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Bolt Torque Estimator

A reasonable starting-point torque for a dry or lubricated bolt — by size, grade, and what’s on the threads. Use it to get in the ballpark, not to override a spec.

Tightening torque depends on three things: the bolt’s size, its grade (how much it can take), and how slick the threads are. Pick those below and we’ll work out a target clamp load and the torque to reach it — using the standard T = K × D × F method engineers use for a first estimate.

Read this first. These are general estimates for a fastener tightened to about 75% of its proof load. They are not a substitute for a manufacturer’s torque spec. When a service manual, joint design, or gasket calls out a value, always use that — especially for engines, flanges, structural connections, and anything safety-critical.

Bolt size (coarse thread)

Grade

Thread condition

Estimated torque

Target clamp load—

In·lb—

Tightening torque—

Based on a target preload of 75% of proof load and the standard nut-factor equation. Real torque varies with surface finish, accuracy of your wrench, reused vs. new hardware, and joint stiffness — treat this as a starting point and back off if a joint or gasket calls for less.

What the numbers mean

T = K × D × F. Torque is only a way of getting clamp load — the squeeze that holds the joint. K is the “nut factor” (friction): slicker threads, lower K, less torque for the same clamp.

Where your wrench effort goes on a dry steel bolt (typical). Most of it fights friction — which is why lube changes the number so much.

Same ½″-13 Grade 5 bolt, same 9,046 lb clamp — only the thread condition changes. Put the dry 75 ft·lb on an anti-seized bolt and it’s pulled to about 18,092 lb — past its 12,062 lb proof load.

How the estimate works

The tool uses T = K × D × F, where D is the bolt diameter, F is the target clamp load, and K is the “nut factor” that captures friction — lower when the threads are lubricated, higher when they’re dry. The clamp load F is set to 75% of the bolt’s proof load (proof strength × the thread’s tensile stress area), which is the usual target for a reusable, non-critical joint.

Lubrication matters more than people expect: the same bolt wants noticeably less torque when the threads are waxed or treated with anti-seize, because more of your effort turns into clamp load instead of fighting friction. Over-torquing a lubricated bolt at its “dry” number can stretch or snap it.

Not sure of the grade? Check the head against the bolt grades & strength chart. Not sure of the size? Start with how to measure a fastener.

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