What the lines and letters on a bolt head mean — and how much load each grade is built to take. Read the head, know the bolt.
The marks stamped on top of a bolt head are its grade — a shorthand for how strong it is and what it’s made of. A higher grade means a stronger bolt, not a bigger one: a Grade 8 and a Grade 2 of the same size look alike until you read the head. The numbers below are the three that matter — proof, yield, and tensile — given in ksi (1 ksi = 1,000 pounds per square inch). When in doubt, match the grade you’re replacing; stepping down can let a joint fail, and stepping up isn’t always free of trade-offs.
Proof load
The stress a bolt can hold and still spring back — the everyday “don’t exceed” working number.
Yield strength
Push past this and the bolt stretches for good. The line between “reusable” and “permanently deformed.”
Tensile strength
The breaking point — where the bolt finally snaps. Always the highest of the three.
| Head mark | Grade | Material | Size range | Proof (ksi) | Yield (ksi) | Tensile (ksi) |
|---|---|---|---|---|---|---|
| Grade 2 | Low / medium carbon steel | ¼″ – ¾″ | 55 | 57 | 74 | |
| Grade 2 | Low / medium carbon steel | >¾″ – 1½″ | 33 | 36 | 60 | |
| Grade 5 | Medium carbon steel, quenched & tempered | ¼″ – 1″ | 85 | 92 | 120 | |
| Grade 5 | Medium carbon steel, quenched & tempered | >1″ – 1½″ | 74 | 81 | 105 | |
| Grade 8 | Medium carbon alloy steel, quenched & tempered | ¼″ – 1½″ | 120 | 130 | 150 |
| Head mark | Spec | Material / use | Size range | Proof (ksi) | Yield (ksi) | Tensile (ksi) |
|---|---|---|---|---|---|---|
| A307 Gr. A | Low carbon — general structural & anchor bolts | ¼″ – 4″ | — | — | 60 | |
| A325 | Medium carbon, heat treated — structural steel connections | ½″ – 1½″ | 85 | 92 | 120 | |
| A490 | Alloy steel, heat treated — high-strength structural | ½″ – 1½″ | 120 | 130 | 150 |
Metric lookalikes
Metric bolts carry a number on the head instead of lines. Two of them sit close to the inch grades:
Close in strength, not interchangeable — the threads are different. A metric bolt takes a metric nut. See metric vs. inch.
Other things people ask at the counter
Stainless isn’t graded this way. Common 18-8 stainless (304-series) is rated by its alloy and how much it’s been work-hardened, not by a Grade number — typical tensile runs roughly 70–100 ksi. Stainless is chosen for corrosion resistance first; if you need both high strength and corrosion resistance, ask us about the right alloy.
Socket head cap screws run higher. Standard alloy-steel socket cap screws (ASTM A574) are heat treated to about 170–180 ksi tensile — stronger than Grade 8 — which is why they show up in machinery and tooling.
Match what you’re replacing. The safest rule on an existing assembly is to put back the same grade. If you’re not sure what came out, bring the old bolt to either counter and we’ll read the head with you.