M8 bolt torque
Tighten an M8 × 1.25 class 8.8 bolt to 25 N·m (19 ft-lb) with dry threads, or 19 N·m when lubricated. Class 10.9 takes 36 N·m and class 12.9 takes 43 N·m.
M8 torque by property class
Tap a row to show it on the dial. The unit switch above changes every value on this page.
| Property class | Dry torque | Lubricated | Clamp load | Proof load | Proof stress |
|---|---|---|---|---|---|
| Class 8.8 | 19 N·m | 15.9 kN | 21.2 kN | 580 MPa | |
| Class 10.9 | 27 N·m | 22.8 kN | 30.4 kN | 830 MPa | |
| Class 12.9 | 32 N·m | 26.6 kN | 35.5 kN | 970 MPa |
Clamp load is 75 % of proof load. Dry: K = 0.20. Lubricated: K = 0.15.
M8 at a glance
Wrench, hex key, drill and clearance sizes for the same bolt, so you can set up the job from one page.
Before you tighten
Lubricant changes the number
Oil, moly paste or anti-seize lowers the nut factor K, so the same torque produces far more clamp load. Use the lubricated value whenever the threads are oiled, or the bolt can yield.
Tighten in a star pattern
On flanges and covers, bring every bolt to about 30 %, then 70 %, then full torque, crossing the pattern each pass. Finish with one round at full torque in sequence.
Give the thread enough grip
Engage at least one diameter of thread in steel, about 1.5 diameters in cast iron and 2 diameters in aluminium, or the internal thread strips before the bolt reaches its clamp load.
How the number is worked out
The M8 class 8.8 figure, step by step. Every size on this site is calculated the same way.
- K
- 0.20, nut factor for dry plain steel
- F
- 15.9 kN, 75 % of the 21.2 kN proof load
- d
- 8 mm, nominal diameter
Where the inputs come from
Proof stress is taken from ISO 898-1: 580 MPa for class 8.8 at this diameter, 830 MPa for 10.9 and 970 MPa for 12.9. Multiply by the tensile stress area of 36.6 mm² to get the proof load.
The stress area comes from As = π/4 × (d − 0.9382 p)². Target clamp load is 75 % of proof load, the usual figure for joints that will be taken apart again.
Lubricated values use K = 0.15. For other coatings or preload targets, use the torque calculator.
Other metric sizes
Common questions
What is the torque for an M8 8.8 bolt?
About 25 N·m (19 ft-lb) with dry threads and about 19 N·m lubricated, based on 75 % of proof load.
What is the torque for an M8 10.9 bolt?
About 36 N·m (27 ft-lb) dry and 27 N·m lubricated.
What is the torque for an M8 12.9 bolt?
About 43 N·m (31 ft-lb) dry and 32 N·m lubricated.
What size wrench fits an M8 bolt?
13 mm for ISO hex bolts. An M8 socket head cap screw takes a 6 mm hex key.
What drill size do I need to tap M8?
Use a 6.8 mm drill for M8 × 1.25 coarse thread.