Extrusion Multiplier/Flow-Rate Calibration for PrusaSlicer

This is a Flow Rate Calibration Test specifically designed for PrusaSlicer, as it does not support per-object settings.
These models are optimized to use as little filament as possible while still providing accurate results.

There are two passes per nozzle size (0.25 mm, 0.4 mm, and 0.6 mm) to allow for both coarse and fine tuning.

These designs are optimized for the standard layer heights of each nozzle size:

• 0.25 mm nozzle → 0.12 mm layer height

• 0.4 mm nozzle → 0.2 mm layer height

• 0.6 mm nozzle → 0.32 mm layer height

I created these tests because calibrating Flow Rate in OrcaSlicer is possible but cumbersome on the Prusa MK4S and CoreOne, especially since the settings don’t match those in PrusaSlicer.

How to Use

1. Set your initial Flow Rate (Extrusion Multiplier) to 1.00 in PrusaSlicer.

   • Go to Filaments > Filament > Extrusion Multiplier and set it to 1.00 before starting the test.

2. Slice and print the corresponding test for your nozzle size:

   • 0.25 mm nozzle → Use the 0.25 mm test files

   • 0.4 mm nozzle → Use the 0.4 mm test files

   • 0.6 mm nozzle → Use the 0.6 mm test files

3. Examine the print and check which section produces the best wall thickness and surface quality.

4. Choose the Flow Rate setting that matches the correct extrusion width.

How to Add Custom GCode in PrusaSlicer

To apply the automated Flow Rate changes per layer, you need to add Custom GCode to your print profile. The provided height values correspond to the model, ensuring that the changes work correctly even if you are not using the standard layer heights.

Steps to Add Custom GCode:

1. Open PrusaSlicer and go to Printers.

2. Navigate to "Custom GCode".

3. In the "Before Layer Change GCode" section, insert the corresponding GCode based on your Nozzle.

4. I recommend copying the GCode into the slicer without saving the profile to ensure that the settings remain unchanged for future prints, while still allowing proper evaluation of the test models.

• For 0.25 mm nozzle - Pass 1:

{if layer_z == 0.12}M221 S115{endif}
{if layer_z == 0.48}M221 S110{endif}
{if layer_z == 0.84}M221 S105{endif}
{if layer_z == 1.20}M221 S100{endif}
{if layer_z == 1.56}M221 S95{endif}
{if layer_z == 1.92}M221 S90{endif}
{if layer_z == 2.28}M221 S85{endif}

• For 0.25 mm nozzle - Pass 2:

{if layer_z == 0.12}M221 S100{endif}
{if layer_z == 0.48}M221 S99{endif}
{if layer_z == 0.84}M221 S98{endif}
{if layer_z == 1.20}M221 S97{endif}
{if layer_z == 1.56}M221 S96{endif}
{if layer_z == 1.92}M221 S95{endif}
{if layer_z == 2.28}M221 S94{endif}
{if layer_z == 2.64}M221 S93{endif}
{if layer_z == 3.00}M221 S92{endif}

• For 0.4 mm nozzle - Pass 1:

{if layer_z == 0.2}M221 S115{endif}
{if layer_z == 0.8}M221 S110{endif}
{if layer_z == 1.4}M221 S105{endif}
{if layer_z == 2.0}M221 S100{endif}
{if layer_z == 2.6}M221 S95{endif}
{if layer_z == 3.2}M221 S90{endif}
{if layer_z == 3.8}M221 S85{endif}

• For 0.4 mm nozzle - Pass 2:

{if layer_z == 0.2}M221 S100{endif}
{if layer_z == 0.8}M221 S99{endif}
{if layer_z == 1.4}M221 S98{endif}
{if layer_z == 2.0}M221 S97{endif}
{if layer_z == 2.6}M221 S96{endif}
{if layer_z == 3.2}M221 S95{endif}
{if layer_z == 3.8}M221 S94{endif}
{if layer_z == 4.4}M221 S93{endif}
{if layer_z == 5.0}M221 S92{endif}

• For 0.6 mm nozzle  - Pass 1:

