3D printer over-extrusion: signs, causes and fixes
Over-extrusion is when a 3D printer pushes out more plastic than the toolpath has room for, so lines overlap, top surfaces turn rough and parts come out slightly oversized. It is almost always a calibration or settings problem, not broken hardware: the flow ratio, e-steps, filament diameter setting or temperature is off, and you can fix it in about an hour.
- Measure the filament with calipers in four or five spots. Standard filament should read close to 1.75 mm, and the slicer's filament diameter must match (2.85 mm machines need 2.85 mm).
- Check the nozzle size in the slicer profile matches the nozzle actually fitted.
- Run a flow ratio test for the filament you're using and lower the flow ratio to the smoothest sample. Typical corrections are a few percent.
- If every filament over-extrudes by the same amount, calibrate e-steps (Marlin) or rotation_distance (Klipper). Skip this on Bambu Lab and recent Prusa printers.
- Drop the nozzle 5–10 °C if walls look glossy, droopy or blobby.
What is over-extrusion in 3D printing?
Definition: over-extrusion is extruding a larger volume of plastic per millimetre of travel than the slicer planned. The excess has nowhere to go, so it squeezes up between lines, bulges out of walls and drags on the nozzle.
Your slicer plans each line as a little ribbon: line width × layer height × length. It converts that volume into millimetres of filament using the filament diameter, multiplies by the flow ratio (also called extrusion multiplier or flow), and the firmware turns it into motor steps using e-steps. If any link in that chain is off, every line gets fatter by the same percentage.
That chain is why over-extrusion is so fixable. Unlike under-extrusion, which is usually a physical restriction like a clog, too much plastic almost always means a number is wrong somewhere.
Over-extrusion symptoms (and look-alikes)
Look at a print with a flat top surface and a few straight walls. Over-extrusion shows up as:
- Top surfaces with raised ridges between lines, or a rough, “ploughed” texture the nozzle dragged through.
- Walls that bulge, look lumpy or have blobs that aren't lined up on the seam.
- Outside dimensions a little too big and holes a little too small, by roughly the same amount.
- Print-in-place hinges, threads and snap fits that fuse or bind.
- Rounded, bulging corners and a build-up of plastic on the outside of the nozzle.
Plenty of other defects look similar. Rule these out before you touch the flow ratio:
| Look-alike | Where it shows up | The giveaway | Read |
|---|---|---|---|
| Wet filament | Everywhere, randomly | Popping or hissing from the nozzle, bubbles, fuzzy surfaces and extra stringing | Filament drying |
| Ringing (ghosting) | Walls, just after corners or text | Evenly spaced ripples that fade with distance from the feature | Ghosting and ringing |
| Elephant's foot | Bottom 0.5–1 mm only | Only the first few layers flare out; the rest of the wall is fine | Elephant's foot |
| Seam zits and corner blobs | On the seam line or at corners | Bumps line up vertically or sit only where the nozzle starts, stops or turns | Blobs and zits |
| True over-extrusion | Every layer, every surface | Uniform roughness on top, bulging walls and oversized parts across the whole print | This page |
What causes over-extrusion?
Six causes cover nearly every case. They're listed roughly from most to least common on hobby printers.
| Cause | How to spot it | Fix |
|---|---|---|
| Flow ratio (extrusion multiplier) too high | One filament or brand looks overstuffed; others are fine | Run a flow ratio calibration and save it to that filament profile |
| E-steps or rotation_distance wrong | Every filament over-extrudes by the same percentage, often after a firmware flash or extruder swap | Do the 100 mm extrusion test below |
| Wrong filament diameter in the slicer | Extreme over- or under-extrusion straight away, often on a new printer profile | Set 1.75 or 2.85 mm to match the filament you actually load |
| Thick filament (out of tolerance) | A cheap or new spool suddenly over-extrudes | Measure; recalibrate flow or swap the spool |
| Nozzle size mismatch | Profile says 0.6 mm but a 0.4 mm nozzle is fitted (lines far too fat, extruder may click) | Match the nozzle in the printer profile |
| Nozzle temperature too high | Glossy, sagging walls, extra ooze and stringing | Lower 5 °C at a time within the filament's range |
| Top surface flow or ironing over-tuned | Only the top layer is rough or ridged | Reset top surface flow to 100% and lower ironing flow |
Why the filament diameter setting matters so much
The slicer converts plastic volume into filament length using the cross-section of the strand, which scales with the diameter squared. Small errors grow fast:
- Slicer set to 1.75 mm but the filament really measures 1.80 mm: (1.80 ÷ 1.75)² = 1.058, so about 6% over-extrusion.
