Bolt torque calculator
Pick the bolt, grade and thread condition. The result uses T = K × F × d with the proof stress from ISO 898-1 or SAE J429.
Picking the nut factor
K matters more than anything else
K bundles thread and under-head friction. Typical values: 0.20 for plain steel as received, 0.22 for dry zinc plating, 0.15 for oiled threads and 0.10–0.12 for moly or anti-seize.
Preload target
75 % of proof load is the common target for joints that will come apart again. Structural and pressure joints usually follow their own specification.
Check the internal thread
High preload needs enough engaged thread: about one diameter in steel and two in aluminium.
Common questions
How do you calculate bolt torque?
Use T = K × F × d: torque equals nut factor times target clamp load times nominal diameter. A common target clamp load is 75 % of the bolt's proof load.
What K factor should I use?
About 0.20 for dry plain steel, 0.15 for oiled threads and 0.10–0.12 for anti-seize or moly paste.