The part-cooling fan controls how quickly each layer of plastic solidifies, which makes or breaks overhangs, bridges, and fine detail. This calculator converts the friendly percentage in your slicer into the real PWM duty cycle the firmware sends, and crucially warns you when a low setting falls below the speed at which the fan can even start spinning.
How it works
Firmware maps the fan command linearly. For an 8-bit controller the formula is:
PWM value = (fan % ÷ 100) × 255
So a 60% command becomes 0.60 × 255 ≈ 153. Klipper often uses a 0–1000 scale
and RepRapFirmware uses 0–1, but the proportion is identical.
The catch is the stall threshold. A brushless blower needs a minimum duty cycle to overcome static friction and begin rotating — commonly 15–25%. Below that the fan draws current but produces no airflow. The tool flags any setting that lands in this dead band and tells you the minimum percentage that keeps the fan reliably spinning.
Example and tips
If your fan stalls below 20% and your slicer drops to 15% on the first solid layer, the part gets zero cooling there even though the display reads “fan on”. Set your slicer’s minimum fan speed to 20% so commands never fall into the dead band, then let the slicer ramp toward 100% over bridges and steep overhangs.
Tune cooling per material: PLA tolerates full cooling, PETG prefers moderate, and ABS or PC need very little — too much part cooling causes warping and layer cracking on high-temperature plastics.