- Slicer set to 1.75 mm on a 2.85 mm machine: (2.85 ÷ 1.75)² = 2.65, so about 2.65× the plastic. The extruder usually grinds or the nozzle blobs within the first layer.
- Slicer set to 2.85 mm while feeding 1.75 mm: you get only about 38% of the plastic, which is severe under-extrusion.
To see the 2.85 versus 1.75 difference for yourself, switch the diameter in PrintPal's filament length and weight converter: the same 1 kg of PLA comes out at roughly 38% of the length, because every millimetre carries 2.65× more plastic.
Your slicer profile says 1.75 mm filament, but the printer is a 2.85 mm machine. Roughly how much plastic comes out compared with what the slicer planned?
Filament cross-section scales with the diameter squared: (2.85 ÷ 1.75)² ≈ 2.65. The slicer asks for the length of thin filament it thinks it needs, and the printer feeds that length of thick filament. The 38% figure is the reverse mistake, which causes under-extrusion.
How to fix over-extrusion, step by step
Do these in order. Each step assumes the one before it is right, so skipping ahead means calibrating around an error you haven't found yet.
- Measure the filament with calipers. Check four or five spots across a metre of filament, rotating the calipers each time in case the strand is oval. Many brands quote a tolerance of ±0.02 to ±0.05 mm. Leave the slicer at the nominal 1.75 mm unless your average is consistently off, and confirm the setting itself is right (PrusaSlicer: Filament Settings → Diameter; Cura: material Diameter; OrcaSlicer and Bambu Studio: filament Diameter).
- Confirm the nozzle size. Check the printer profile against the nozzle on the machine, especially after swapping nozzles. A 0.6 mm profile on a 0.4 mm nozzle forces 0.6 mm-wide lines through a smaller hole.
- Dry the filament if it pops or hisses. Moisture makes surfaces look overstuffed and throws off any flow test. Dry first, calibrate second.
- Calibrate e-steps or rotation_distance (once per machine). Only needed on printers where you control it, such as Marlin or Klipper machines, or after an extruder upgrade. The procedure is in the next section.
- Calibrate the flow ratio for each filament. Use your slicer's built-in test if it has one, then save the result into that filament's profile. See flow ratio calibration below.
- Tune temperature. Print a temperature tower and pick the coolest band that still bonds well. Stay inside the filament's range; PLA usually runs 200–220 °C and PETG 225–245 °C.
- Check the finishing settings. If only the top layer is rough, look at top surface flow (OrcaSlicer, Bambu Studio), Top/Bottom Flow (Cura) and ironing flow. Ironing should lay down only a small fraction of normal flow; if ironed tops look ridged, lower it a few percent.
Calibrate e-steps (Marlin) or rotation_distance (Klipper)
E-steps (steps per millimetre) tell Marlin how far the extruder motor must turn to push 1 mm of filament. Klipper stores the same thing the other way round as rotation_distance, the millimetres of filament per motor turn. If either is wrong, every filament is off by the same percentage.
Skip this on Bambu Lab and recent Prusa printers. Their extruders are factory set and don't expose e-steps in normal use. Tune flow ratio in the slicer instead.
Marlin: the 100 mm test
- Heat the nozzle to printing temperature with filament loaded. Firmware won't extrude cold.
- Send
M503and note theM92 Evalue in the reply. That's your current e-steps. - Mark the filament 120 mm above the extruder inlet with a fine marker.
- Extrude 100 mm slowly with the commands below. Slow matters: fast extrusion builds pressure and skews the result.
- Measure from the inlet to the mark. Perfect is 20 mm. Actual extruded = 120 − your measurement.
- Calculate and save: new e-steps = current × 100 ÷ actual. If it pushed 104 mm, you're over-extruding and the value goes down. Send
M92 E<new>, thenM500.
