Torque Converter Stall Calculator
A torque converter's stall speed is governed by its capacity factor and the engine torque fed into it. For a given converter, stall speed rises with the square root of input torque, and the converter is characterised by a capacity factor K equal to stall RPM divided by the square root of torque. Enter K and your engine torque to estimate stall speed, or enter a measured stall and torque to solve for K. Because K is specific to each converter, it is a user-editable input.
Stall speed formula
Stall speed (RPM) = K * sqrt(engine torque)
Capacity factor K = measured stall / sqrt(engine torque)
sqrt torque = square root of engine torque (lb-ft)
Estimated vs measured = estimated stall / measured stall
K is an empirical property of the specific converter. Stall scales with the square root of input torque, so increasing torque at stall raises stall RPM for the same converter.
Choosing a converter
- K comes from the converter manufacturer or a measured stall test, not from theory.
- Match stall speed to the engine's torque peak and your driving use.
- Higher stall aids launch but adds slip, heat, and reduced efficiency in normal driving.
- Solve for K from one stall test, then predict stall at other torque levels.
- Confirm engine torque at the stall RPM from a dyno or factory torque curve.
Torque converter stall: frequently asked questions
What is torque converter stall speed?
Stall speed is the engine RPM at which a torque converter stops slipping enough to start driving the transmission firmly under load, roughly the highest RPM the engine can reach against a held output. A higher stall converter lets the engine spin up further before coupling, which can put a high-torque or high-revving engine into its power band off the line.
How is stall speed related to engine torque?
For a given converter, stall speed scales with the square root of the input torque. The converter's behaviour is captured by a capacity factor K, defined as stall RPM divided by the square root of torque. Stall speed therefore equals K times the square root of engine torque, so more torque at stall raises the stall RPM for the same converter.
Where does the capacity factor K come from?
K is a property of the specific converter, set by its internal geometry and fluid coupling characteristics. It is determined empirically by the manufacturer or from a measured stall test, not derived from first principles. Because it is converter-specific, this calculator takes K as a user-editable input, or lets you solve for K from a known stall and torque.
How do I find K from a stall test?
Measure the stall RPM the engine reaches against the brakes and note the engine torque at that RPM from a dyno or factory curve. Then K equals stall RPM divided by the square root of that torque. Once you have K for your converter, you can predict stall at other torque values.
Why does stall speed matter when choosing a converter?
Matching stall speed to the engine's torque peak and the vehicle's use improves launch and drivability. Too low a stall bogs a high-strung engine off the line; too high wastes energy as heat and slip during normal driving. Stall selection balances launch performance against efficiency, towing, and heat.
Official sources
- National Institute of Standards and Technology: NIST units and mechanical measurement.
- U.S. Department of Energy: DOE vehicle drivetrain efficiency.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.