Torque Wrench Extension Calculator (Setting with an Extension or Crow's Foot)
Setting to dial on a torque wrench so the bolt gets the torque you want when an extension is fitted in line with the wrench.
Results
The dashed line marks the value you entered.
What this tool does
An extension that lengthens the wrench in a straight line increases the leverage on the bolt without the wrench knowing: the wrench clicks at its set torque, but the bolt receives that torque multiplied by (L + E) ÷ L. To get the wanted torque at the bolt, set the wrench to the wanted torque times L ÷ (L + E). A 100 mm extension on a 400 mm wrench means setting 80 N·m to get 100; ignoring it over-tightens the bolt by a quarter. The correction holds only for an extension in line with the wrench — at right angles to it, no correction is needed. Use the same unit for L and E, measured from the centre of the drive.
Formula
setting = wanted torque × L ÷ (L + E) (L = wrench length, E = length added in line)
Variables
| Symbol | Meaning | Unit |
|---|---|---|
tq | Torque | N·m |
l | Effective length of the wrench (drive centre to hand grip) | mm |
ex | Length of the extension (in line with the wrench) | mm |
ST | Setting to dial on the wrench | N·m |
FC | Correction factor L ÷ (L + E) | — |
OV | Over-tightening if you ignore the extension | % |
Worked example
- Torque100 N·m
- Effective length of the wrench (drive centre to hand grip)400 mm
- Length of the extension (in line with the wrench)100 mm
- Setting to dial on the wrench80.0 N·m
- Correction factor L ÷ (L + E)0.800
- Over-tightening if you ignore the extension25.0 %
Limitations
- Mixing units is the most common source of error. Convert every input to the units shown next to each field before calculating.
- The formula assumes ideal conditions: no friction losses, no air resistance and no efficiency losses unless you enter them.