Tightening torque from target clamp force and nut factor (T = K×D×F) — the standard fastener relationship. The nut factor K genuinely varies with lubrication and thread condition, so it's a direct input here, not a hidden assumption.
Standard method — stated scope limits apply
Direct Torque Calculation
K ≈ 0.2 for dry, as-received steel; ≈ 0.12–0.15 well-lubricated/plated — use the bolt's own datasheet if you have one.
Or: Estimate Clamp Force from Bolt Grade
Enter values and calculate.
About This Calculation
Bolt tightening torque and clamp force are related by one simple, universal formula (T = K×D×F) — but the K in that formula (the "nut factor") is not a universal constant. It genuinely depends on whether the threads are dry, lubricated, or plated, and can easily vary by 30-50% between conditions. Getting the formula right is easy; the real engineering judgment is choosing the correct K for your actual bolt and lubrication condition, which is why this calculator treats K as a direct input rather than a hidden assumption.
Frequently Asked Questions
What K value should I use if I don’t know it?
0.2 is the widely-used default for dry, as-received (non-lubricated, non-plated) steel fasteners. Lubricated or plated fasteners commonly run lower, around 0.12–0.15 — if the bolt manufacturer specifies a K factor, use theirs instead of a generic default.
Why does the same bolt need less torque when lubricated?
Most of the tightening torque (typically 80-90%) goes into overcoming friction, not into actually stretching the bolt to create clamp force. Lubrication reduces that friction, so a lubricated bolt reaches the same clamp force at a noticeably lower torque — which is exactly why using a dry-condition torque spec on a lubricated bolt under-tightens it.
What is "proof load" and why does the grade calculator use a percentage of it?
Proof load is the load a bolt can withstand without permanent (plastic) deformation, defined by its property class (e.g. 8.8, 10.9) in ISO 898-1. Real bolted joints are commonly tightened to 65-90% of proof load — tightening to 100% leaves no margin for the joint to be re-torqued or for minor overload before yielding.
Are the bolt sizes and grades in this calculator standard values?
Yes — the tensile stress areas come from ISO 724 and the proof stresses from ISO 898-1, both published, tabulated standards, not estimates. A size or grade outside this table should be looked up from its own governing standard.