{if layer_z == 0.32}M221 S115{endif}
{if layer_z == 1.28}M221 S110{endif}
{if layer_z == 2.24}M221 S105{endif}
{if layer_z == 3.20}M221 S100{endif}
{if layer_z == 4.16}M221 S95{endif}
{if layer_z == 5.12}M221 S90{endif}
{if layer_z == 6.08}M221 S85{endif}

• For 0.6 mm nozzle - Pass 2:

{if layer_z == 0.32}M221 S100{endif}
{if layer_z == 1.28}M221 S99{endif}
{if layer_z == 2.24}M221 S98{endif}
{if layer_z == 3.20}M221 S97{endif}
{if layer_z == 4.16}M221 S96{endif}
{if layer_z == 5.12}M221 S95{endif}
{if layer_z == 6.08}M221 S94{endif}
{if layer_z == 7.04}M221 S93{endif}
{if layer_z == 8.00}M221 S92{endif}

How to Evaluate the Results

Step 1: Coarse Adjustment (Pass 1)

1. Slice and print the Pass 1 calibration test.

2. Examine the printed sections and determine which one has the smoothest and most even walls with no gaps or bulging.

3. Update the Extrusion Multiplier in PrusaSlicer using the following formula: Old Flow Ratio × (100 + Modifier) / 100 = New Flow Ratio

   • Example: If your previous flow ratio was 1 and you selected the block with a +5% modifier, the new value is: 100 × (100 + 5) / 100 = 105

4. Save the filament profile in PrusaSlicer.

Step 2: Fine Adjustment (Pass 2)

1. Slice and print the Pass 2 calibration test.

2. Examine the printed sections and identify the most accurate flow rate range.

3. Apply the fine-tuning formula to adjust the flow ratio: Previous Flow Ratio × (100 + Modifier) / 100 = New Flow Ratio

   • Example: If your previous Extrusion Multiplier was 1.05 and you selected the block with a -6% modifier, the new value is: 105 × (100 - 6) / 100 = 0.987

4. Save the updated filament profile in PrusaSlicer.

Evaluating Results

Each field of the model is printed with a different Flow Rate.
The evaluation is based primarily on surface quality, not on dimensional accuracy.

Choose the field with:

• Smooth, even top surface

• Extrusion lines just touching or slightly overlapping

• No visible gaps (under‑extrusion)

• No ridges or material buildup (over‑extrusion)

If two neighboring fields look equally good, always choose the one with the higher Flow Rate. Fine tuning is done in Pass 2.

The hole has a nominal CAD diameter of 3.00 mm, but will typically print around 2.5–2.7 mm.
This is normal and expected due to extrusion width and slicer geometry handling.
The hole is used only to observe trends (higher flow → smaller hole, lower flow → larger hole) and must not be used as a target dimension. It is more of a visual aid than a test of dimensions.

This evaluation method follows the same principles as the OrcaSlicer Flow Rate Calibration: https://github.com/OrcaSlicer/OrcaSlicer/wiki/flow-rate-calib

Key point:
You do not measure anything with calipers.
You visually compare the sections and choose the one with the best extrusion quality.

• Hold the print up to a bright light source and look in the direction of the lines. If you can see the infill under the top layer, your Extrusion Multiplier might be too low.

• Shining a light from behind can also help detect gaps or inconsistent extrusion.

• The best result will have slightly overlapping infill lines without visible gaps or bulging.

• If you can not decide between two take the one with the higher Modifier.

Here is a visual aid from Orca to help:

Source

Why Use This Flow Rate Test?

✅ Designed specifically for PrusaSlicer – since per-object settings are not possible.
✅ Uses minimal filament – making it quick and efficient to run.
✅ Supports different nozzle sizes – tests available for 0.25 mm, 0.4 mm, and 0.6 mm.
✅ Two levels of accuracy – a coarse test (Pass 1) and a fine test (Pass 2).
✅ Fully automated adjustments – Flow Rate changes automatically via Custom GCode.

Conclusion

This is a fast and precise method to calibrate your Flow Rate in PrusaSlicer, using minimal filament while still getting accurate results.
Let me know if you have any feedback, and happy printing! 🎉

Edit

• I updated the multipliers for 0.25n.

• I also updated the extrusion width test for the 0.6n.

• All corresponding files were updated.

• Further expanded on how to evaluate.