Glossary/Figures Activity Monitor leaves out
Fan curve
Also: fan control
Quick answer
A fan curve is the rule that maps a Mac's temperature to its fan speed: cool means slow or off, hot means fast. macOS sets the curve itself through the system management controller, and no built-in app shows or changes it. Fan control apps replace the curve with one you draw.
Every Mac with a fan runs one, and the Apple silicon MacBook Air has no fan at all: it slows the chip instead when it gets hot. The curve is tuned for quiet, so the fans start late and ramp gently. That is why a MacBook Pro under a long export feels hot to the touch before the fans are audible, and why thermal throttling can begin before the fans reach full speed.
A fan control app lets you trade the other way: fans earlier and louder for a cooler chip and, sometimes, sustained performance under load. It can also do the opposite, which is the risk: a curve set too low lets the Mac run hot for hours, and heat is what shortens a battery's life and makes kernel_task climb.
How to see fan speed without a fan control app
Activity Monitor does not show fan speed or temperature. Apple Diagnostics checks the fan but reports only a pass or a fault. For a live figure you need a monitor that reads the sensors: fan speed in revolutions per minute, and the CPU and GPU temperatures the curve is responding to.
Reading is harmless. The sensors are there for anyone to read, and looking at them changes nothing.
Whether to change the fan curve
Most people should not. The default keeps the Mac inside its limits, and the throttling that people hope to avoid is macOS protecting the chip, not a fault. A custom curve helps in one case: a Mac that runs at full load for hours, a render farm on a desk or a long compile, where a louder fan keeps clocks up.
If you do, set a floor rather than a ceiling: a curve that never lets the fan drop below the default at a given temperature. Apps that change the curve include iStat Menus, TG Pro and Macs Fan Control; the change lives in software and stops when the app quits, so it cannot brick a Mac, but it can let one run hot while it is on.
Questions people ask
Can I control the fan speed on a Mac?
Not with anything built in. macOS sets it from the temperature. Third-party apps such as iStat Menus, TG Pro and Macs Fan Control can set a curve of your own, and Vitals can hold a fan at a speed you choose. The change lasts only while the app runs.
Why do my Mac's fans not come on when it is hot?
The default curve starts the fans late, because Apple tunes it for quiet. The Mac is inside its limits; if it gets too hot it slows the chip rather than the other way round. A MacBook Air has no fan and only slows down.
Does a fan control app damage the Mac?
Setting the fans faster than the default does not. Setting them slower can let the Mac run hot for long periods, which ages the battery and brings on throttling. The safe use is a floor above the default, not a ceiling below it.
Related
Step by step: How to control fan speed on a Mac.
Want to watch this all the time? See the best system monitors for Mac, or the best Activity Monitor alternatives. Or browse every term in the glossary.