Bolt Torque Calculator finds the tightening torque for a bolt from its size, grade and thread condition. It also shows the clamp load, the bolt's tensile stress and a torque chart.
Pick the size and the grade marked on the bolt head. Torque is set for 75% of proof load.
Thread and Joint Details
Coarse thread is loaded with each size. Change the pitch for fine threads.
Torque Chart, Class 8.8
| Coarse Thread Size | Clamp Load | Torque |
|---|---|---|
| M5 × 0.8 | 6.17 kN | 6.17 N·m |
| M6 × 1 | 8.75 kN | 10.5 N·m |
| M8 × 1.25 | 15.92 kN | 25.5 N·m |
| M10 × 1.5 | 25.23 kN | 50.5 N·m |
| M12 × 1.75 | 36.66 kN | 88.0 N·m |
| M14 × 2 | 50.22 kN | 141 N·m |
| M16 × 2 | 68.15 kN | 218 N·m |
| M18 × 2.5 | 86.61 kN | 312 N·m |
| M20 × 2.5 | 110.2 kN | 441 N·m |
| M22 × 2.5 | 136.5 kN | 601 N·m |
| M24 × 3 | 158.6 kN | 761 N·m |
| M27 × 3 | 206.7 kN | 1,116 N·m |
| M30 × 3.5 | 252.3 kN | 1,514 N·m |
| M33 × 3.5 | 312.1 kN | 2,060 N·m |
| M36 × 4 | 367.5 kN | 2,646 N·m |
| M39 × 4 | 439.1 kN | 3,425 N·m |
What the Bolt Torque Calculator Works Out
The Bolt Torque Calculator starts from one of two things you know. With Bolt Size and Grade, it finds the torque that loads the bolt to a set share of its proof load. With Target Clamp Load, it finds the torque that produces the clamp force your joint needs.
Both routes return the tightening torque in N·m, lb-ft, lb-in and kgf·m. The cards add the clamp load per bolt and for the whole pattern, the tensile stress in the bolt, and the torque for other thread conditions. A torque chart for the chosen grade sits under the cards.
Bolt Torque Formula
Torque is the product of three numbers. K is the nut factor, which stands for friction in the threads and under the head. D is the nominal bolt diameter and F is the clamp load, the tension the tightened bolt holds.
The clamp load comes from the bolt's strength. The Bolt Torque Calculator multiplies the proof strength of the grade by the tensile stress area of the thread. It then takes 75% of that proof load, unless you change the percentage.
This is the method behind the Fastenal Torque-Tension Reference Guide, which lists clamp loads at 75% of proof load and torque from T = KDF. The stress area depends on diameter and pitch, and the Bolt Area Calculator shows that step on its own.
Take an M12 × 1.75 Class 8.8 bolt with dry threads. The stress area is 84.27 mm² and the proof strength is 580 MPa, so the proof load is 48.87 kN. At 75% the clamp load is 36.66 kN. With K at 0.20, the torque is 0.20 × 12 mm × 36.66 kN, or 88.0 N·m.
Metric Bolt Torque Chart
The chart below is what the Bolt Torque Calculator returns for coarse metric threads at 75% of proof load with dry threads, K 0.20. Proof strengths are 580 MPa for Class 8.8 up to M16 and 600 MPa above it, 830 MPa for Class 10.9 and 970 MPa for Class 12.9.
| Size | Class 8.8 | Class 10.9 | Class 12.9 |
|---|---|---|---|
| M6 × 1 | 10.5 N·m | 15.0 N·m | 17.6 N·m |
| M8 × 1.25 | 25.5 N·m | 36.5 N·m | 42.6 N·m |
| M10 × 1.5 | 50.5 N·m | 72.2 N·m | 84.4 N·m |
| M12 × 1.75 | 88.0 N·m | 126 N·m | 147 N·m |
| M14 × 2 | 141 N·m | 201 N·m | 235 N·m |
| M16 × 2 | 218 N·m | 312 N·m | 365 N·m |
| M20 × 2.5 | 441 N·m | 610 N·m | 712 N·m |
| M24 × 3 | 761 N·m | 1,053 N·m | 1,231 N·m |
So a 10 mm Class 8.8 bolt takes about 50 N·m and an M20 takes about 441 N·m. These are ceilings for the bolt, not for the parts it clamps. A service manual figure for the same bolt is often lower, and it takes priority.
Fine threads have a larger stress area, so they carry more torque at the same grade. Type the pitch as a Custom value under Thread and Joint Details in the Bolt Torque Calculator. An M12 × 1.25 Class 8.8 bolt comes out at 96.1 N·m against 88.0 N·m for the coarse thread.
