Volumetric flow calculator
Under-extrusion at high speed is usually a hotend limit, not a slicer bug. This calculator turns your speed settings into a volumetric flow rate so you can see whether the hotend can melt that much plastic.
Limits are rough averages for PLA at typical temperatures. Higher temperatures and larger nozzles raise the limit; PETG and filled materials lower it.
The maths
Volumetric flow (mm³/s) = speed (mm/s) × layer height (mm) × line width (mm). A 0.4 mm nozzle at 0.2 mm layers and 0.45 mm width printing at 150 mm/s needs 13.5 mm³/s. A stock MK8 hotend tops out around 10–12 mm³/s with PLA, so that speed will under-extrude.
Typical ceilings with PLA: stock MK8/V6 ≈ 10–12 mm³/s, Volcano ≈ 20, high-flow ≈ 25+. If you need more, raise the nozzle temperature, fit a larger nozzle, or use a high-flow hotend.
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FAQ
What is a good volumetric flow limit?
For a 0.4 mm nozzle with PLA: about 10–12 mm³/s on a stock hotend, 20 with a Volcano, and 25 or more with a modern high-flow hotend.
Why does my print under-extrude only on infill?
Many slicers print infill faster than walls, so infill crosses the flow limit first. Cap infill volumetric speed to the hotend limit.