Fastener Property Classes

What a property class or grade represents

A metric property class (such as 8.8, 10.9, or 12.9) or an SAE grade (such as Grade 5 or Grade 8) is a manufacturer marking system that identifies a fastener's minimum mechanical strength without requiring a lab test of every part. Each class or grade corresponds to a specified minimum proof strength and minimum tensile strength that a fastener of that marking must meet, set by the governing standard — ISO 898-1 for metric classes, SAE J429 for SAE grades.

Proof strength is the stress a fastener can sustain without taking a permanent set — for static engineering design, it is the strength value that governs allowable load, which is why BoltLab's calculator uses it as the primary basis for capacity, not the higher tensile (ultimate) strength.

Nominal designation versus specified minimum

A metric class's two-part number is itself a designation, not a direct strength readout: the first digit × 100 gives the nominal tensile strength in MPa (used to construct the class name), and the second digit relates to the yield-strength ratio. The specified minimum strength a class must actually meet can run slightly above this nominal figure — for example, class 10.9's nominal figure is 1000 MPa, but its specified minimum tensile strength is 1040 MPa. BoltLab's Bolt Strength Grades guide was corrected to reflect this distinction after a source-reconciliation review; the table below states the same specified minimums.

ISO metric property classes in this calculator

Some ISO classes are diameter-dependent: the same class can carry a different specified minimum for larger fasteners.

SAE grades in this calculator

SAE grades are also diameter-dependent — the same grade can require a different minimum strength above or below a stated diameter break.

Values render directly from the same data file the calculator uses, so this table and the calculator can never disagree.

The role of proof strength in the calculator

The Bolt Load Capacity Calculator multiplies a fastener's tensile stress area by the selected class's minimum proof strength to estimate its proof-based tensile capacity, then divides by your own safety factor for an allowable value. Minimum tensile (ultimate) strength is shown only as a secondary, reference figure — it is not the governing basis for the calculator's allowable-capacity result, consistent with standard fastener design practice.

Source and provenance boundaries

Neither ISO 898-1 nor SAE J429 was directly read to build this dataset — both remain proprietary, purchasable standards. Every value shown above instead traces to at least one freely-published manufacturer or industry-association engineering data sheet that itself cites the governing standard, and every class or grade listed here was independently cross-checked against a second, separately-operated source with full numeric agreement before being marked verified. See BoltLab's Engineering Data Methodology for how this fits the site's broader data-classification system.

Why some classes are not included yet

BoltLab's dataset also holds ISO classes 4.8, 5.6, 5.8, 6.8, and 9.8, and the SAE Grade 2 requirement for diameters over 3/4 inch — none of these appear in the calculator or the tables above. Classes 4.8/5.6/5.8/6.8/9.8 were each confirmed from only one independent source, not two, so they have not yet cleared BoltLab's cross-verification bar. The SAE Grade 2 over-3/4-inch case is different: two sources were checked, but they disagreed on its minimum yield strength, and that disagreement has not been resolved. Rather than publish a class with an unresolved number attached to it, BoltLab holds it back entirely until it can be independently confirmed. A property class only appears in the calculator once it clears this bar — nothing is estimated or filled in to make the list look more complete than the evidence supports.