Estimated tightening torque
N·m
Mechanics · Bolts, preload and tightening
Bolt tightening torque calculator
Estimate torque for a target preload with a simple model whose result depends strongly on actual friction conditions.
Created v1.0
By Thibaut Grzelak, Mechanical Analysis Engineer
03
MetricResults
Target preload used
kN
Nominal diameter used
mm
Nut factor used
—
View calculation detailsFormulas, SI conversion, numerical substitution and resultsOpen ↓
Calculation note
Full calculation details
Overall estimate using T = KFd. K groups thread, bearing-surface, coating and lubrication effects. Preload acts in one bolt with no relaxation or loss. For a critical joint, use manufacturer data or a qualified torque-tension procedure.
01Formulas and symbols
Formulas used
T
K × F × dEstimated tightening torqueK
T / (F × d)Factor grouping friction effects02Assumptions and limits
Scope of validity
- Overall estimate using T = KFd.
- K groups thread, bearing-surface, coating and lubrication effects.
- Preload acts in one bolt with no relaxation or loss.
- For a critical joint, use manufacturer data or a qualified torque-tension procedure.
03Validation example
Reference numerical case
- d = 12 mm, F = 35 kN and K = 0.20.
- T = 0.20 × 35,000 × 0.012.
- Estimated torque = 84 N·m.
04Frequently asked questions
Questions about the calculation
Is K = 0.20 always suitable?
No. It is only an illustrative starting value. Lubrication, coating, thread condition and bearing surfaces can change K substantially.
Why does torque not set preload exactly?
Most applied torque is dissipated by friction. Its variability creates substantial preload scatter.
Can this result be used for a critical joint?
Not by itself. Check the manufacturer specification, applicable code and a qualified tightening procedure.
05References
Technical references
- ISO 16047:2005, Fasteners — Torque/clamp force testing (with 2012 amendment).
- NASA RP-1228, Fastener Design Manual, simplified torque/preload relation.