Maximum volumetric flow is the ceiling on how much plastic a hotend can melt per second, measured in mm³/s. It is a property of the hotend, not of the nozzle alone, and it is the single number that decides whether a speed setting is real.
The equation your slicer is solving
Flow demanded = layer height × line width × head speed.
At 0.2 mm layers and 0.45 mm lines, 60 mm/s asks for 5.4 mm³/s. The same geometry at 150 mm/s asks for 13.5 mm³/s. Push to 300 mm/s and it asks for 27 mm³/s.
What happens above the ceiling
Not an error message. Put that 27 mm³/s demand into a hotend that can supply 12 and the machine runs at an effective 133 mm/s instead of the 300 you set — or, on a machine that does not compensate, it under-extrudes and prints a weak, gappy part.
Either way, the speed setting has stopped describing what the machine is doing. That is why prints routinely take longer than a slicer predicted, and why the estimator on this site reports whether your inputs are flow-limited.
Typical bands
| Nozzle | Typical flow |
|---|---|
| 0.2 mm | 1–3 mm³/s |
| 0.25 mm | 2–4 mm³/s |
| 0.4 mm | 8–25 mm³/s |
| 0.6 mm | 15–40 mm³/s |
| 0.8 mm | 20–55 mm³/s |
| 1.0 mm | 25–70 mm³/s |
Those are wide because the hotend matters more than the orifice. A 2019 Bowden machine and a modern high-flow one wearing identical 0.4 mm nozzles sit at opposite ends of that row.
Measuring your own
Extrude at a known rate with the nozzle in free air, weigh what comes out over a fixed time, and step the rate up until the extruder starts skipping or the extruded weight falls short. The last rate that kept up is your real ceiling.
It is a ten-minute test, it is specific to your machine and your filament, and it will change every profile decision you make afterwards — because it tells you whether to spend on speed or on nozzle diameter.
Repeat it per material. A stiff PLA and a soft TPU do not reach the same ceiling on the same hotend, and filled grades sit lower again because the fibre does not melt at all.
Enter your measured ceiling alongside your layer height, line width and speed in the print time estimator; it reports the demand, the ceiling and the speed you will actually get.