Bolt Grade Converter

Bolt grades indicate tensile strength (how much pulling force the bolt can handle).

SAE to Metric equivalents:

• SAE Grade 2 = Metric Class 4.6 (low strength, general purpose)
• SAE Grade 5 = Metric Class 8.8 (medium strength, most common)
• SAE Grade 8 = Metric Class 10.9 (high strength, heavy duty)
• No SAE equivalent = Metric Class 12.9 (very high strength, aerospace)

These are equivalents, not equalities. SAE and metric are separate standards written by different bodies, so a Grade 5 and a Class 8.8 are close enough to swap in most work but are not the same specification. The table gives each its own numbers rather than pretending one row covers both.

Proof strength is the number that actually matters when you torque a bolt. Tensile is where it snaps; proof is the load it carries and still springs back from with no permanent stretch. Torque specs normally aim to put a bolt somewhere around 75 to 90% of proof, so a joint designed around the tensile figure is designed around the wrong number.

Identification marks:

• Grade 2: no marks on head
• Grade 5: 3 radial lines on head
• Grade 8: 6 radial lines on head
• Metric: class number stamped on head (e.g., “8.8”)

The metric class number is a code you can read once you know the trick. In “8.8,” the first digit times 100 is the tensile strength in megapascals, so 8.8 means about 800 MPa. The number after the dot is the ratio of yield strength to tensile, so 0.8 means the bolt starts to deform permanently at roughly 80% of its breaking point. That’s why a 10.9 is meaningfully stronger than an 8.8, not just a bigger label.

A few practical notes the chart won’t tell you. Grade matters most when a joint is torqued tight or takes shock loads; using a Grade 2 where the design calls for a Grade 8 invites a sheared bolt under load. Match the nut to the bolt, since a soft nut on a hard bolt strips and throws away the strength you paid for. And grade says nothing about rust: a high-grade bolt with no coating still corrodes, and the hardest grades can actually be more prone to hydrogen embrittlement, so for outdoor or marine work the finish matters as much as the number.