Bolt grades explained
A bolt’s grade tells you how strong it is. Here’s how to read metric, stainless and imperial markings — and what each grade is used for.
Metric (e.g. 8.8)
First number × 100 = tensile MPa. First × second × 10 = yield MPa. 8.8 → 800 / 640 MPa.
Imperial SAE (2 / 5 / 8)
Higher number = stronger. Radial head lines: none = Gr2, three = Gr5, six = Gr8.
Stainless (A2 / A4)
A2 = 304, A4 = 316 (added Mo). Number after dash = tensile ÷ 10. A2-70 → 700 MPa.
Metric — carbon & alloy steel (ISO 898-1)
| Grade | Standard | Proof | Yield | Tensile | Typical use |
|---|---|---|---|---|---|
| 4.6 | ISO 898-1 | 225 MPa | 240 MPa | 400 MPa | Low-stress general fastening — mild-steel frames, brackets, non-critical assemblies, fencing, general engineering. |
| 4.8 | ISO 898-1 | 310 MPa | 340 MPa | 420 MPa | Light-duty machinery, sheet-metal fastening, appliances, general assembly hardware. |
| 5.8 | ISO 898-1 | 380 MPa | 420 MPa | 520 MPa | Medium-duty general fastening, machine building, conveyor & equipment frames. |
| 8.8 | ISO 898-1 | 580 MPa | 640 MPa | 800 MPa | The workhorse structural grade — steel construction, machinery, automotive chassis, plant & equipment, pipe flanges. |
| 9.8 | ISO 898-1 | 650 MPa | 720 MPa | 900 MPa | Higher-strength small-diameter fasteners in machinery and automotive where 8.8 is marginal. |
| 10.9 | ISO 898-1 | 830 MPa | 940 MPa | 1040 MPa | High-strength joints — automotive engines & drivetrains, heavy machinery, cranes, structural steel, pressure equipment. |
| 12.9 | ISO 898-1 | 970 MPa | 1100 MPa | 1220 MPa | Highest standard grade — aerospace, motorsport/high-performance automotive, dies & tooling, socket-head cap screws, critical high-load joints. |
Metric stainless (ISO 3506)
| Grade | Standard | Proof | Yield | Tensile | Typical use |
|---|---|---|---|---|---|
| A2-70 | ISO 3506-1 | — | 450 MPa | 700 MPa | Corrosion-resistant general use — food & beverage, architectural, mild outdoor, pumps, light marine. |
| A4-70 | ISO 3506-1 | — | 450 MPa | 700 MPa | Marine, coastal & chemical exposure — offshore, shipbuilding, chemical plant, medical, water treatment. |
| A4-80 | ISO 3506-1 | — | 600 MPa | 800 MPa | Higher-strength corrosion service — structural marine, desalination, chemical process equipment. |
Imperial — SAE J429 & ASTM
| Grade | Standard | Proof | Yield | Tensile | Typical use |
|---|---|---|---|---|---|
| Grade 2 | SAE J429 | 379 MPa | 393 MPa | 510 MPa | General light-duty hardware — non-critical assemblies, appliances, furniture, general shop use. |
| Grade 5 | SAE J429 | 586 MPa | 634 MPa | 827 MPa | Automotive, agricultural & general machinery, medium-duty structural — the common mid-strength imperial bolt. |
| Grade 8 | SAE J429 | 827 MPa | 896 MPa | 1034 MPa | High-stress duty — vehicle suspension, heavy equipment, off-highway, plant machinery, high-load structural. |
| A307-A | ASTM A307 Gr A | — | — | 414 MPa | General construction & non-structural steel-to-steel, anchor bolts, low-strength bolting. |
| A325 | ASTM F3125 A325 | 85 MPa | 634 MPa | 827 MPa | Structural steel connections — buildings, bridges, towers (high-strength structural bolting). |
| A490 | ASTM F3125 A490 | 827 MPa | 896 MPa | 1034 MPa | Highest-strength structural steel bolting — heavy bridges, high-rise, transmission towers, heavy connections. |
| B7 | ASTM A193 B7 | — | 724 MPa | 862 MPa | High-temperature / high-pressure bolting — flanges, pressure vessels, refineries, petrochemical & power plant. |
Frequently asked
What does 8.8 mean on a bolt?
On a metric bolt, the first number × 100 is the minimum tensile strength in MPa, and the first × second × 10 is the minimum yield strength. So 8.8 means 800 MPa tensile and 640 MPa yield.
What is the difference between 8.8 and 10.9?
10.9 is a higher-strength alloy-steel grade — 1040 MPa tensile vs 800 MPa for 8.8 — used for high-load joints such as engines, cranes and heavy machinery. 8.8 is the general structural workhorse.
What is the equivalent of Grade 8 in metric?
SAE Grade 8 (150 ksi / ~1034 MPa tensile) is broadly comparable to metric class 10.9. SAE Grade 5 is broadly comparable to 8.8.
What do A2 and A4 mean?
A2 is 304 stainless; A4 is 316 stainless with added molybdenum for better corrosion resistance (marine/chemical). The number after the dash is the tensile strength ÷ 10 — so A2-70 is 700 MPa.
Values are specified minimums per ISO 898-1, ISO 3506 and SAE J429 / ASTM. Verify against the current standard edition and your engineering spec for critical joints.