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.