What does this calculator do?
Calculate requested extrusion volume per second. This is demand, not measured melting capability or guaranteed layer bonding.
How to use the calculator
- Collect measurements and check units.
- Match slicer geometry and test actual nozzle, temperature and filament. Poor adhesion can precede visible under-extrusion.
- Record assumptions and check the result before applying it.
How the calculation works
Flow = width × height × speed (rectangular approximation)
Rectangular Q = width × height × speed. Stadium A = h × (w − h) + πh²/4 for w ≥ h, then Q = A × speed.
Practical calculation example
0.45 × 0.20 mm at 100 mm/s requires 9 mm³/s rectangularly, or 8.14 mm³/s with rounded geometry.
Understanding the results
Match slicer geometry and test actual nozzle, temperature and filament. Poor adhesion can precede visible under-extrusion.
What the flow rate describes
Volumetric flow is the volume of plastic that must leave the nozzle each second while an extrusion move is running. It links geometry to speed. A wall printed slowly and a thick infill line printed quickly can place very different demands on the same hotend. The calculation is useful before choosing a speed, but it cannot establish the hotend's capacity. That requires a controlled print test with the relevant material, nozzle and temperature. Treat the calculated value as demanded throughput, then compare it with a separately measured limit.
Measure the extrusion path, not the nozzle opening
Enter the extrusion width selected for the path you want to examine. A 0.4 mm nozzle does not mean every printed line is exactly 0.4 mm wide. Your slicer may use wider infill and first-layer paths or vary the width of thin walls. Enter layer height in millimetres and the speed of that specific extrusion feature in millimetres per second. Travel speed is irrelevant because travel does not deposit the same continuous extrusion. For adaptive layers, calculate the tallest and widest demanding combination rather than averaging all layers together.
How the rectangular approximation works
The tool uses Q = w × h × v, where w is line width in mm, h is layer height in mm and v is print speed in mm/s. Multiplying the first two gives an approximate cross-sectional area in mm²; multiplying by speed gives mm³/s. Actual deposited lines have rounded sides and neighbouring lines overlap, so this is a deliberately simple planning approximation. Some slicers account for a rounded cross-section or effective line spacing. Compare like with like when interpreting a slicer's displayed flow rather than expecting exact numerical identity.
A worked flow comparison
At 0.45 mm width, 0.20 mm height and 100 mm/s speed, the rectangular demand is 0.45 × 0.20 × 100 = 9 mm³/s. Raising only the height to 0.30 mm produces 13.5 mm³/s, a 50% increase. Keeping the original height but raising speed to 150 mm/s also produces 13.5 mm³/s. Doubling both height and speed multiplies demand by four. This makes the calculation especially helpful when a seemingly modest profile change suddenly produces under-extrusion: more than one flow-driving variable may have increased at once.
Filament feed is a different speed
The head's movement speed and the incoming filament speed are not the same quantity. For an ideal 1.75 mm cylindrical filament, area is π × 1.75² / 4, approximately 2.405 mm². A demanded output of 9 mm³/s corresponds to about 3.742 mm/s of filament feed before any extrusion correction. That conversion does not require density because both quantities describe volume. Density is needed when converting volume to mass. A high-density material does not automatically demand more volumetric flow for identical extrusion geometry, although its melting behaviour may make its practical limit different.
Plan a physical capacity test
Start with a stable temperature, clean nozzle and dried filament appropriate to the material. Use a test that increases demanded flow while retaining enough straight extrusion to reach the intended speed. Record the first repeatable loss of quality and the settings used. Repeat around the suspected threshold instead of adopting a single visually ambiguous step. Clicking, discontinuous lines, poor bonding and reduced extrusion can indicate insufficient throughput, but they can also reflect an extruder obstruction, wet material or temperature instability. Diagnose those causes before declaring a hotend limit.
Separate transient demand from sustained demand
Short features often finish before the printer reaches its nominal speed. A profile setting of 200 mm/s therefore does not prove every line demands its calculated steady-state flow. Conversely, long infill moves can sustain high demand and expose a problem that small calibration parts conceal. Corners and acceleration introduce transient extrusion behaviour, which pressure advance addresses differently from the sustained melting limit. Use a representative part with long paths after the calibration test, and inspect strength as well as appearance. A surface that looks acceptable can still have weak layer bonding.
Applying the number responsibly
Compare the demand with a conservative tested capacity and configure the slicer's material-specific volumetric cap. Keep machine speed, acceleration, cooling and quality constraints alongside that cap. If a 12 mm³/s tested limit is appropriate, a 9 mm³/s calculated path leaves 3 mm³/s of nominal headroom; this is a difference in throughput, not a guaranteed safety percentage for every geometry. Record the material brand, colour, nozzle and temperature with the test. Retest after a substantial hotend change, and avoid transferring a high-temperature result directly to a cooler production profile.
Advanced tips
- Match slicer geometry and test actual nozzle, temperature and filament. Poor adhesion can precede visible under-extrusion.
Common mistakes
- Using travel speed or nozzle diameter instead of actual extrusion-path speed and width.
Frequently asked questions
Does a wider nozzle increase the calculated flow?
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Only when the selected extrusion width, layer height or speed increases. Nozzle diameter itself is not an input to this geometry model.
Why is the slicer number different?
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The tool uses a rectangular cross-section. Slicer line geometry, overlap, extrusion modifiers and transient speed can produce another value.
Sources & methodology
Mathematical results depend on the supplied inputs. Material properties and machine limits need confirmation for your exact equipment. Editorial specialist approval remains pending.