Volumetric flow rate and extrusion width calculator

A printer does not have a top speed, it has a top flow rate. The hotend can melt so many cubic millimetres of plastic a second, and how fast the head may move depends entirely on how much plastic each millimetre of travel lays down. That is why the same machine prints happily at 300 mm/s with thin layers and starves at 120 mm/s with a fat nozzle. Bambu Lab and Polymaker both publish the arithmetic — layer height times line width times speed, and the same expression rearranged to give a speed — and this page runs it in both directions. The one number it will not invent is the limit itself. Melt rate belongs to your hotend, your nozzle, your material and your temperature, so the figures in the list are the ones Prusa and Bambu print with their names on them, and the last option is the one you measured.

How it is calculated

flow = layer height × extrusion width × speed · max speed = limit ÷ (height × width)

Every millimetre the nozzle travels lays down a bead as tall as the layer and as wide as the extrusion width, so the cross-section in square millimetres times the speed in millimetres a second is a volume in cubic millimetres a second. Bambu Lab writes it exactly that way and works an example: 0.45 mm wide, 0.2 mm tall, 200 mm/s comes to 18 mm³/s. Polymaker publishes the same relation solved for speed, which is what a slicer does when a flow ceiling is set — it divides the ceiling by the cross-section and slows the head down to fit.

Source: Bambu Lab Wiki — Volumetric speed and how it impacts 3D printing — Volumetric Speed (mm3/s) = Layer height (mm) X Layer width (mm) X Print speed (mm/s). In a real-world example, if you are printing with a 0.4mm Nozzle and use a 0.45mm layer width at 0.2mm layer height with a 200mm/s speed, the machine will be outputting 18mm3/s. If the layer settings are kept the same, but we increase the speed to 300mm/s, it will result in 27mm3/s

Questions people ask

Why are there two flow figures?

Because the bead is not a rectangle and the slicer knows it. Extruded plastic squashed between the nozzle and the layer below has straight sides and rounded ends, and PrusaSlicer computes the area as a rectangle with semicircles at the ends — its own source writes the shortcut as h (w − 0.215 h). For a 0.45 by 0.2 bead that is about nine percent less plastic than width times height. The plain product is what the wikis publish and what every other flow calculator shows, so it is the headline here; the slicer figure sits beside it because that is the number your slicer will actually compare against its limit.

What is my hotend's real limit?

Nobody can tell you from a page, and the manufacturers say so themselves. Bambu Lab writes that the maximum volumetric speed "can only be determined through practical testing"; Prusa gives ranges rather than numbers and tells you to start conservative and increase gradually. The ranges Prusa publishes are 8–12 mm³/s for a standard all-metal hotend such as an E3D V6, 15–20 for its own Nextruder, 20–35 for a high-flow nozzle like the Bondtech CHT and 25–40 for a high-flow hotend such as a Revo HF or Rapido HF. The list here takes the bottom of each range, because the bottom is the one that does not ruin a print.

Does the material have a limit of its own?

Yes, and the effective ceiling is whichever of the two is lower — Prusa states this plainly: the effective limit is set by the weaker link, either the hotend or the material. Its published per-material ranges are PLA 12–20 mm³/s, PETG 8–15, ABS and ASA 10–16, PA nylon 8–12, PC 6–10 and TPU 2–5. Flexibles are the extreme case for a mechanical reason rather than a thermal one: TPU buckles in the extruder before the hotend is anywhere near its melting limit. If your hotend is rated 25 and you are printing TPU, the number that matters is 2 to 5.

Where does the extrusion width come from?

From your slicer, and if you have never touched it, from a multiplier in the slicer's source. PrusaSlicer calls its default "a sane extrusion width default" and sets perimeters, solid infill and infill to 1.125 times the nozzle diameter — 0.45 mm on a 0.4 mm nozzle, which is exactly the width Bambu uses in its own worked example. Supports, support interfaces and top solid infill get the nozzle diameter itself instead. That is the figure in the last box on this page; anything you have set by hand overrides it.

Why does this say I am over the limit when my prints come out fine?

Because a slicer silently protects you. A flow ceiling never raises a speed, it only lowers one, so a requested 200 mm/s that would need 18 mm³/s on a hotend set to 15 simply prints at about 167 mm/s and you never see the difference except in the estimated time. This page shows the flow your settings ask for and the speed the ceiling would allow, which is the pair worth knowing before you conclude that a bigger nozzle made your prints faster.

Can I get more flow by turning the temperature up?

A little, and it is the last adjustment rather than the first. Bambu Lab suggests 5 to 10 °C as the range worth trying, and only after checking the print for gaps in the walls, rough infill and layers that separate — the signs that the hotend is already at its limit. Prusa makes the same point from the other side: with a larger nozzle you can run hotter and lift the ceiling, but past a point you trade stringing, drooping overhangs and discolouration for the speed. Lowering the layer height buys the same headroom and costs nothing but time.

Sources

The documents this page reads its numbers out of, linked so you can check them yourself.

1. Bambu Lab Wiki — Volumetric speed and how it impacts 3D printing — Volumetric Speed (mm3/s) = Layer height (mm) X Layer width (mm) X Print speed (mm/s). In a real-world example, if you are printing with a 0.4mm Nozzle and use a 0.45mm layer width at 0.2mm layer height with a 200mm/s speed, the machine will be outputting 18mm3/s. If the layer settings are kept the same, but we increase the speed to 300mm/s, it will result in 27mm3/s
2. Polymaker Wiki — Max Volumetric Speed Limits Your Print Speed — Print Speed (mm/s) = Volumetric Flow Rate (mm3/s) / (Layer Height (mm) × Line Width (mm))
3. Prusa Research, Prusa Knowledge Base — Max volumetric speed — The calculation used for the max speed is Max speed = MVS / cross-section area. PrusaSlicer uses a stadium-shaped cross-section model for accurate volume calculation
4. Bambu Lab, Bambu Studio filament preset "Bambu PLA Basic @base" — "filament_max_volumetric_speed": ["21"], "filament_density": ["1.26"]
5. Prusa Research, PrusaSlicer source, Flow::mm3_per_mm() — Rectangle with semicircles at the ends. ~ h (w - 0.215 h) — float(m_height * (m_width - m_height * (1. - 0.25 * PI)))

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