152 torque figures — M8 in grade 8.8 takes 26 N·m
A target clamp force and a tightening torque for every meeting of 19 bolt sizes and 8 grades. Nothing is copied from a table: the grade number gives the strength, the thread gives the area, and the rest follows.
Torque is not the goal
What you actually want is clamp force — the bolt stretching and pressing two parts together. A bolt is a very stiff spring: it stretches a little and pushes hard. Torque is only how you produce that stretch, so the same clamp force needs less torque on a slipperier thread.
The grade number is the strength
In 8.8, the first digit means 800 MPa tensile; the second says the yield is eight tenths of that — 640 MPa. A 12.9 is 1200 MPa with a yield of 1080. There is no table to look up: the two digits stamped on the head say it. Only stainless marks like A2-70 follow a different rule.
Nine tenths of the effort goes to friction
Roughly half the effort is lost to friction in the threads and another half under the head. Only about a tenth actually stretches the bolt. Oil the thread and the same clamp force needs a quarter less torque — and applying a dry-spec torque to an oiled bolt overtightens it by that same quarter.
We pull to seven tenths of yield
Pull a bolt all the way to yield and it stays stretched, so it will not give the same force the next time it is loosened and retightened. Common practice is therefore 70% of yield. Every clamp force here is that figure, and the torque is worked back from it.
These figures are a starting point
Whenever the manufacturer specifies a torque, that number wins. Bolts into aluminium, bolts squeezing a gasket and bolts with thread locker are all set differently from this calculation.
Find it by size and grade
Tightening torque · plain dry steel
M3Pitch 0.5mm · 5.03mm²
M3.5Pitch 0.6mm · 6.77mm²
M4Pitch 0.7mm · 8.78mm²
M5Pitch 0.8mm · 14.18mm²appliances and electronics
M6Pitch 1mm · 20.12mm²
M7Pitch 1mm · 28.86mm²
M8Pitch 1.25mm · 36.61mm²bicycle cranks
M10Pitch 1.5mm · 57.99mm²car suspension
M12Pitch 1.75mm · 84.26mm²
M14Pitch 2mm · 115.44mm²
M16Pitch 2mm · 156.67mm²machine frames
M18Pitch 2.5mm · 192.47mm²
M20Pitch 2.5mm · 244.79mm²
M22Pitch 2.5mm · 303.4mm²
M24Pitch 3mm · 352.49mm²heavy equipment and structures
M27Pitch 3mm · 459.39mm²
M30Pitch 3.5mm · 560.59mm²
M33Pitch 3.5mm · 693.56mm²
M36Pitch 4mm · 816.74mm²
How to read this
- Torque turns the bolt; what you want is the clamp force the stretched bolt provides.
- The slipperier the thread, the less torque the same clamp force needs — pick the condition you have.
- Clamp force is set at 70% of yield. A manufacturer figure always wins over this one.
- These are coarse threads. A fine thread has slightly more area, so its numbers differ.
Frequently asked questions
Q. What torque does an M8 bolt take?
26 N·m in grade 8.8, dry. Oiled, drop it to 20 N·m to reach the same clamp force.
Q. What does the number 8.8 mean?
The first 8 is 800 MPa tensile strength; the second says yield is eight tenths of that, 640 MPa. It is stamped on the head.
Q. Should I lower the torque if the thread is oiled?
Yes. Less friction means the same torque pulls harder, so a dry-spec figure overtightens an oiled bolt by roughly a quarter.
Q. What if I have no torque wrench?
There is no accurate substitute. Knowing the clamp force and grade at least tells you how far off a guess would be.
Q. Do stainless bolts use the same table?
No. A2-70 is 700 MPa tensile with 450 MPa yield, a different rule, and it galls easily so lubrication matters more.