Bolt Strength Chart — SAE Grade 2/5/8, ASTM A325/A490 & Metric Class 8.8/10.9/12.9

Imperial Bolt Strength Chart (SAE & ASTM)

Standard strength ratings for carbon and alloy steel bolts up to 1″ diameter. These are the grades you’ll encounter on 99% of US engineering drawings.

Grade	Head Marking	Material	Proof (psi)	Yield (psi)	Tensile (psi)
SAE Grade 2	No markings	Low carbon steel	55,000	57,000	74,000
SAE Grade 5	3 radial lines (120°)	Medium carbon, heat treated	85,000	92,000	120,000
SAE Grade 8	6 radial lines (60°)	Medium carbon alloy, heat treated	120,000	130,000	150,000
ASTM A325	“A325” stamped	Carbon steel (structural)	85,000	92,000	120,000
ASTM A490	“A490” stamped	Alloy steel (structural)	120,000	130,000	150,000
Socket Head (SHCS)	Knurled head	Alloy steel	140,000	153,000	180,000

Note: Grade 2 and Grade 5 values are for bolts up to 1″ diameter. Larger sizes have reduced ratings (e.g., Grade 5 drops to 105,000 psi tensile above 1″).

Metric Bolt Strength Chart (ISO 898-1)

Metric bolts use a property class system. The class number isn’t arbitrary — it encodes the strength directly.

Property Class	Head Marking	Proof (MPa)	Yield (MPa)	Tensile (MPa)
Class 8.8	“8.8” embossed	600	640	800
Class 10.9	“10.9” embossed	830	940	1,040
Class 12.9	“12.9” embossed	970	1,100	1,220

The Metric Hack: Decode Strength from the Head Marking

Unlike SAE grades, metric property classes tell you the exact strength just by looking at the number:

Example: Class 8.8

First number (8) → Ultimate Tensile Strength = 8 × 100 = 800 MPa
Second number (.8) → Yield Strength = 80% of Tensile = 800 × 0.8 = 640 MPa

Example: Class 10.9

Tensile = 10 × 100 = 1,000 MPa (actual is 1,040 — close enough for quick reference)
Yield = 1,000 × 0.9 = 900 MPa (actual is 940)

Imperial vs. Metric Cross-Reference

When you need to substitute across systems:

SAE/ASTM	≈ Metric Class	Notes
Grade 5 / A325	Class 8.8	Most common structural grade in both systems
Grade 8 / A490	Class 10.9	High-strength applications
Socket Head (SHCS)	Class 12.9	Highest common grade

Understanding the Strength Terms

Proof Strength

The maximum stress a bolt can handle without any permanent deformation. This is your design limit for pre-loaded joints. Think of it as the “safe working load” — the bolt can take this stress indefinitely and spring back perfectly every time.

Yield Strength

The stress at which the bolt begins to permanently stretch. Beyond this point, the bolt won’t return to its original length. In torque-to-yield (TTY) applications, bolts are intentionally tightened past yield for maximum clamping force — but those bolts are single-use.

Ultimate Tensile Strength (UTS)

The maximum stress the bolt can withstand before it breaks. There’s always a safety margin between yield and UTS, but you should never design to this number in a static application.

Pro Tip: In most joint design, you care about proof strength, not tensile. A properly torqued joint should have bolt preload at 75-90% of proof load. If you’re designing to UTS, something has gone wrong in your analysis.