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Torque Wrench Basics for Machinists

Guide · Updated October 28, 2026

Torque wrenches set fasteners, vise bolts, chuck jaws and fixture clamps to a known preload. A click wrench is the most common, a beam wrench never loses calibration, and a dial wrench reads continuously. All of them need correct units and smooth pulls.

Calibrate measuring tools too

Like gage blocks, torque wrenches need periodic checks. Keep a calibration log for both. The free Gage Block Calculator builds the stack.

Step by step

Choose the type: Click for production, beam for simplicity, dial for audits and breakaway checks.
Pick the range: Use the middle 20–80% of the wrench range for best accuracy.
Set units: Check ft·lb, in·lb or N·m. 1 ft·lb = 12 in·lb = 1.356 N·m.
Pull smoothly: Pull on the handle center at 90°; stop at the click, do not jerk.
Account for extensions: A crowfoot or extension in line changes the reading; see the formula.
Store at lowest setting: Back a click wrench to its minimum after use.
Check calibration: Check yearly or about every 5,000 cycles against a tester.

Worked example

A fixture bolt needs 40 ft·lb, but you use a 2 in crowfoot in line with a 14 in wrench (handle center to drive). Setting = desired × L ÷ (L + E) = 40 × 14 ÷ (14 + 2) = 35 ft·lb. In N·m, 40 ft·lb × 1.356 = 54.2 N·m. If the crowfoot is turned 90° to the handle, E is effectively 0 and you set 40 ft·lb.

Dry vs lubricated threads

Torque specs usually assume a condition. Oiled threads can reach about 20–25% more preload at the same torque, so follow the spec for dry or lubed.

Common problems

Overtorque from a jerky pull past the click. Wrong units. A wrench left at high setting drifting. Reading at the extreme low end of its range.

Tips

Calibration: calibration interval thinking applies.
Vises: set up a milling vise.
Fixtures: use a fixture plate.

Quick shop check

Example: 30 ft-lb = 360 in-lb = about 40.7 N·m. Use the middle of the wrench range for best accuracy; ISO 6789-1 allows an adjustable click wrench (Type II, Class A) ±4% of the set value when its maximum torque is above 10 N·m, and ±6% at or below 10 N·m; a maker can claim tighter. Store click wrenches at their lowest setting.

FAQ

How do I convert ft·lb to N·m?

Multiply by 1.356.

Do extensions change torque?

In-line extensions do; use setting = T × L ÷ (L + E).

Should I store a click wrench at zero?

At its lowest setting, not below.

How often should I calibrate?

About yearly or every 5,000 cycles.

Can I use a torque wrench to loosen?

Avoid it on click wrenches; it can upset calibration.

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