Tightening Torque Table by Bolt Diameter and Class

After detailing the torque-tension relationship and the Kellermann & Klein formula, here's a reference table of typical tightening torques by bolt diameter and class. Useful for a first order of magnitude — must be adjusted for your actual application (see the assumptions below).

Calculation assumptions — read before the table

These values are generated by directly running BoltCore's own calculation engine — the same one used in the application — for each diameter/class combination, not from a reconstructed external formula. Parameters used, explicitly:

• Adjustable friction coefficient (0.09 / 0.12 / 0.14 via the selector below) — assumed identical for the thread and under the head, a common simplification that doesn't always reflect reality (see our article on friction, which shows a realistic range of 0.08 to 0.30 depending on surface treatment, and where thread µ and under-head µ can differ).
• Target tightening margin = 5% — i.e. a tightening Von Mises stress equal to Re/1.05 (yield strength divided by 1.05). A target closer to commonly published "classic" torque tables than the more conservative 75% of proof strength (Sp) convention used in an earlier version of this article.
• ISO 273 medium-series clearance hole, sized per diameter.
• Standard ISO 4014 hex head, standard ISO 724 coarse-pitch metric thread.

Change any of these parameters (different actual friction, different target margin, different bolt head) and the torques change significantly.

Tightening torque table (M4 to M24)

Indicative tightening torques, generated directly by BoltCore's calculation engine for a 5% tightening margin:

Values generated on August 7, 2026 by running BoltCore's calculation engine for each diameter, class and friction coefficient (target tightening margin 5%, ISO 273 medium-series clearance hole), with the displayed margin directly verified in the interface (5% confirmed across all 90 combinations). Does not replace a calculation specific to your application.

Why this table doesn't replace a dedicated calculation

This table gives a useful order of magnitude for preliminary sizing or a quick sanity check — but it relies on generic assumptions that almost never exactly match your actual application. The friction coefficient in particular can change the required torque by more than 50% between a dry bolt and one with a MoS2 coating, as detailed in our dedicated article. For a justification file that needs to hold up in design review, a calculation with your actual parameters remains essential.

How to adjust these values

• Friction different from 0.14 — torque varies almost proportionally with the friction coefficient used; a µ of 0.10 reduces torque by about 25% versus the table values, a µ of 0.20 increases it by about 35%.
• Target margin different from 5% — torque varies proportionally with the target stress; targeting a 10% margin instead of 5% reduces torque by about 4.3%, targeting 0% margin (stress = Re exactly) increases it by about 5%.
• Different head type (socket head instead of hex) — the bearing diameter changes, which alters the torque needed for the same preload.
• Stainless bolts or specific surface treatment — A2/A4 classes have different yield strengths than steel classes; see our article on strength classes.

Automating this with BoltCore

Frequently asked questions