What this calculator does
Print speed is the setting everyone changes, but it is not the thing that limits a printer. The hotend can only melt so much plastic a second, and that limit is in cubic millimetres per second rather than millimetres per second. A 0.42 mm line at 0.2 mm layers running 120 mm/s asks for 10.08 mm³/s, which is 84 per cent of a typical 12 mm³/s hotend. There is room, but not much.
The mistake is treating speed and layer height as separate dials. They multiply. Going from 0.2 mm layers to 0.28 mm at the same 120 mm/s takes the demand from 10.08 mm³/s to 14.11, which is 118 per cent of the limit, and drops the fastest speed the hotend can feed from 143 mm/s to 102. Under-extrusion that appears after a layer height change is almost always this and not a clogged nozzle.
The formula
Extrusion is a rectangle of plastic laid down at a speed, so the flow is line width × layer height × print speed, which gives cubic millimetres a second. Rearranged for speed, the fastest the hotend allows is flow limit ÷ (line width × layer height). The filament feed rate is the same volume seen from the other end: divide the flow by the cross-section of the filament, which is 2.405 mm² for 1.75 mm stock.
| Term | Meaning |
|---|---|
| Volumetric flow | Plastic melted per second, in mm³/s. The figure a hotend is actually rated in. |
| Line width | Width of one extruded line, usually a little wider than the nozzle. |
| Hotend flow limit | The most that hotend can melt per second at your temperature and material. |
| Filament feed rate | How fast the filament itself moves into the extruder, in mm/s. |
The inputs explained
| Field | What to enter |
|---|---|
| Line width (mm) | Extrusion width for the lines you care about, which is usually the slicer default rather than the nozzle size. |
| Layer height (mm) | Layer height. This multiplies flow exactly as speed does. |
| Print speed (mm/s) | The print speed you want to run, for the feature that moves fastest. |
| Hotend flow limit (mm³/s) | Your hotend flow limit. Measure it rather than guess: it varies with hotend, nozzle, material and temperature. |
| Filament diameter (mm) | Filament diameter, 1.75 mm or 2.85 mm. Only affects the feed rate output. |
When to use it
Checking a speed before you run it
Enter the width, layer height and the speed you want. If the flow is over the limit the print will under-extrude no matter how good the rest of the profile is.
Setting a maximum volumetric speed in the slicer
Most slicers have a field for this. Put your measured limit in and the slicer will cap speeds for you, which is more reliable than capping each feature by hand.
After changing nozzle or layer height
A wider nozzle and a taller layer both raise flow. Re-check the fastest allowed speed, because the old speed may no longer fit.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How much flow does a given print speed need?
Only the print speed changes.
| Print speed | Volumetric flow needed | Within the hotend limit | Filament feed rate |
|---|---|---|---|
| 60 mm/s | 5.04 mm³/s | yes, with 6.96 mm³/s spare | 2.10 mm/s |
| 100 mm/s | 8.40 mm³/s | yes, with 3.60 mm³/s spare | 3.49 mm/s |
| 120 mm/s | 10.08 mm³/s | yes, with 1.92 mm³/s spare | 4.19 mm/s |
| 150 mm/s | 12.60 mm³/s | no, over by 0.60 mm³/s | 5.24 mm/s |
| 200 mm/s | 16.80 mm³/s | no, over by 4.80 mm³/s | 6.98 mm/s |
What does layer height do to the flow?
The speed is held constant and only the layer height changes.
| Layer height | Volumetric flow needed | Fastest speed the hotend allows | Flow as a share of the limit |
|---|---|---|---|
| 0.12 mm | 6.05 mm³/s | 238 mm/s | 50.4% |
| 0.16 mm | 8.06 mm³/s | 179 mm/s | 67.2% |
| 0.2 mm | 10.08 mm³/s | 143 mm/s | 84.0% |
| 0.28 mm | 14.11 mm³/s | 102 mm/s | 117.6% |
| 0.32 mm | 16.13 mm³/s | 89 mm/s | 134.4% |
Questions
How do I measure my hotend flow limit?
Print a single wall with the flow stepped up in stages and find where the extrusion visibly thins or the extruder starts skipping. That number is specific to the material and temperature you tested at, so repeat it when either changes.
Why does the nozzle size not appear here?
Because line width does the work, and line width is a slicer setting rather than the nozzle diameter. A larger nozzle matters because it lets you use a wider line and a taller layer, and both of those are already inputs.
Does a hotter nozzle raise the limit?
Usually yes, up to the point where the material degrades or stringing becomes unmanageable. That is why the limit is an input: it belongs to your hotend at your temperature, not to the printer model.
What happens if I just run over the limit?
The extruder commands more plastic than the melt zone can supply, so you get under-extrusion: thin walls, weak layer bonding, gaps in the top surfaces. Some extruders will also skip or grind the filament.
Is the first layer a problem?
Rarely, because first layers are slow. The features to check are infill and the outer wall on long straight runs, which are where the slicer actually reaches the speed you set.
For what the print will weigh and cost, see filament weight and cost. Print time estimate turns speed into hours, and layer count covers the layer height side.