[SpecCalcs]

M20x1.5 Bolt Torque

A M20x1.5 bolt in Class 10.9, dry, torques to about 676 N·m (499 lb-ft) at 75% of proof load.

20 mm major dia · 1.5 mm pitch · clean, undamaged threads · 75% of proof load

Torque calculator
Tightening torque
T=K⋅D⋅FT = K \cdot D \cdot F
T torque · K nut factor (friction) · D nominal diameter · F preload
Preload (target clamp load)
F=u⋅Sp⋅AtF = u \cdot S_p \cdot A_t
u utilisation (0.75) · Sp proof strength · At tensile stress area

M20x1.5 torque — all grades & lubrication (N·m)

GradeDry / as-received steel
K=0.2		Zinc plated
K=0.22		Lubricated (light oil)
K=0.15		Molybdenum disulfide
K=0.1		Anti-seize compound
K=0.13		Waxed / PTFE
K=0.1
Class 8.8489538367244318244
Class 10.9676744507338439338
Class 12.9790869593395514395
A2-70 Stainless367403275183238183

Why one number isn't enough: the same bolt can need half the torque lubricated versus dry, because K drops from ~0.20 to ~0.10. A dry-torque figure on a lubricated bolt over-tightens it and can snap it. Match the column to your assembly.

How the M20x1.5 torque value is worked out

Torque isn't the goal — clamp load is. The whole reason you tighten a bolt is to stretch it into a controlled tension that squeezes the joint together. For a M20x1.5 Class 12.9 bolt at 75% of proof load, that target tension is about 198 kN. Torque is just the indirect lever we use to reach it, and the relationship runs through the nut factor K.

The tensile stress area of a M20x1.5 thread is 272 mm² — this is the effective cross-section carrying the load, based on the mean of the pitch and minor diameters, not the nominal 20 mm. Multiply that area by the grade's proof strength and by 0.75, and you have the target preload F. Feed F into T = K·D·F and you get the torque figures in the table above.

Choosing the right friction condition

This is the input people get wrong most often. K bundles together every friction loss in the joint — under the bolt head and across the threads. It swings more than any other variable, and it changes the torque dramatically:

  • Dry / as-received (K≈0.20): a M20x1.5 Class 12.9 bolt wants about 790 N·m.
  • Lubricated with light oil (K≈0.15): the same bolt now needs only about 593 N·m to reach the same tension.
  • Moly / anti-seize (K≈0.10): down to roughly 395 N·m.

Notice the tension target never changed — only the torque needed to get there. If you look up a dry-torque number and then apply it to a bolt you've coated in anti-seize, you drive the tension far past proof load and risk yielding or snapping the fastener. Always dial the lubrication selector to match what's actually on the threads.

Grade matters as much as size

A M20x1.5 bolt isn't one spec — the grade sets the proof strength, and that scales the whole result. At the low end, Class 8.8 tolerates roughly 489 N·m dry; Class 12.9 in the same diameter takes about 790 N·m because it can safely hold far more tension. Torquing a lower-grade bolt to a higher-grade spec is another common way to fail a joint — check the head markings before you pick a row.

Assumptions behind these figures

  • Clean, undamaged threads. Rust, grit, or damaged threads raise K unpredictably.
  • 75% of proof load — the usual target for reusable structural joints. Gasketed or fatigue-critical joints may call for a different utilisation; adjust the preload field in the calculator.
  • Torque control has real scatter: even done carefully, achieved preload commonly varies ±25%. For anything safety-critical, angle-control or tension-measuring methods beat a torque wrench.

Tapping the mating hole? → M20x1.5 tap drill size