How to use the torque wrench extension calculator
- Enter the torque you want at the bolt and choose the unit.
- Enter the wrench length from the center of the drive square to the center of the handle grip, and the extension length from the drive square to the center of the fastener. Use the same length unit for both.
- Enter the angle between the extension and the wrench line. Use 0 when the extension points straight along the wrench and 90 when it sticks out sideways.
- Set the wrench to the value shown. The check line gives back your target.
Formula
effective length = wrench length + extension length x cos(angle)
wrench setting = target torque x wrench length / effective length
check: setting x effective length / wrench length = target torque
The wrench measures the force at the handle times the wrench length. An extension that lies along the wrench makes the lever longer, so the bolt sees more torque than the wrench shows. Only the part of the extension that lies along the wrench counts.
A 15 inch wrench with a 3 inch extension
To put 100 lbf-ft on a bolt with an extension in line with the wrench (angle 0), the effective length is 15 + 3 = 18 inches. The setting is 100 x 15 / 18 = 83.33 lbf-ft. Setting the wrench to 100 would over-tighten the bolt to 120 lbf-ft.
At 90 degrees, the extension adds nothing to the lever and the setting equals the target, 100. At 60 degrees with a 12 inch wrench and a 4 inch extension, the effective length is 12 + 4 x 0.5 = 14, so a target of 80 needs a setting of 68.57.
Why the setting is lower
Torque is force times distance. The wrench clicks when the force at the handle reaches the value that gives the set torque over the wrench length. An extension moves the working point farther from the handle, so the same force produces a larger torque at the bolt. To get the target at the bolt, the wrench must click earlier, at a lower setting.
Safety and limits
Always start from the torque value in the maker's service manual or fastener specification, and use a wrench rated for it. This page does only the lever arithmetic. A flexible extension, a loose fit or a pull off the center of the grip changes the real torque, so use the shortest, stiffest extension you can. For related mechanics, see the Horsepower from Torque Calculator and the Gear Ratio Calculator.
Assumptions and limits
- The extension and wrench are rigid and the pull is at the center of the grip.
- Lengths use the same unit. The unit itself does not matter, since only the ratio is used.
- Adapters that change the lever length without changing the lever direction are covered by the angle.
- Gear reductions, multipliers and torque angle methods are not modeled.
Frequently asked questions
What is the angle for a crowfoot?
It is the angle between the crowfoot and the wrench line. A crowfoot at 90 degrees to the wrench adds nothing to the lever, so no correction is needed. At 0 degrees it adds its full length.
Does the unit of length matter?
No, as long as both lengths use the same unit, because the formula uses a ratio. Inches and millimeters give the same setting for the same real wrench.
Why do I see an error about the angle?
If the angle is near 180 degrees and the extension is as long as the wrench, the extension cancels the lever and no setting can give the torque. Change the angle or the lengths.
Can I use this with a beam wrench or a digital wrench?
The lever arithmetic is the same. A digital wrench measures torque at the drive, so the reading may already include the extension. Check the manual for your tool.