Bolt Grades Explained: What the Markings on a Bolt Head Mean

Two bolts can look nearly identical and still have very different strength ratings. The difference is the grade, and choosing the wrong one can mean anything from a stripped connection to a failed assembly. The good news is that most bolts tell you their grade right on the head, once you know what to look for.

Grade 2: no markings

A plain hex head with no radial lines is typically a Grade 2 bolt. Made from low or medium carbon steel, it's the most economical option and works well for light-duty applications like household projects, light framing, and non-critical assemblies where loads are modest.

Grade 5: three lines

Three evenly spaced radial lines on the head mark a Grade 5 bolt. It's heat treated for significantly higher strength than Grade 2, with a minimum tensile strength of about 120,000 psi in common sizes. Grade 5 is a popular all-around choice for automotive, machinery, and equipment work.

Grade 8: six lines

Six radial lines indicate a Grade 8 bolt, made from heat-treated alloy steel with a minimum tensile strength of about 150,000 psi. You'll find Grade 8 in heavy equipment, suspension components, and high-stress connections where maximum strength matters.

Metric property classes

Metric bolts use numbers stamped on the head instead of lines. Class 8.8 is roughly comparable to Grade 5, and Class 10.9 is roughly comparable to Grade 8. The first number relates to tensile strength, and the second tells you the ratio of yield strength to tensile strength, so an 8.8 bolt yields at about 80 percent of its tensile strength.

Stainless steel markings

Stainless bolts are usually marked by alloy rather than grade, such as 18-8, 304, or 316, or with metric designations like A2-70 and A4-80. Stainless is chosen for corrosion resistance, not maximum strength. Standard stainless bolts are generally not as strong as Grade 5 and should not be substituted for Grade 8 in high-stress applications.

Stronger isn't always better

It's tempting to default to the highest grade, but harder bolts are also less ductile, meaning they bend less before breaking. In some applications, a bolt that flexes slightly is safer than one that snaps. Nuts and washers should also be matched to the bolt's grade, since a Grade 8 bolt paired with a weaker nut is only as strong as the nut. When a project calls for a specific grade, always follow the engineer's or manufacturer's specification.

Need a specific grade?

Let us know the grade or property class your project requires, along with size, finish, and quantity, and we'll quote fasteners that match. Visit our Get in Touch page to get started.