Inch Bolt Torque by Grade
Switch Units to US and the Bolt Torque Calculator changes its grades to ASTM A307 Grade A, SAE Grade 5 and SAE Grade 8. Grade 5 uses a proof strength of 85,000 psi up to 1 in and 74,000 psi above it. Grade 8 uses 120,000 psi. A307 has no proof load in its standard, so 36,000 psi is used, as Fastenal does.
| Size | Grade 5 | Grade 8 |
|---|---|---|
| 1/4-20 | 8.45 lb-ft | 11.9 lb-ft |
| 5/16-18 | 17.4 lb-ft | 24.6 lb-ft |
| 3/8-16 | 30.9 lb-ft | 43.6 lb-ft |
| 1/2-13 | 75.4 lb-ft | 106 lb-ft |
| 5/8-11 | 150 lb-ft | 212 lb-ft |
| 3/4-10 | 267 lb-ft | 376 lb-ft |
| 7/8-9 | 429 lb-ft | 606 lb-ft |
| 1-8 | 644 lb-ft | 909 lb-ft |
These match the dry column of the Fastenal chart, which lists 75 lb-ft for a 1/2-13 Grade 5 bolt and 106 lb-ft for Grade 8. Each inch size also has a fine thread option in the pitch list. A 1/2-20 Grade 5 bolt takes 85.0 lb-ft against 75.4 lb-ft for 1/2-13.
The same guide adds a condition that is easy to miss. The torque can only be reached if the nut or tapped hole is at least as strong as the bolt. A Grade 8 bolt in a soft nut, or in threads tapped into aluminum, can strip before it reaches these numbers.
How the Nut Factor Changes Torque
K is the least certain input. The Bolt Torque Calculator offers 0.20 for dry or zinc plated threads, 0.17 for anti-seize or threadlocker, 0.15 for oiled threads and 0.12 for PTFE coatings. Fastenal uses 0.20 for plain dry bolts and 0.15 for lubricated ones.
Published values for zinc plating do not agree. One Fastenal chart uses 0.17 for zinc plated and dry, while Brighton-Best uses 0.22. On the M12 example that spread moves the torque from 74.8 N·m to 96.8 N·m. If you have the fastener maker's figure, enter it as a Custom nut factor.
The last card shows why this matters. An oiled M12 bolt needs only 66.0 N·m for the same 36.66 kN. Tighten that oiled bolt to the dry figure of 88.0 N·m and the clamp load rises by 33%, which puts a bolt meant for 75% of proof at 100%.
Even with the right K, torque is a rough way to set tension. EDT Engineers puts the torque method at about ±25% of the intended bolt tension. That scatter is the reason the default target is 75% of proof and not more. To find K from a measured torque and tension, use the Bolt Friction Coefficient Calculator.
Bolt Force From a Target Clamp Load
Choose Target Clamp Load when the joint design gives you a force per bolt. Enter that load, the bolt size and the nut factor. A 25 kN clamp load on an M12 bolt with K 0.20 needs 60.0 N·m, and the bolt runs at 51% of its proof load.
The grade is used only to check the bolt in this mode. Above 90% of proof the stress card turns to a warning. Above 100% the Bolt Torque Calculator tells you to use a larger bolt, a higher grade or more bolts. Below 50% it notes that a lightly loaded joint can loosen under vibration.
Set Number of Bolts in the Bolt Torque Calculator to see the total clamp force of the pattern. Four M12 Class 8.8 bolts at 36.66 kN each hold 146.6 kN together. To go the other way, from a torque you already applied to the bolt force it produced, use the Bolt Preload Calculator.
Bolt Torque FAQ
How do I tighten a bolt without a torque wrench?
Use the turn-of-nut method. Bring the nut snug, mark it, then turn it a set fraction of a turn further. Structural practice uses about one third to two thirds of a turn past snug, depending on bolt length.
The method depends far less on friction, so it is more consistent than torque. EDT Engineers rates turn-of-nut at about ±15% of the intended tension against ±25% for a torque wrench. It suits large structural bolts better than small machine screws.
How tight is 5 N·m by hand?
5 N·m is 3.69 lb-ft, or 44 lb-in. On a wrench held 200 mm from the bolt, that is a pull of 25 N, about 5.6 lbf. It is a firm twist of a short hex key, not a hard pull.
For scale, the Bolt Torque Calculator gives 6.17 N·m for an M5 Class 8.8 bolt at 75% of proof with dry threads. A 5 N·m figure is typical for fasteners of that size.
Can I use it as a flange bolt torque calculator?
Only for the bolt's own limit. Torque for a gasketed pipe flange is set by the gasket stress and the tightening pattern, which ASME PCC-1 covers. The grade mode here would overload many gaskets.
If the gasket supplier gives a required load per bolt, Target Clamp Load converts that load into torque. Enter the stud diameter and the nut factor for the lubricant you use on the studs.