risk)
SAE Grade 2	~380 MPa	~241 MPa	Light hardware, unhardened
SAE Grade 5	~827 MPa	~635 MPa	Common automotive and machinery
SAE Grade 8	~1034 MPa	~896 MPa	High-load structural and suspension

Strength values are nominal; always verify against the applicable standard (ISO 898-1 for metric, SAE J429 for inch).

Calculating proof load from stress area

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

proof load (N) = proofStress(MPa) * stressArea(mm²)

reported in kilonewtons for convenience.

For example, an M12 coarse-thread bolt has a tensile stress area of approximately 84.3 mm². At a proof stress of 600 MPa (class 8.8), the proof load is roughly 600 × 84.3 = 50,580 N ≈ 50.6 kN. Tightening the bolt beyond this will permanently stretch it and reduce its clamping force.

Tips and engineering notes

Proof load is the working ceiling — keep preload below it so the joint never yields under tightening plus service load. Most torque specifications target 70–80% of proof 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 absolute load capacity.
Plated and stainless fasteners differ. A304 and A2-70 stainless bolts have their own strength classes that do not match the carbon-steel property class table. Never substitute by appearance.
Stress area is smaller than nominal shank area because it is taken at the thread root; using nominal area overstates capacity.
Class 12.9 bolts must not be hot-dip galvanised. The hydrogen introduced during the galvanising process causes hydrogen embrittlement and brittle fracture at stresses well below proof load — a serious safety risk.