Bolt Grades Explained: Markings, Strength, and What to Use
Two bolts can look identical and be worlds apart in strength. One is fine holding a bracket to a fence; the other holds a control arm to a moving car. The difference is stamped right on the head: a set of lines on an inch bolt, or a pair of numbers on a metric one. Once you can read those marks, you stop guessing and start matching the bolt to the load.
This guide covers both systems: the SAE grades you see on inch hardware in North America, and the metric property classes used almost everywhere else. It explains what the marks mean, what the strength numbers translate to, and how the two systems line up.
Why grade matters more than size
People fixate on diameter, but a bolt fails on strength, not just thickness. A joint clamps because the bolt is stretched slightly and pulls the parts together, holding a preload. Push past the bolt's yield point and it stops springing back; push past its tensile strength and it snaps. Grade tells you how hard you can pull before either happens. Fit a low-grade bolt where a high-grade one belongs and it can quietly yield, lose clamp load, and let the joint work loose. Often the first sign is a failure, not a warning.
Inch bolts: the SAE grade system
On inch (Unified) fasteners, grade is shown by radial lines on the head. Count the lines and add three. That shortcut works because the common grades read as a tidy progression: no lines is the weakest, six lines is the strongest of the everyday grades.
The everyday inch grades
- Grade 2: unmarked, low-carbon steel. Fine for light, low-stress work. Tensile strength about 74,000 psi (and lower on larger diameters).
- Grade 5: three lines, medium-carbon steel, quenched and tempered. The general-purpose workhorse. Tensile strength around 120,000 psi.
- Grade 8: six lines, medium-carbon alloy steel, quenched and tempered. For high-stress joints. Tensile strength about 150,000 psi.
Head markings also carry a maker's stamp, and stainless bolts are marked differently again (often with an "18-8", "A2", or "A4" stamp) because their strength comes from a different family of specs. If a head is completely blank apart from a logo, treat it as grade 2 and keep it out of anything that matters.
Metric bolts: property classes
Metric fasteners skip the lines and print two numbers separated by a dot, such as 8.8, 10.9, or 12.9. It looks cryptic, but it is one of the most useful markings in fastening because you can read the strength straight off the head.
How to read a metric class: multiply the first number by 100 to get the nominal tensile strength in MPa. The second number is the yield-to-tensile ratio in tenths. So 8.8 means 800 MPa tensile, and yield at 0.8 of that, about 640 MPa. 10.9 means 1000 MPa tensile with yield near 900 MPa.
- Class 8.8: 800 MPa tensile. The metric equivalent of grade 5 and by far the most common structural bolt.
- Class 10.9: 1040 MPa tensile. High-strength, close to grade 8. Common in engines, suspension, and machine tools.
- Class 12.9: 1220 MPa tensile. The top of the common range, used in socket-head cap screws and demanding assemblies.
How the two systems line up
You cannot swap an inch bolt for a metric one, because the threads never interchange, which the metric vs imperial guide covers in detail. But you can compare them on strength, which is handy when a design calls out one system and your hardware is in the other.
| SAE grade | Metric class | Tensile strength | Typical use |
|---|---|---|---|
| Grade 2 | Class 4.6 / 4.8 | ~60–74,000 psi (400–500 MPa) | Light, low-stress fastening |
| Grade 5 | Class 8.8 | ~120,000 psi (800 MPa) | General automotive and machinery |
| Grade 8 | Class 10.9 | ~150,000 psi (1040 MPa) | Suspension, drivetrain, high load |
| — | Class 12.9 | ~177,000 psi (1220 MPa) | Socket-head cap screws, tooling |
The pairings are close matches in strength, not exact twins, so treat them as practical substitutes rather than identical parts. When a joint is safety-critical, follow the spec the designer called out.
Choosing the right grade
Higher is not automatically better. Stronger bolts are harder, and hardness brings a little more brittleness. A grade 8 or 12.9 is less forgiving of shock loads and notches than a mid-grade bolt. The goal is to match the grade to the load, not to overbuild blindly.
- Structural or moving joints (suspension, engine, machinery under load): grade 8 or class 10.9. Do not drop below what the design specifies.
- General assembly (brackets, frames, guards): grade 5 or class 8.8 covers the vast majority of jobs.
- Light, non-critical work: grade 2 or class 4.8 is fine and cheaper.
Whatever grade you pick, torque it correctly. A strong bolt left loose is no stronger than a weak one. Our torque specs tool gives target values by size and grade, and the bolt grades reference lists the full strength figures for every class.
The bottom line
Read the head before you reach for the wrench. On an inch bolt, count the lines: none, three, or six for grades 2, 5, and 8. On a metric bolt, read the two numbers: first times 100 is tensile strength in MPa. Match that grade to the load, torque it properly, and the joint does its job quietly, which is exactly what a fastener should do.
Frequently asked questions
What do the lines on a bolt head mean?
On an inch (SAE) bolt, the radial lines are the grade marking. No lines is grade 2, three lines is grade 5, and six lines is grade 8. More lines mean a stronger bolt. Metric bolts use printed numbers such as 8.8 or 10.9 instead of lines.
What is the difference between grade 5 and grade 8 bolts?
Grade 8 is stronger and harder. Grade 5 runs about 120,000 psi tensile; grade 8 is about 150,000 psi. Grade 5 (three lines) suits general automotive and machinery work, while grade 8 (six lines) is for high-stress joints like suspension and drivetrain parts.
What does 8.8 mean on a metric bolt?
It is a property class. The first number times 100 is the tensile strength in MPa (8 × 100 = 800 MPa), and the second is the yield-to-tensile ratio (0.8, so yield near 640 MPa). Class 8.8 is roughly the metric equivalent of SAE grade 5.
Is a metric 10.9 bolt the same as a grade 8?
Close, not identical. Class 10.9 is near 1040 MPa tensile (about 150,000 psi), which lines up well with grade 8. They substitute for each other on strength, but you must still match thread standard, diameter, and pitch, since metric and inch threads never interchange.