Bolt tightening torque: 8.8, 10.9, 12.9 chart and calculator
Published September 28, 2026 — torques and preloads from M3 to M36, a calculator that accounts for friction, and why a torque that's right on paper can still give a wrong joint.
Tightening torque calculator
Pick the bolt, its property class and your joint's friction coefficient: you get the permissible preload and the matching tightening torque (VDI 2230 method, 90% of yield strength used in equivalent stress, ISO 4017 hex head bolt, medium clearance hole).
Maximum torque, not a number to apply blindly. These values use 90% of the bolt's yield strength: it's the maximum permissible tightening. If the clamped part is aluminium, plastic or cast iron, or the thread engagement is short, the part is the limit — not the bolt.
1. Chart: torques and preloads, M3 to M36 (µ = 0.12)
ISO metric coarse thread, friction of 0.12 under the head and in the thread (oiled steel, the most common case). Torques in newton-metres, preloads in kilonewtons.
Values calculated with the VDI 2230 method and the minimum yield strengths of ISO 898-1 (640 MPa for 8.8 up to M16, 660 MPa above; 940 MPa for 10.9; 1,100 MPa for 12.9). They match fastener manufacturers' published charts to within rounding.
2. Friction changes everything
About 90% of the applied torque goes into friction (under the head and in the thread); only about 10% actually stretches the bolt. As a result, at the same torque, an MoS2-greased bolt is far more stretched than a dry one. Applying a “dry” torque to a lubricated bolt can make it yield.
- Plain, phosphated or zinc-plated steel, oiled: µ = 0.12 is the reference value of the charts, and of this page.
- Molybdenum disulphide (MoS2) paste or assembly lubricant: lower friction, so torque must be reduced. The lubricant maker gives the value.
- Dry A2 / A4 stainless: high and widely scattered friction, with a risk of galling. Use anti-seize paste and charts specific to stainless bolts (their yield strengths differ from classes 8.8 to 12.9).
These ranges are ballpark figures. For a critical joint, the coefficient is measured on a test rig with the actual fasteners and lubricant, or taken from the manufacturer's data sheet.
3. Tightening right: what really matters
- The torque wrench has its own tolerance (± 4 to ± 6% depending on type, ISO 6789) and, above all, friction scatter makes the resulting preload vary by ± 20% or more. For critical joints, use angle tightening or measure elongation.
- A 12.9 bolt in an aluminium tapped hole is pointless: the part's thread fails first. Check thread engagement (at least 2 × d in aluminium, see the thread chart) or use a thread insert.
- Tighten in a cross pattern, in two or three passes on flanges and covers: tightening in sequence makes the flange seat unevenly.
- Put the torque on the drawing when it matters, with the lubrication condition. A torque without its associated µ is incomplete data.
4. FAQ
What tightening torque for an M8 8.8 bolt?
About 24.6 N·m with a friction of 0.12 (oiled steel), for a preload of about 18.6 kN. In class 10.9, about 36.1 N·m.
What tightening torque for an M10 bolt?
With µ = 0.12: about 48 N·m in 8.8, 71 N·m in 10.9 and 83 N·m in 12.9. These are maximum torques: the tapped part may require less.
What does property class 8.8 mean?
The first number is the nominal tensile strength divided by 100 (8 → 800 MPa), the second is the ratio of yield to tensile strength (0.8). An 8.8 bolt therefore has a nominal yield strength of 640 MPa.
Should a bolt be greased before tightening?
Only if the torque was defined for a greased bolt. Lubricant lowers friction: at the same torque the bolt is stretched much more and can exceed its yield strength. Torque and lubrication condition always go together.
How do I convert N·m to kgf·m?
Divide by 9.81: 48 N·m ≈ 4.9 kgf·m (shops often use daN·m: 48 N·m = 4.8 daN·m).