Bolt torque calculator.
Compute recommended tightening torque from bolt grade, diameter, and lubrication state. Based on the K-factor formula T = K × D × P with proof loads from ISO 898-1 and SAE J429.
How this works
Torque does not tighten a bolt directly — it generates a clamping force (preload) that holds the joint. The relationship is:
T = K × D × P
where T is torque, K is the friction-dependent "nut factor," D is the bolt nominal diameter, and P is the target preload (typically 75 % of the bolt's proof load).
Why lubrication changes the answer so much
The K-factor lumps together thread friction, head bearing friction, and a small "geometry" term. Lubrication primarily reduces friction. Going from K = 0.20 (dry) to K = 0.10 (anti-seize) cuts the required torque in half for the same preload. If you don't account for that, you'll either snap the bolt (over-torque) or end up with a loose joint (under-torque).
Why this is hard for AI
The same bolt has 3–5 valid torque numbers depending on the lubricant, surface finish, and target preload — and the "right" answer depends on the joint design. A general-purpose AI gives you one number, often without specifying which conditions it assumed. The right tool shows the work.
When this calculator is wrong
- Torque-to-yield (TTY) bolts. Cylinder head bolts and many engine fasteners are torque-to-yield: torque to a value, then turn an additional angle. The torque alone is not the spec. Don't use this calculator for TTY bolts.
- Bolt-up of gasket joints. Pressure vessel and flange bolt-up follows a different process (cross-pattern multiple passes, target stress not preload). Use ASME PCC-1.
- High-temperature service. Bolts that operate hot (turbines, exhaust) need a different torque to account for thermal expansion and creep. Use manufacturer spec.
- Wood, plastic, or other soft material under the head. The preload will settle quickly; this calculator assumes both joint members are metal of comparable hardness.
- Used or galled fasteners. K-factor varies wildly on used bolts. Use a new bolt and a new nut for accurate preload.
Sources
- Metric grades (proof loads): ISO 898-1 — Mechanical properties of fasteners made of carbon steel and alloy steel.
- Imperial grades: SAE J429 — Mechanical and Material Requirements for Externally Threaded Fasteners.
- K-factor formula: Bickford, J. H. An Introduction to the Design and Behavior of Bolted Joints, 4th ed.
Disclaimer. Torque values shown are guidelines for typical metal-on-metal joints. For any joint where failure has consequences (structural, automotive, machinery), use the manufacturer's torque spec, not a general calculator.