Bolt Grades: Picking the Right Strength (Without Overthinking It)

Bolt grades trip people up. Here's the short version that actually helps you pick the right one without going deep into metallurgy.

How metric grades work

Two numbers on the head: 10.9 for example. First number (10) = tensile strength in hundreds of MPa. So 10 x 100 = 1000 MPa. Second number (9) = yield strength as a percentage of tensile. 9 x 10 = 90%. So yield is 900 MPa.

The common grades

Grade 4.8

Tensile 400 MPa, yield 320 MPa. General purpose, non-critical work. Sheet metal, electronics enclosures, light fixtures. Don't use where there's real structural load.

Grade 8.8

Tensile 800 MPa, yield 640 MPa. Machinery, automotive, moderate structural. This is what most engineers reach for when 4.8 isn't enough but 10.9 is overkill.

Grade 10.9

Tensile 1000 MPa, yield 900 MPa. Heavy machinery, automotive suspension, structural connections. When the joint carries real load.

Grade 12.9

Tensile 1200 MPa, yield 1080 MPa. Aerospace, high-stress precision machinery. Expensive and often unnecessary.

US grades (SAE)

Grade 2 = low strength (no marking). Grade 5 = medium (3 radial lines on head). Grade 8 = high strength (6 radial lines). Structural bolts: A325 is common for structural work, 190,000 psi. A490 is stronger, 200,000 psi. These are specified in building codes — don't substitute.

Rule of thumb

Don't overspecify. A grade 10.9 bolt in a grade 4.8 application is a waste of money and can cause problems with thread engagement and proper torqueing.