Introduction:
Do you find yourself working on projects, grabbing Grade 5 or Grade 8 bolts from the hardware store, but not knowing what to torque them to?
This should help.
Suggested Starting Values:
The below estimated torque calculations are only offered as a guide. Use of its content by anyone is the sole responsibility of that person and they assume all risk. Due to many variables that affect the torque-tension relationship like human error, surface texture, and lubrication the only way to determine the correct torque is through experimentation under actual joint and assembly conditions.
Grade 5 Bolts:
| Bolt Size | TPI (Threads Per Inch) | Tightening Torque (ft lbs) | ||
|---|---|---|---|---|
| Galvanized | Plain | |||
| 1⁄4 | 20 | 11 | 8 | |
| 5⁄16 | 18 | 22 | 17 | |
| 3⁄8 | 16 | 39 | 31 | |
| 7⁄16 | 14 | 62 | 49 | |
| 1⁄2 | 13 | 94 | 75 | |
| 9⁄16 | 12 | 136 | 109 | |
| 5⁄8 | 11 | 188 | 150 | |
| 3⁄4 | 10 | 333 | 266 | |
| 7⁄8 | 9 | 537 | 429 | |
| 1 | 8 | 805 | 644 | |
| 11⁄8 | 7 | 992 | 794 | |
| 11⁄4 | 7 | 1,400 | 1,120 | |
| 13⁄8 | 6 | 1,836 | 1,469 | |
| 11⁄2 | 6 | 2,438 | 1,950 | |
| 13⁄4 | 5 | 2,857 | 2,286 | |
| 2 | 41⁄2 | 4,297 | 3,438 | |
| 21⁄4 | 41⁄2 | 6,284 | 5,027 | |
| 21⁄2 | 4 | 8,594 | 6,875 | |
| 23⁄4 | 4 | 11,651 | 9,321 | |
| 3 | 4 | 15,391 | 12,313 | |
| * SAE J429 grade 5 bolts do not exceed 1-1/2″ diameter. | ||||
Grade 8 Bolts:
| Bolt Size | TPI (Threads Per Inch) | Tightening Torque (ft lbs) | |
|---|---|---|---|
| Plain | Lubricated | ||
| 1⁄4 | 20 | 12 | 6 |
| 5⁄16 | 18 | 25 | 12 |
| 3⁄8 | 16 | 44 | 22 |
| 7⁄16 | 14 | 70 | 35 |
| 1⁄2 | 13 | 107 | 53 |
| 9⁄16 | 12 | 154 | 77 |
| 5⁄8 | 11 | 212 | 106 |
| 3⁄4 | 10 | 376 | 188 |
| 7⁄8 | 9 | 606 | 303 |
| 1 | 8 | 909 | 454 |
| 11⁄8 | 7 | 1,287 | 644 |
| 11⁄4 | 7 | 1,875 | 938 |
| 13⁄8 | 6 | 2,382 | 1,191 |
| 11⁄2 | 6 | 3,161 | 1,581 |
| 13⁄4 | 5 | 4,988 | 2,494 |
| 2 | 41⁄2 | 7,500 | 3,750 |
| 21⁄4 | 41⁄2 | 10,969 | 5,484 |
| 21⁄2 | 4 | 15,000 | 7,500 |
| 23⁄4 | 4 | 17,794 | 8,897 |
| 3 | 4 | 23,507 | 11,753 |
| 31⁄4 | 4 | 30,286 | 15,143 |
| 31⁄2 | 4 | 38,266 | 19,133 |
| 33⁄4 | 4 | 47,545 | 23,773 |
| 4 | 4 | 58,100 | 29,085 |
| * SAE J429 grade 8 bolts do not exceed 1-1/2″ diameter. | |||
Notes:
Values calculated using industry accepted formula T = KDP where T = Torque, K = torque coefficient (dimensionless), D = nominal diameter (inches), P = bolt clamp load, lb.
K values: waxed (e.g. pressure wax as supplied on high strength nuts) = .10, hot dip galvanized = .25, and plain non-plated bolts (as received) = .20.
Torque has been converted into ft/lbs by dividing the result of the formula by 12
All calculations are for Coarse Thread Series (UNC).
Grade 2 calculations only cover fasteners 1⁄4“-3⁄4” in diameter up to 6″ long; for longer fasteners the torque is reduced significantly.
Clamp loads are based on 75% of the minimum proof loads for each grade and size.
Proof load, stress area, yield strength, and other data is based on IFI 7th Edition (2003) Technical Data N-68, SAE J429, ASTM A307, A325, A354, A449, and A490.