Bolt Torque & Preload Calculator
Tightening torque from target preload, or clamp force from a known torque, for metric coarse/fine, UNC and UNF bolts: property class or SAE grade, nut factor K or thread and head friction coefficients, preload scatter, bolt stretch and a torque chart.
How it works
Short form: T = K × F × d · Preload F = % of proof × proof stress × As · As = π/4 × (d − 0.9382 p)² metric, (d − 0.9743 p)² inch · Friction form (ISO 16047 / VDI 2230): T = F × (p ÷ 2π + 0.577 μ_th d₂ + μ_h D_km ÷ 2), d₂ = d − 0.6495 p · Reverse: F = T ÷ (K × d) · Stretch ΔL = F × L_grip ÷ (As × E)
A bolted joint works by stretching the bolt like a very stiff spring so it clamps the parts together. The torque you apply is only a proxy for that stretch: typically 85–90% of it is spent overcoming friction in the threads and under the head or nut. The short-form equation T = K × F × d folds all of that friction into one nut factor K, from about 0.10 for a waxed fastener to 0.30 for a dry, as-received one — which is why lubricating a bolt and then applying the “dry” torque can stretch it past yield. For more control, switch to the friction-coefficient form used by ISO 16047 and VDI 2230, which splits torque into the part that stretches the bolt (thread pitch), thread friction and under-head friction. Preload is set as a share of the bolt’s proof load, which depends on the property class or SAE grade and the thread’s tensile stress area; you can also work backwards from a known torque to the clamp force it produces. Because a torque wrench leaves roughly ±25% preload scatter, the calculator shows the expected preload range after embedding losses, warns when the high end reaches yield, and lists bolt stretch over your grip length, a torque chart for every size in the series and a comparison across friction conditions.
Worked example
M12 × 1.75 class 8.8 (As 84.3 mm², proof 580 MPa) to 75% of proof: F = 0.75 × 580 × 84.3 = 36.7 kN. With K = 0.2, T = 0.2 × 36,670 N × 0.012 m = 88 N·m (65 lbf·ft). The bolt stretches about 53 µm over 25 mm of grip, roughly 11° of nut rotation past snug. Using friction coefficients instead (μ = 0.12 in the threads and under the head), the torque per newton is 0.279 + 0.752 + 0.936 = 1.967 mm, so T = 72 N·m — equivalent to K = 0.164. Working backwards, 88 N·m on an oiled (K = 0.18) M12 gives 40.7 kN, 83% of proof.
Sources: ISO 898-1 — Mechanical properties of fasteners (property classes) · NIST SP 811 — Guide for the Use of the SI (conversion factors)
Frequently asked questions
Why do published torque tables differ?
Different K or friction assumptions (0.17–0.22 is common for plated bolts), different preload targets (65–90% of proof or yield) and rounding. Always check which lubrication condition a table assumes — this calculator shows the torque for each condition side by side.
Should I use the nut factor K or friction coefficients?
K is quick and matches most torque charts. Thread and under-head friction coefficients (μ) are what ISO 16047 tests measure and what VDI 2230 uses; they account for the thread pitch and the bearing diameter of the head, so they are better when the fastener supplier gives μ values (often 0.08–0.16).
How much clamp force does a given torque produce?
Switch to “Preload from torque” and enter the torque in N·m, lbf·ft, lbf·in or kgf·m. Preload is F = T ÷ (K × d); the calculator also shows it as a share of proof load and flags torques that would yield the bolt.
What happens if I lubricate a bolt but use the dry torque?
Less friction means more of the torque turns into tension. Going from K = 0.20 to 0.12 at the same torque raises preload by 67%, which can yield or snap the bolt. Reduce the torque in proportion to K when you lubricate.
Should I re-torque after a while?
Joints with gaskets or soft materials relax (embedment); re-torquing after the first thermal cycle or 24 h is standard for flanges and cylinder heads. The embedding loss setting shows how much preload you might lose.
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