Too much plastic (over-extrusion) and how to calibrate flow

Symptom

More plastic than needed comes out: the top surface is uneven, with build-up next to the perimeters, bottom surfaces look scarred, and parts that should fit or be glued together don't seat properly.

Likely causes, from most to least likely

1. Slicer extrusion multiplier (flow) too high for that filament: each material, colour and even spool may need a different value.
2. Flow percentage changed on the printer (M221 on Marlin) and left above 100%.
3. Miscalibrated extruder: it pushes more filament than requested.
4. Slicer profile meant for another printer, nozzle or filament.

Checks to tell which one it is

• Check the flow percentage on the printer screen: it should be 100% unless you changed it on purpose.
• Print a test cube and look at the top surface: build-up next to the perimeters means too much material; gaps between lines mean too little.
• To measure precisely, print a cube in vase mode (single wall), measure the thickness at three or more points in the middle area and compare it with the profile's extrusion width.
• Check the extruder calibration: mark the filament, extrude 50 mm slowly and measure what went in.

Before you start

Before taking anything apart, cleaning or adjusting it, switch the printer off and unplug it, unless a step says otherwise, and do not touch the nozzle, the heater block or the bed until they have cooled down. If in any doubt about electricity, heat or moving parts, stop and ask a professional.

Step-by-step fix

1. Set printer flow back to 100%

   If the printer's flow has been changed, set it back to 100% from the menu (on Marlin, M221 S100).

   Check: The screen shows flow at 100%.

   If that does not fix it: Calibrate the extruder.

2. Calibrate the extruder

   With the hotend hot, extrude 50 mm slowly and measure. If it doesn't match, correct it: on Marlin, the extruder steps with M92 E and M500 to save them; on Klipper, new rotation_distance = previous × actual distance / requested distance.

   Check: Repeating the test feeds 50 mm.

   If that does not fix it: Adjust the extrusion multiplier.

3. Adjust the multiplier by eye

   Print a test cube with 0.20 mm layers, look at the top surface and lower the filament's extrusion multiplier in 1-2% steps until it is smooth, with no build-up or gaps. According to Prusa, typical values are between 0.9 and 1.1.

   Check: The top surface is smooth and even.

   If that does not fix it: Use the precise method with calipers.

4. Precise method with calipers

   Print a cube in vase mode, measure the wall thickness at no fewer than three points in the middle area and calculate the multiplier as the profile's extrusion width divided by the average measured thickness (Prusa uses 0.45 mm, the width in its 0.4 mm nozzle profile).

   Check: The measured thickness matches the profile's extrusion width.

   If that does not fix it: Check that the profile matches your nozzle and filament.

5. Save the value in the filament profile

   Store the multiplier in that filament's profile and repeat the calibration when changing brand, colour or spool if you notice differences.

   Check: Later prints with that filament have a smooth top surface.

   If that does not fix it: If over-extrusion comes back, check the printer's flow and the extruder calibration again.

Prevention

• Keep one profile per filament with its calibrated multiplier.
• If you change flow from the printer during a print, set it back to 100% when it finishes.

Still not fixed?

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Printers it applies to

Any FDM (filament) 3D printer, from any brand.

Official sources

This guide summarises the official documentation linked here:

• Extrusion multiplier calibration · Prusa Research
• M221 - Set Flow Percentage · Marlin
• M92 - Set Axis Steps-per-unit · Marlin
• Rotation distance · Klipper