What volumetric flow means
A hotend can only melt so much plastic per second. That limit, the maximum volumetric flow, is measured in cubic millimeters per second (mm³/s). Print speed, layer height and line width together decide how much plastic you ask for. Push past the limit and the extruder can't keep up: you get thin, weak or gappy walls even though everything looks right in the slicer.
The formula
Slicers model an extruded line as a rectangle with rounded ends, not a plain rectangle:
cross-section = (line width − layer height × (1 − π/4)) × layer height
volumetric flow = cross-section × speed
This is the same model PrusaSlicer and OrcaSlicer use, so the numbers match what your slicer reports.
Finding your hotend's real limit
- OrcaSlicer has a built-in Max flowrate calibration test.
- Manually: extrude 100 mm of filament at increasing speeds and weigh or measure the result; the speed where output stops increasing linearly is your limit.
- Once you know it, set it as the max volumetric speed in your filament profile. The slicer will then slow down automatically where needed.
Frequently asked questions
What is a good volumetric flow rate?
It depends on the hotend. Standard hotends are commonly limited to the low teens of mm³/s with PLA, while high-flow designs can reach several times that. Test your own setup for a reliable number.
Why are my fast prints under-extruded?
Most likely you are asking for more volumetric flow than the hotend can melt. Check the "hotend capacity used" value above. Raising temperature slightly helps a bit; a high-flow hotend helps a lot.
Does layer height affect max speed?
Yes. Doubling layer height roughly doubles the plastic per millimeter of travel, which roughly halves the speed you can sustain.