How this instrument works
When you tighten a bolt, the torque you apply on the wrench doesn't translate directly into clamp force — most of that torque is actually consumed overcoming thread friction and friction under the bolt head or nut, and only a fraction converts into the axial preload that actually clamps the joint together. The relationship between applied torque and resulting clamp force is captured in the short-form torque-tension equation: T = K x D x F, where K is a single number, the "nut factor," that lumps together all of that friction behavior.
The nut factor K isn't a fixed physical constant — it depends on the bolt and nut material, surface finish, plating, thread condition, and especially lubrication. Dry, plated steel threads commonly run around K=0.20, while lubricated threads can drop to K=0.10-0.15, meaning a lubricated bolt reaches the same clamp force at meaningfully lower applied torque. Because K varies so much with real-world joint conditions, always use the fastener manufacturer's specified K value when one is given — the default here is a commonly assumed planning figure, not a guarantee for your specific hardware.
This equation is a widely used approximation, not an exact physical law — actual achieved clamp force from a given torque can vary meaningfully (some references cite variation as high as plus or minus 25-30%) due to the many friction factors K only approximates. For critical or safety-relevant joints, follow the torque specification from the fastener manufacturer, equipment manual, or governing engineering standard rather than relying on this calculator alone.
- Enter the bolt's nominal diameter in inches.
- Enter your target clamp force (preload) in pounds-force.
- Enter the nut factor K — use the fastener manufacturer's specified value if given, or the default for a rough dry-steel estimate.
- Read the resulting tightening torque in both inch-pounds and foot-pounds.
Tightening a 1/2 in bolt to an 8,000 lbf preload
A 1/2 in bolt needs to reach a target clamp force of 8,000 lbf, and the joint's dry, plated steel threads suggest the commonly assumed nut factor K=0.20. Torque: T = 0.20 x 0.5 x 8,000 = 800 lbf-in, or 800/12 = 66.667 lbf-ft — a torque setting a torque wrench can dial in directly.
Switch to a larger 3/4 in bolt at a higher 15,000 lbf preload, but with lubricated threads bringing K down to 0.15: T = 0.15 x 0.75 x 15,000 = 1,687.5 lbf-in, or 140.625 lbf-ft. Notice how much torque the lubrication saved — the same bolt dry (K=0.20) would need roughly 187.5 lbf-ft for that preload, nearly 47 lbf-ft more, purely from the friction difference lubrication makes.
Questions
What nut factor (K) should I actually use?
Always use the value specified by the fastener manufacturer or your engineering drawing when one is given — it accounts for the exact plating, lubrication, and thread condition of that specific hardware. As a rough planning default in the absence of a spec, K=0.20 is commonly assumed for dry, plated steel threads, dropping to roughly 0.10-0.15 for lubricated threads.
Why does lubrication change the torque needed for the same clamp force?
Most of the torque you apply to a bolt is consumed by friction — between the threads, and under the bolt head or nut — rather than converting into clamp force. Lubrication reduces that friction, which lowers the nut factor K and means less applied torque is needed to reach the same target preload.
How accurate is torque as a way to control clamp force?
Less accurate than direct tension measurement. Because the nut factor K only approximates real friction behavior, torque-based tightening of the same bolt under nominally identical conditions can still produce meaningfully different actual clamp forces — commonly cited scatter is on the order of plus or minus 25-30%. For high-precision or safety-critical joints, methods like turn-of-nut, direct tension indicators, or ultrasonic bolt-tension measurement are more reliable than torque alone.
Is this safe to use for structural or safety-critical bolted connections?
Use this only as a general planning reference. Structural, pressure-vessel, automotive, and other safety-critical bolted joints should always be tightened to the torque (or tension) specification given by the governing engineering drawing, equipment manual, or applicable code — not to a value estimated from a generic default nut factor.