Bolt Grade & Strength Chart (Metric and SAE)
What the numbers on a bolt head mean — proof load, tensile and yield strength for the metric property classes and the SAE grades.
| Grade / class | System | Size range | Min. proof (MPa) | Min. tensile (MPa) | Min. yield (MPa) | Typical material |
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Standard: ISO 898-1 / ISO 3506-1 / SAE J429 · Data checked: 2026-10-14
Frequently asked questions
How do I read the number 8.8 on a bolt head?
The figure before the point is one hundredth of the minimum tensile strength in MPa, so 8.8 means 800 MPa. The figure after the point is ten times the ratio of yield to tensile strength, so yield is 0.8 x 800 = 640 MPa. A 10.9 bolt is 1040 MPa tensile and 940 MPa yield.
Is stainless as strong as a graded steel bolt?
Usually not. A2 and A4 stainless cannot be age-hardened and only gains strength from cold work, so A4-80 reaches about 800 MPa where an alloy steel 12.9 reaches 1220 MPa. Stainless is chosen for corrosion resistance, not strength, and substituting it into a design sized for 8.8 can be a significant loss.
What do SAE grade markings look like?
Grade 5 bolts carry three or five radial dashes on the head. Grade 8 carries six radial dashes plus the letters SAE, or three radial dashes and the number 8. Unmarked bolts are usually grade 2 or unrated — treat them as low strength.
Do I need a washer with a high-grade bolt?
Yes, and with the right hardness. A soft washer under a class 12.9 bolt collapses before the bolt reaches its preload, so the torque you apply goes into crushing the washer. Match the washer hardness class to the bolt class.