Stall torque is the maximum torque a motor produces when its shaft is not rotating. It represents the upper limit of twisting force the motor can generate, occurring at the exact moment the shaft is either held completely still or loaded so heavily it stops turning. Think of it as the motor pushing as hard as it possibly can against an immovable resistance.
How Stall Torque Works
Every motor has a tradeoff between speed and torque. When a motor spins freely with no load attached, it reaches its maximum speed but produces very little torque. As you add resistance to the shaft, the motor slows down and produces more torque to compensate. Stall torque is the endpoint of that relationship: zero speed, maximum force.
This tradeoff follows a surprisingly simple pattern in DC motors. If you plotted torque on one axis and speed on the other, you’d get a straight line sloping downward. Stall torque sits at one end of that line (where speed is zero), and no-load speed sits at the other end (where torque is essentially zero). Engineers call this the torque-speed curve, and it’s one of the first things they look at when selecting a motor for a project.
When a motor stalls, its internal coils act as a fixed resistance rather than a spinning generator. Normally, a turning motor creates a voltage that pushes back against the power supply, which limits how much current flows through the windings. At stall, that opposing voltage disappears entirely. The only thing limiting current is the physical resistance of the copper wire in the coils. The result is a massive surge of current, and that current is what creates the peak torque.
Stall Torque vs. Starting Torque
These two terms describe the same physical measurement, just viewed from different directions. Starting torque is the maximum force the motor can produce to begin moving a load from rest. Stall torque is the maximum force that can be applied to a spinning motor’s shaft before it stops. In both cases, the shaft is at zero RPM, and the motor is drawing maximum current. The numbers are identical.
One important nuance: temperature changes the value. A cold motor produces higher stall torque than a warm one. As the motor heats up, the resistance in its windings increases and the magnetic strength of its permanent magnets decreases slightly. Both effects reduce the current flowing through the motor, which directly reduces torque. So a motor’s rated stall torque depends on whether it was measured cold or after running for a while.
How to Calculate It
For a DC motor, you can estimate stall torque with two pieces of information from the motor’s datasheet: the supply voltage and the terminal resistance (the resistance of the motor’s internal wiring). Dividing the voltage by the resistance gives you the stall current, following Ohm’s Law. Multiply that current by the motor’s torque constant (a value that tells you how much torque the motor produces per amp of current), and you get the stall torque.
For example, if a motor runs on 12 volts, has 2 ohms of terminal resistance, and a torque constant of 0.05 newton-meters per amp, the stall current would be 6 amps. Multiply 6 amps by 0.05, and you get a stall torque of 0.3 Nm. Most motor datasheets list stall torque directly, but knowing how to calculate it helps when you need to estimate performance at a different voltage.
Typical Values for Common Motors
Stall torque is measured in newton-meters (Nm) in metric units, or in ounce-inches and pound-feet in imperial. The values vary enormously depending on the motor’s size and design.
Small stepper motors commonly used in 3D printers and CNC machines give a good reference point. A NEMA 17 stepper (the compact size found in most desktop 3D printers) produces a holding torque between 0.35 and 0.65 Nm at continuous current, reaching up to about 1.05 Nm at peak current for the largest variants. Step up to a NEMA 23 (used in more powerful CNC routers and laser cutters), and you’re looking at 1.1 to 2.6 Nm at continuous current, with peak values reaching 3.25 Nm. For stepper motors specifically, “holding torque” is the equivalent concept to stall torque: it’s the force needed to move the shaft when the motor is energized but stationary.
Hobby servo motors for robotics projects might produce anywhere from 0.1 to 2 Nm. Industrial motors used in conveyor systems or heavy machinery can generate hundreds or thousands of newton-meters at stall.
Why Running at Stall Is Dangerous
A motor at stall draws the highest current it will ever see. All of that electrical energy converts directly into heat, because there’s no mechanical work being done and no spinning rotor to help cool the windings. Two things happen quickly.
First, the current surge can be many times higher than the motor’s normal operating current. In some motors, stall current is five to ten times the rated running current. This can burn out the copper windings if the motor isn’t protected by a fuse, circuit breaker, or current-limiting controller. Second, the temperature inside the motor rises rapidly. Under normal operation, the spinning rotor acts like a fan, circulating air through the motor housing. At stall, that cooling disappears. The heat buildup damages the insulation around the windings, which can cause short circuits and permanent failure.
This is why motors in real applications almost never operate continuously at stall torque. The stall value on a datasheet is a peak rating, not a sustained one. Most motor controllers include thermal protection or current limits that reduce power before damage occurs.
Stall Torque in Electric Vehicles
Stall torque has a direct impact on how electric vehicles perform in everyday driving. Pulling away from a stoplight, starting on a steep hill, or towing a heavy load all push the motor into the high-torque, low-speed range where stall conditions become relevant. In these situations, the motor draws very high current and operates inefficiently, with a significant portion of the electrical energy becoming heat rather than motion.
EV motor controllers manage this by carefully limiting the current to protect the inverter and motor from thermal damage. The tradeoff is that acceleration from a standstill can feel weaker than expected, especially under heavy loads or on inclines. A fully loaded EV climbing a hill may deliver noticeably less responsiveness than the same vehicle accelerating on flat ground. Performance EVs address this with oversized cooling systems and controllers rated for brief bursts of very high current, allowing them to deliver strong launch acceleration without overheating.
Choosing a Motor Based on Stall Torque
When selecting a motor, stall torque tells you the absolute upper limit of what the motor can deliver. Your actual operating torque needs to stay well below that number. A common rule of thumb is to choose a motor whose stall torque is at least two to three times higher than the maximum torque your application requires during normal use. This keeps the motor running in a comfortable range on its torque-speed curve, with reasonable current draw and manageable heat.
If your application involves frequent starts and stops, like a robotic arm that repositions repeatedly, the brief spikes of high torque during each start matter more than steady-state running torque. In that case, you’ll want a larger safety margin above your required torque. If the motor runs at relatively constant speed with a predictable load, like a fan or a pump, you can operate closer to the middle of the torque-speed curve and the stall torque rating becomes less critical to your selection.