Bolt Grade Chart
Yield strength, tensile strength, and head markings for SAE, ASTM, and metric fasteners. Use this to look up what you have, then run the Flange Torque Calculator for a torque value.
SAE Inch Bolts (low-carbon and alloy steel)
Common on automotive, machine equipment, and general fabrication. Head markings are radial lines: count the lines and add 2 to get the grade (3 lines = Grade 5, 6 lines = Grade 8).
| Grade | Size range | Yield (ksi) | Tensile (ksi) | Head marking |
|---|---|---|---|---|
| SAE Grade 2 | 1/4"–3/4" | 57 | 74 | No markings |
| SAE Grade 2 | over 3/4"–1-1/2" | 36 | 60 | No markings |
| SAE Grade 5 | 1/4"–1" | 92 | 120 | 3 radial lines |
| SAE Grade 5 | over 1"–1-1/2" | 81 | 105 | 3 radial lines |
| SAE Grade 8 | 1/4"–1-1/2" | 130 | 150 | 6 radial lines |
ASTM Bolts — Structural & High-Strength
The standards used on civil structures, pressure vessels, and flange bolting.
| Standard | Use | Size | Yield (ksi) | Tensile (ksi) |
|---|---|---|---|---|
| A307 Gr A | General service | 1/4"–4" | — | 60 |
| A325 (now F3125) | Structural steel | 1/2"–1" | 92 | 120 |
| A325 (F3125) | Structural steel | 1-1/8"–1-1/2" | 81 | 105 |
| A490 (F3125) | HS structural | 1/2"–1-1/2" | 130 | 150–170 |
| A193 B7 | Pressure flanges | ≤ 2-1/2" | 105 | 125 |
| A193 B7 | Pressure flanges | 2-5/8"–4" | 95 | 115 |
| A193 B7 | Pressure flanges | 4-1/8"–7" | 75 | 100 |
| A193 B8M Cl 1 | 316 SS, corrosive | all | 30 | 75 |
| A193 B8M Cl 2 | 316 SS, hardened | ≤ 3/4" | 100 | 110 |
| A193 B16 | High-temp alloy | ≤ 2-1/2" | 105 | 125 |
| A574 | Socket head cap screws | ≤ 1/2" | 180 | 200 |
Metric Bolts (ISO Property Classes)
Marked on the head as two numbers separated by a dot — e.g., "8.8". The first number is 1/100 of the tensile strength in MPa; the second multiplied by 10% gives the yield ratio.
| Class | Yield (MPa) | Yield (ksi) | Tensile (MPa) | Tensile (ksi) |
|---|---|---|---|---|
| 4.6 | 240 | 35 | 400 | 58 |
| 5.8 | 420 | 61 | 520 | 75 |
| 8.8 | 640 | 93 | 800 | 116 |
| 10.9 | 900 | 130 | 1040 | 151 |
| 12.9 | 1080 | 157 | 1220 | 177 |
Metric class 8.8 is roughly equivalent to SAE Grade 5. Class 10.9 ≈ SAE Grade 8. Class 12.9 is one step above anything common in US inch bolting — rare outside European-spec machinery.
What the strengths mean in practice
- Yield strength: stress at which the bolt starts to permanently stretch. Design preload is typically 50–75% of yield to keep the bolt elastic.
- Tensile (ultimate) strength: stress at which the bolt fails. Provides margin above yield.
- Proof load: ~90% of yield — the stress a bolt can take repeatedly without permanent set. Important for retorquing.
Tension capacity (lb) = stress area (in²) × yield (psi). For a 3/4" SAE Grade 5 bolt: 0.334 × 92,000 = 30,728 lb at yield. Design preload at 75% yield = ~23,000 lb.