The calculator
Runs in your browser. Torque in any unit: the correction is a ratio of lengths.
How the correction works
A click wrench measures the moment about its own drive, at the handgrip it was calibrated to. With an adapter on the drive, the fastener sits at the adapter's drive instead. Your hand still pulls at the same point, but the lever from your hand to the fastener is now L + E, so the fastener gets more torque than the wrench shows.
AC 43.13-1B, paragraph 7-40h: “If special adapters are used which will change the effective length of the torque wrench, the final torque indication or wrench setting must be adjusted accordingly.” Figure 7-2 gives Y = T × L / (L + E) for an adapter that extends the wrench and Y = T × L / (L − E) for one turned back under it, where T is the torque wanted, Y the wrench setting, L the effective length of the lever and E the effective length of the extension.
At an angle. The AC covers the two in-line cases. Between them the effective lever is the straight line from the handgrip to the adapter drive: √(L² + E² + 2·L·E·cos θ). At 90° that is √(L² + E²), barely longer than L, which is why a crowfoot set at right angles needs almost no correction.
Worked example — AC 43.13-1B Figure 7-2
The AC's own example: 135 lb-in wanted, a 10.0 in wrench, a 1.5 in adapter.
- Adapter extending the wrench: Y = 135 × 10 / (10 + 1.5) = 1350 / 11.5 = 117.4 lb-in.
- Same adapter turned back: Y = 135 × 10 / (10 − 1.5) = 1350 / 8.5 = 158.8 lb-in.
- At 90°: effective lever √(10² + 1.5²) = 10.11 in, Y = 1350 / 10.11 = 133.5 lb-in, 1.1 % under.
Set 117 lb-in with the adapter extending the wrench, 159 lb-in turned back. These are the AC's figures.
Limits — when NOT to use this
- L is the calibrated length, not the overall length. Measure from the drive centreline to the handgrip centreline. Some wrenches mark it; otherwise take it from the wrench's data.
- Hold the handgrip where it was calibrated. The formula assumes your hand stays on that point. Pulling at the end of a long handle changes L.
- Beam and dial wrenches read the moment differently from click wrenches of some designs. Check the manufacturer's instructions for adapter use.
- The torque value itself comes from the manufacturer's data or AC 43.13-1B Table 7-1, with friction drag added where the AC requires it. This calculator only converts it for the adapter.
Questions people actually ask
Do I need to correct for a crowfoot at 90 degrees?
Very little. At 90° the lever grows from L to √(L² + E²): for a 2 in crowfoot on a 14 in wrench that is 14.14 in, about 1 %. The calculator gives the exact figure; whether to apply it is a matter of your procedure.
Does the torque unit matter?
No. The correction is the ratio L / (L ± E), so it works the same in lb-in, lb-ft or N·m. Keep L and E in the same unit.
Why is the setting higher when the adapter is turned back?
Because the fastener is now between the wrench drive and your hand: the lever is shorter, so you need more pull, and the wrench reads more than the fastener receives.
Sources
- FAA AC 43.13-1B CHG 1, Chapter 7, par. 7-40h and Figure 7-2 — the requirement to adjust the setting for adapters, both formulas, the legend for T, Y, L and E, and the 135 lb-in worked example reproduced above.