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Bolt torque calculator

From bolt grade and size to the wrench setting, dry or lubricated — SAE grades in ft·lb, ISO classes in N·m.

System
Bolt size (UNC coarse)
Grade

Head marking: 3 radial head marks.

Thread condition

Result

75.4 ft·lb
1/2"-13 UNC · SAE Grade 5 · dry threads · ≈ 102.2 N·m
Target clamp force9046 lb
Basis75% of proof (85 ksi) × 0.1419 in²
FormulaT = K · d · F

Torque is a stand-in for what actually matters — the clamp force stretching the bolt. Friction eats most of the wrench's work, which is why lube changes the number by a quarter and why the manufacturer's spec sheet always wins.

Same clamp force, different wrench
Dry (K = 0.20)75.4 ft·lbLubricated (K = 0.15)56.5 ft·lbBoth bars aim at the SAME clamp force: 9046 lb. Lube lowers friction, so less wrench does the same job.
How is this calculated?

Using the tool

From grade to wrench setting, the honest way: the grade sets the proof strength, the size sets the stress area, 75% of proof becomes the target clamp force — and friction (dry or lubricated) decides how much torque it takes to get there. Works in both worlds: SAE grades in ft·lb, ISO classes in N·m.

The engineering

Tightening a bolt is stretching a spring: the wrench turns the thread, the thread pulls the bolt, and the stretch clamps the joint. The engineering chain runs grade → proof strength → clamp target → torque: SAE marks the grade with radial lines on the head (3 = Grade 5, 6 = Grade 8) while ISO stamps the class as a number (8.8, 10.9, 12.9 — roughly Grade 5, 8, and above), proof strength times the thread's stress area gives what the bolt can hold, 75% of that is the standard preload target, and T = K·d·F converts it to a wrench setting. The K factor is the friction reality — about 0.20 for dry steel, 0.15 lubricated — and since most of your torque is spent on friction, not stretch, changing the lube without changing the number over-tightens the joint.

New to the math? Learn the concept: Industrial mechanic fundamentals

Formula sources: SAE J429 proof strengths (Grades 2, 5, 8) and UNC tensile stress areas, ISO 898-1 proof stresses (classes 8.8, 10.9, 12.9) and metric-coarse stress areas, Short-form torque equation T = K·d·F, K ≈ 0.20 dry / 0.15 lubricated

Reference values from the standard short-form equation — actual K varies with plating, thread condition, and washers. The equipment manufacturer's torque spec ALWAYS overrides this table; never re-torque critical or torque-to-yield fasteners by generic values.