Bolt torque calculator.
Bolt torque calculator: Compute the recommended tightening torque for any bolt given grade, diameter, and lubrication state.
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.
Common questions
How is bolt torque calculated?
With T = K × D × P, where K is the nut factor (friction), D is the nominal bolt diameter, and P is the target preload (clamp force, usually about 75% of proof load). Most of the torque overcomes friction rather than creating tension directly.
Why does lubrication change the required torque so much?
Most of your torque fights friction under the head and in the threads. Lubrication lowers K (from about 0.20 dry to 0.15 or less), so the same torque produces much more clamp force. Applying dry-spec torque to a lubricated bolt can over-stretch or snap it — typically reduce torque 15–25% when lubricated.
What is bolt preload and why does it matter?
Preload is the tension (clamp force) the bolt holds after tightening. It is what actually keeps the joint together and resists loosening and fatigue. Torque is only the indirect means of reaching the right preload.
What is the difference between Grade 5 and Grade 8 bolts?
Higher grades are stronger steel with higher proof and tensile strength, so they accept more preload and more torque. SAE Grade 5 is roughly equivalent to metric class 8.8, and Grade 8 to class 10.9.
Should I use dry or lubricated torque values?
Match the spec to the fastener's actual condition. Torque tables state whether they assume dry, plated, or lubricated threads — mixing them up is the most common torque mistake.
Is a torque wrench accurate enough for critical joints?
Torque control only sets preload to roughly ±25% because friction varies bolt to bolt. For critical joints, use torque-angle (turn-of-nut) or a direct tension-indicating method instead of torque alone.
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.