Calculate recommended bolt tightening torque from preload, nominal diameter, and nut factor for metric and imperial fasteners.
This page turns a preload you already know into a torque and does not check it against the bolt's proof load, so it accepts a preload the bolt cannot carry. Check the preload first in the Bolt Clamp Force Calculator, or start from a bolt size and strength class in the Screw Torque Calculator.
Example Result
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The calculated result will appear here after you press Calculate.
The most widely used formula for estimating bolt tightening torque is T = K × F_i × d, where T is the applied torque, K is the dimensionless nut factor (also called torque coefficient), F_i is the desired bolt preload (clamping force), and d is the nominal bolt diameter. The nut factor K accounts for friction on the thread flanks and under the nut bearing face. For clean dry steel threads K is typically 0.20; for lightly oiled or zinc-plated threads K is approximately 0.15–0.18; for PTFE (dry-film) lubricants K may fall as low as 0.10–0.13. The relationship is an estimate: actual preload scatter at a given torque is typically ±25–30% even in controlled conditions. Always consult manufacturer torque tables, joint-design standards (VDI 2230, ISO 16047, ASME PCC-1), and applicable fastener specifications before specifying torque on safety-critical joints. This calculator supports SI (mm / N / N·m) and Imperial (in / lbf / lb·ft) input.
Select your unit system — SI (metric) or Imperial.
Enter the target bolt preload — the clamping force you want the bolt to develop.
Enter the nominal bolt diameter (the thread major diameter, e.g. 12 for M12).
Enter the nut factor K. Use 0.20 for dry clean steel, 0.15–0.18 for oiled or plated threads. Check manufacturer data for accurate values.
Press Calculate. The result is the estimated tightening torque.
FlowUtil uses the widely applied short-form equation T = K × F_i × d, where T is the estimated tightening torque, K is the dimensionless nut factor (torque coefficient), F_i is the target bolt preload (clamping force), and d is the nominal bolt diameter. In SI units the diameter entered in millimetres is divided by 1000 to metres, so T comes out in N·m; in Imperial units the diameter in inches is divided by 12 to feet, giving T in lb·ft.
The nut factor K bundles thread-flank friction and nut-bearing-face friction into one empirical constant. It is not a material property — it depends on surface finish, plating, lubrication, and installation method. Typical values are about 0.20 for clean dry steel, 0.15–0.18 for lightly oiled or zinc-plated threads, and 0.10–0.13 for PTFE dry-film lubricant. Because K varies, the formula predicts preload only to roughly ±25–30% even under controlled conditions, so the result is an engineering estimate, not an exact value.
| Symbol | Meaning |
|---|---|
| T | Estimated tightening torque (N·m or lb·ft) |
| K | Nut factor / torque coefficient (dimensionless, typically 0.10–0.25) |
| F_i | Target bolt preload — desired clamping force (N or lbf) |
| d | Nominal bolt diameter (mm in SI, in in Imperial) |
1SI: M12 bolt, target preload 30,000 N, dry steel threads (K = 0.20).
T = K × F_i × d = 0.20 × 30,000 × (12/1000) = 0.20 × 30,000 × 0.012
T ≈ 72.0 N·m · ≈ 53.1 lb·ft
About 72 N·m of wrench torque is needed to develop 30 kN of clamping force on a dry M12 bolt. If the same bolt were oiled (K = 0.15) the torque would drop to 54 N·m for the same preload — applying the dry value to an oiled bolt would over-tension it.
2SI: M20 bolt, target preload 80,000 N, oiled threads (K = 0.15).
T = 0.15 × 80,000 × (20/1000) = 0.15 × 80,000 × 0.020
T = 240 N·m · ≈ 177.0 lb·ft
A large M20 bolt needs 240 N·m to reach 80 kN of preload when lubricated. The torque scales linearly with both diameter and preload, so doubling the bolt from M12 to M20 (at the same K and preload) more than doubles the required torque because diameter and the chosen preload both rise.
3Imperial: 1/2 in bolt, target preload 6,700 lbf, dry threads (K = 0.20).
T = K × F_i × d / 12 = 0.20 × 6,700 × 0.500 / 12
T ≈ 55.8 lb·ft · ≈ 75.6 N·m
A 1/2-inch bolt torqued to about 56 lb·ft develops roughly 6,700 lbf of clamp load under dry conditions. The /12 factor converts the inch·pound-force product into pound-feet, the unit most torque wrenches display in Imperial practice.
The result is an estimated tightening torque in N·m (SI) or lb·ft (Imperial), with conversions to the other common torque units. It is the wrench setting expected to produce the target preload F_i for the given diameter and nut factor — not a guaranteed clamp force. Because the nut factor varies with surface condition, the actual preload achieved at that torque typically scatters by ±25–30%.
A lower K means less torque is needed for the same preload, which is why lubricated bolts must be torqued to a lower setting than dry ones. Applying a dry-thread torque specification to a lubricated bolt can yield or fracture the fastener; always confirm whether a published torque value assumes dry or lubricated conditions.
For safety-critical joints — pressure-boundary flanges, structural connections, lifting hardware — torque control alone is not precise enough. Direct-tension indicators, bolt elongation measurement, or hydraulic tensioning are used where the clamp force must be controlled tightly. This calculator supports preliminary sizing and checking, not final specification of such joints.
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