numvana

Bolt Preload Calculator (Torque to Clamp Force)

Calculate the clamp force (preload) a tightening torque produces in a threaded fastener, F = T ÷ (K·D), and the resulting axial stress in the bolt shank, using the standard nut-factor torque-tension equation.

Clamp force (preload)
25,000 N
Resulting bolt stress
431 MPa

How it works

The standard nut-factor ('short-form') torque-tension equation used in fastener engineering relates applied tightening torque T to the resulting clamp force F through T = K × D × F, where D is the bolt's nominal diameter and K is a dimensionless 'nut factor' lumping together thread friction, under-head friction, and lubrication condition. Rearranged for the clamp force actually being asked about here, F = T ÷ (K × D).

Dividing that clamp force by the bolt's tensile stress area At (a published value per bolt size and thread pitch — e.g. an M10 bolt is about 58mm², a 1/2"-13 UNC bolt about 0.1419in²) gives the resulting axial stress in the bolt shank, σ = F ÷ At — the figure that actually determines how close the joint is to the bolt material's proof or yield strength, which is what the design limit is actually set against.

Neither K nor At is invented here — K genuinely varies with plating, lubricant, and surface condition (typical reference values are offered in the FAQ, not assumed), and At depends on the specific bolt size and thread standard, so both are entered directly rather than guessed from a hardcoded table.

FAQ

What nut factor (K) should I actually use?

It depends on the fastener's plating, lubrication, and surface finish — commonly cited reference ranges are about 0.2 for plain, dry, uncoated steel threads and about 0.15 for lightly lubricated or zinc-plated threads, but the only way to know a specific bolted joint's actual K is to measure it (torque applied vs. clamp force achieved) or use the value published by the bolt or lubricant manufacturer for that exact combination. This calculator takes K as a direct input rather than assuming one, since no single number is correct for every fastener.

Where do I find my bolt's tensile stress area?

From a standard metric (ISO 898) or unified (SAE/UNC/UNF) thread chart for your bolt's exact diameter and thread pitch — it isn't something this calculator can infer from the diameter alone, since the same nominal diameter can come in a coarse or fine thread pitch with a slightly different stress area. Thread charts publishing this value are widely available from fastener manufacturers and engineering references.

Related calculators