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Bolt Torque to Tension Estimator – Online Simple Calculator

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Bolt Torque to Tension Estimator

Estimate bolt preload tension from applied torque using the standard engineering formula T = K × F × D

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Note: This is an estimate based on the simplified formula. Actual preload may vary ±25% due to friction, surface condition, and other factors. Always verify critical applications.

K-Factor Reference Table
Bolt Condition Typical K-Factor Range
Dry Steel (Plain, as-received) 0.20 0.15 – 0.25
Lubricated Steel (oil/grease) 0.15 0.10 – 0.18
Zinc-Plated Steel 0.20 0.15 – 0.25
Stainless Steel (Dry) 0.30 0.20 – 0.35
Stainless Steel (Lubricated) 0.25 0.18 – 0.28
Copper Alloy / Brass 0.18 0.12 – 0.22
Cadmium-Plated 0.16 0.12 – 0.20
PTFE-Coated / Anti-Seize 0.12 0.08 – 0.16

* Values are approximate. Actual K-factor depends on specific materials, coatings, and lubrication conditions.

Formula & Explanation
T = K × F × D

T = Applied Torque (N·m or lb·in)
K = K-Factor (nut factor, dimensionless)
F = Bolt Preload / Tension (N or lbf)
D = Nominal Bolt Diameter (m or in)

Rearranged for tension:

F = T / (K × D)

The torque-tension relationship is an approximation. In practice, only about 10–15% of the applied torque actually produces bolt tension; the rest overcomes friction under the bolt head and in the threads. The K-factor accounts for these combined frictional effects.

Frequently Asked Questions

The torque applied to a bolt is converted into tension (preload) through the inclined plane of the threads. The relationship is approximated by the formula T = K × F × D, where T is torque, K is the nut factor, F is the resulting tension, and D is the bolt diameter. However, friction accounts for roughly 85–90% of the applied torque, making the K-factor critical for accurate estimation.

The K-factor (also called the nut factor or torque coefficient) is a dimensionless value that represents the combined effects of friction in the threads and under the bolt head. It typically ranges from 0.10 to 0.35 depending on materials, surface finish, lubrication, and coatings. A small change in K-factor can significantly alter the resulting tension, so choosing the correct value is essential for accurate torque-to-tension conversion.

Even under controlled laboratory conditions, torque-based tension estimates can vary by ±15% to ±25%. In real-world applications with unknown friction conditions, variations can exceed ±35%. For critical joints, direct tension measurement methods (such as load-indicating washers, ultrasonic bolt meters, or strain gauges) are strongly recommended over torque-based estimates.

Dry steel bolts typically use K ≈ 0.20 (range 0.15–0.25).
Lubricated steel bolts typically use K ≈ 0.15 (range 0.10–0.18).
Lubrication reduces friction, meaning less torque is lost to friction and more goes into bolt tension. This is why lubricated bolts achieve higher tension at the same torque—or require less torque to achieve the same tension. Always check your specific bolt manufacturer's recommendations when available.

Stainless steel is prone to galling—a form of adhesive wear where material transfers between sliding surfaces under pressure. This increases thread friction significantly, resulting in higher K-factors (typically 0.25–0.35 dry). Using proper lubrication or anti-seize compounds on stainless fasteners is highly recommended to reduce galling risk and achieve more consistent tension.

Yes! Simply rearrange the formula: T = K × F × D. Multiply your desired tension (F) by the K-factor and the bolt diameter (D) to find the required torque. For example, to achieve 20,000 N of tension with an M12 bolt (D=12mm=0.012m) and K=0.20: T = 0.20 × 20,000 × 0.012 = 48 N·m. Use consistent units throughout your calculation.

Metric: Torque in N·m, diameter in meters (e.g., 12mm = 0.012m), tension in Newtons (N).
Imperial: Torque in lb·in, diameter in inches, tension in pounds-force (lbf).
Our calculator handles unit conversions automatically, so you can mix and match common units like N·m with mm or lb·ft with inches and get accurate results instantly.
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