Idle speed is the minimum operating speed (generally measured in revolutions per minute, or rpm, of the crankshaft) of a combustion engine. At idle speed, the engine generates enough power to run reasonably smoothly and operate its ancillaries (water pump, alternator, and, if equipped, other accessories such as power steering), but usually not enough to perform useful work, such as moving an automobile. For a passenger-car engine, idle speed is customarily between 600 rpm and 1,000 rpm.

Because an engine at idle may not produce enough power to overcome the inertia of the drivetrain, the engine must either be disconnected from the drivetrain by shifting to neutral or disengaging the clutch or through use of a fluid coupling whose slippage will allow the engine to run without moving the transmission gears.

If the engine is operating a large number of accessories, particularly air conditioning, the idle speed must be raised to insure that the engine generates enough power to run smoothly and operate the accessories. Most air conditioning-equipped engines have an automatic adjustment feature in the carburetor or fuel injection system that raises the idle when the air conditioning is running.

Engines optimized for power at high engine speeds, such as auto racing engines, frequently have poor low-speed power, and tend to be very rough at idle unless the idle speed is raised significantly.