Bolt Torque Calculator
Pick the bolt, its grade, and how the threads are lubricated, and get the tightening torque and the clamp load it produces. Uses the standard T = K × D × P method at 75% of minimum yield strength, the same basis as the SST Torque References poster.
How Bolt Torque Is Calculated.
T is torque, K is the nut factor, D is the nominal bolt diameter, and P is the clamp load. Clamp load is the target tension in the bolt, usually 75% of its yield strength multiplied by the tensile stress area of the thread. K lumps together friction under the head and in the threads: plain dry steel is about 0.20, zinc plating 0.20 to 0.22, black oxide with oil 0.13 to 0.17, and lubricated threads 0.10 to 0.14.
Most of the torque you apply goes into friction, not stretch. Only about 10 to 15% becomes clamp load, which is why the condition of the threads changes the answer so much and why a torque wrench on a rusty bolt is still a guess. For critical joints, measure bolt stretch or use a torque-angle method.
Bolt Torque Chart, Dry Threads, 75% Of Yield
Inch, ft·lb
| Size | Grade 5 | Grade 8 |
|---|---|---|
| #4-40 | 0.8 | 1.1 |
| #6-32 | 1.4 | 2.0 |
| #8-32 | 2.6 | 3.7 |
| #10-24 | 3.8 | 5.4 |
| 1/4-20 | 9.1 | 13 |
| 5/16-18 | 19 | 27 |
| 3/8-16 | 33 | 47 |
| 7/16-14 | 53 | 76 |
| 1/2-13 | 82 | 115 |
| 9/16-12 | 118 | 166 |
| 5/8-11 | 162 | 230 |
| 3/4-10 | 288 | 408 |
| 7/8-9 | 465 | 657 |
| 1-8 | 697 | 984 |
| 1-1/8-7 | 987 | 1395 |
| 1-1/4-7 | 1393 | 1969 |
| 1-3/8-6 | 1826 | 2580 |
| 1-1/2-6 | 2424 | 3425 |
Metric, N·m
| Size | Class 8.8 | Class 10.9 |
|---|---|---|
| M3 x 0.5 | 1.1 | 1.5 |
| M4 x 0.7 | 2.5 | 3.5 |
| M5 x 0.8 | 5.0 | 7.1 |
| M6 x 1 | 8.5 | 12 |
| M8 x 1.25 | 21 | 29 |
| M10 x 1.5 | 41 | 58 |
| M12 x 1.75 | 72 | 101 |
| M14 x 2 | 114 | 161 |
| M16 x 2 | 177 | 250 |
| M18 x 2.5 | 245 | 345 |
| M20 x 2.5 | 347 | 487 |
| M22 x 2.5 | 473 | 665 |
| M24 x 3 | 599 | 842 |
| M27 x 3 | 878 | 1235 |
| M30 x 3.5 | 1191 | 1675 |
| M36 x 4 | 2082 | 2928 |
Coarse threads shown, K = 0.20 for plain dry steel. Halve the value for lubricated threads (K = 0.10). Fine threads and other grades are in the calculator above and on the SST Torque References poster.
FAQs
What is the torque for a 1/4-20 bolt?
About 9 ft·lb (108 in·lb) for a Grade 5 bolt with dry threads. A Grade 8 bolt takes about 13 ft·lb. Lubricated threads cut those roughly in half.
What is the torque for a 3/8-16 Grade 8 bolt?
About 47 ft·lb dry. Grade 5 is about 33 ft·lb.
What is the torque for a 1/2-13 Grade 8 bolt?
About 115 ft·lb dry. Grade 5 is about 82 ft·lb.
What is the torque for an M8 bolt?
About 28 N·m (21 ft·lb) for a class 8.8 bolt with dry threads. Class 10.9 is about 40 N·m (29 ft·lb).
What is the torque for an M10 bolt?
About 56 N·m (41 ft·lb) for class 8.8 dry. Class 10.9 is about 78 N·m (58 ft·lb).
Why do lubricated bolts need less torque?
Torque is mostly spent overcoming friction. Lubricant cuts friction, so the same torque stretches the bolt further and produces more clamp load. Applying a dry-thread torque to an oiled bolt can push it past yield. Use the K factor that matches the thread condition.
What is the K factor or nut factor?
An empirical number that combines friction under the bolt head, in the threads, and against washers. Plain dry steel is about 0.20, zinc plated 0.20 to 0.22, black oxide with oil 0.13 to 0.17, and lubricated 0.10 to 0.14. A small change in K makes a large change in bolt tension.
What percent of yield should I use?
75% is the common target for bolts that will be removed again. Use 50 to 65% for soft joints, gaskets, or aluminum threads, and up to 90% for permanent structural joints where the bolt will not be reused.
How do I know the grade of a bolt?
Inch bolts carry radial marks on the head: none for Grade 1 or 2, three for Grade 5, six for Grade 8. Metric bolts have the class stamped on the head, such as 8.8 or 10.9.
