This bolt tightening torque calculator helps you estimate the required torque when manufacturer specifications are unavailable. It computes the preload from the bolt's strength and stress area, then the torque needed to reach it for screws, bolts, and nuts.
1) Pick a system (metric or inch), then the bolt or screw diameter.
2) Strength class: choose the closest matching grade/material. Higher yield strength generally means higher allowable preload and torque. Our bolt strength calculator supports ISO, SAE, ASTM, and stainless steel grades.
3) Thread condition / lubrication: this is the biggest real-world uncertainty in bolt tightening torque calculation. Lubrication, coatings, thread condition, and washers can change friction significantly.
4) % of yield load: for non-critical bolts, conservative values (for example 60-70%) are often safer than pushing to the limit. This affects the bolt stress and resulting preload.
5) Thread type: affects the pitch and stress area (As) used in the estimate. Fine threads have slightly larger stress areas than coarse threads.
The well-known bolt torque equation is T = K × D × F: torque equals the nut factor K times the nominal diameter D times the preload F. It is easy to use, but a single K is only exact for mid-size bolts. The thread pitch and the head size do not grow in proportion to the diameter, so with a fixed K large bolts come out too high and small screws too low.
The calculator therefore uses the formula from ISO 16047 and VDI 2230:
T = F × (0.16·P + 0.58·μ·d₂ + μ·Dₖₘ/2)
The three terms are the torque that stretches the bolt through the thread pitch P, the friction in the threads (pitch diameter d₂) and the friction under the head or nut (effective bearing diameter Dₖₘ). Each lubrication option has a friction coefficient μ chosen so that an M10 bolt gives the same result as the familiar K value shown next to it.
Practical notes:
- For critical joints (brakes, steering, suspension, engine internals, safety-related fasteners), use manufacturer procedures and applicable standards.
- Torque is not the same as preload; torque is just a proxy and depends heavily on friction.
- This fastener torque calculator works for both bolt heads and nuts—use the same diameter and grade for accurate nut torque calculations.
- When unsure, use a lower percentage and verify by inspection and re-torque checks if appropriate.
- Need a quick reference? The bolt torque charts list every size and grade.
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