Bolt Torque Calculator
Get the tightening torque for a metric bolt from its size, grade and lubrication state — with the preload, stress area and K-factor assumptions documented.
Open calculatorLast updated: 2026-09-15
How the calculation works
- The tensile stress area is computed from the nominal diameter and the standard coarse pitch (ISO 898 approximation).
- Preload is set at 75% of the proof load for the selected grade — the standard design practice for reusable fasteners.
- Torque follows T = K × F × d with K = 0.20 dry or 0.15 lubricated, the widely used nut-factor model.
Worked example
Interpreting the result
The torque achieves a target stretch (preload), not the torque itself — lubrication changes the torque needed for the same preload dramatically. If a joint leaks or loosens, check lubrication and reused-bolt conditions before increasing torque. Yield-torque (TTI) bolts and critical gasketed joints must follow manufacturer specs.
Assumptions
- ISO metric coarse threads, steel bolt and nut, standard washer.
- 75% of proof load (JOINT NOT PERMANENT). Joints torqued to yield use ~90%.
Limitations
- The nut factor varies 0.10–0.25 with coating, plating and washers — real preload scatter is ±25–35% even with controlled torque.
- Fine threads, flanged heads and prevailing-torque nuts need specific data.
- Aluminum or cast housings may limit clamp load below the bolt's capacity.
Torque to preload: why lubrication changes the answer by 40%
Only about 10–15% of the torque you apply stretches the bolt; the rest is friction under the head and in the threads. That is why the same torque value produces wildly different preload depending on lubrication — a dry, galvanized or anti-seize thread each maps torque to tension differently, and the calculator's friction assumption matters as much as the numbers themselves. The correct habit: pick the friction condition that matches your assembly, and use a torque wrench — the calculation is worthless if the tightening is done by feel.
Grade markings and what they authorize
- Metric 8.8, 10.9 and 12.9 (and SAE grade 5/8) have different yield strengths — applying a 12.9 torque table to an 8.8 bolt stretches it past yield, permanently.
- Gasketed joints, aluminum castings and plastic housings may have torque limits set by the sealed component, not the bolt — a lower value wins.
- Critical joints (brakes, pressure boundaries, structural flanges) follow the engineer's spec and a calibrated procedure, not a generic table.
Frequently asked questions
What torque for an M8 8.8 bolt?
About 24 N·m dry (18 lb·ft). Lubricated, roughly 18 N·m achieves the same preload — always note the condition used.
Why does lubrication reduce torque?
Most tightening torque is consumed by thread and underhead friction, not bolt stretch. Reducing friction sends more torque into stretch, so less input torque reaches the same preload.