M503 ; report settings, note the M92 E value
M83 ; relative extrusion
G1 E100 F100 ; extrude 100 mm at 100 mm/min
M92 E93.00 ; example only: enter your calculated value
M500 ; save to EEPROMKlipper: rotation_distance
Klipper's docs use a 50 mm test at 1 mm/s. Mark the filament about 70 mm from the extruder inlet, extrude, measure, then calculate new rotation_distance = old × actual ÷ requested. Over-extruding means the value goes up, the opposite direction to Marlin's e-steps. Put the result in the [extruder] section and RESTART.
G91
G1 E50 F60
[extruder]
rotation_distance: 22.900 # example only: old x actual / requestedFlow ratio calibration in OrcaSlicer, Bambu Studio, PrusaSlicer and Cura
Flow ratio is the per-filament trim. Every slicer calls it something slightly different, but they all scale the same number.
| Slicer | Setting name | Built-in test |
|---|---|---|
| OrcaSlicer | Filament → Flow ratio | Calibration → Flow ratio → YOLO (Recommended): one print, 11 blocks from −0.05 to +0.05. Add the best block's modifier to your current ratio. The older Pass 1 / Pass 2 method is still there. |
| Bambu Studio | Filament → Flow ratio | Calibration → Flow Rate. Manual coarse (5% steps) then fine (1% steps). Models with a lidar, such as the X1 series, can also run it automatically. |
| PrusaSlicer | Filament Settings → Extrusion multiplier | No built-in wizard. Use the wall or top-surface method below. Prusa's own filament profiles are already tuned. |
| UltiMaker Cura | Material → Flow | No built-in wizard. Use the wall method. Cura also has per-feature flows such as Outer Wall Flow and Top/Bottom Flow. |
The top-surface method (works in any slicer)
Print a few small, flat squares at different flow ratios (for example 0.92, 0.95, 0.98 and 1.00) and look at the top under a low, raking light. Pick the lowest value that has no gaps between lines; anything higher starts to ridge. This is the same logic as OrcaSlicer's and Bambu Studio's tests, and it's harder to fool than calipers.
The single-wall measurement method
Print a hollow cube with one wall, no top layers and no infill (vase mode is fine). Measure the wall with calipers in several places and average the readings. Then: new flow = current flow × expected line width ÷ measured width. A 0.45 mm line that measures 0.48 mm at 100% flow gives 93.75%.
Measure the middle of each wall, away from corners and the seam, where bulges skew the reading.
Don't fix one bad spool by changing e-steps. That shifts every other filament too.
Free PrintPal toolFlow Rate CalculatorGet a corrected flow ratio from your measured wall widthOpen toolIf you're unsure what line width your slicer is using, the guide to layer heights, walls and line widths explains how those numbers are set.
Over-extrusion vs under-extrusion
They're opposite ends of the same dial, but the usual causes are very different. Over-extrusion is mostly a settings problem; under-extrusion is mostly a hardware or speed problem.
| Over-extrusion | Under-extrusion | |
|---|---|---|
| Top surface | Ridged, rough, nozzle drag marks | Gaps, pinholes, see-through lines |
| Walls | Bulging, blobby, rounded corners | Thin, gappy, infill visible through them |
| Dimensions | Parts too big, holes too small | Parts close to size but weak |
| Strength | Strong but ugly; moving parts fuse | Weak; layers split apart |
| Extruder sound | Usually quiet; clicks only in extreme cases | Clicking or grinding is common |
| Most common cause | Flow ratio, e-steps or diameter setting | Partial clog, speed beyond the hotend's flow limit, slipping extruder |
| First thing to try | Flow ratio test, then check settings | Slow down, then clear the nozzle |
Tip: a slightly fatter first layer is normal and helpful. Many profiles deliberately use a wider first-layer line for adhesion. If only the first layers bulge, that's a Z-offset or bed temperature issue, covered in elephant's foot.
How to keep over-extrusion from coming back
- Save the calibrated flow ratio in a separate filament profile for each brand and type, not in the print profile.
- Re-check flow after changing nozzle size or material, or after a hotend swap.
- Measure every new brand of filament once with calipers before trusting it.
- Keep filament dry so your flow tests stay valid; see the filament drying guide.
- Work through the full 3D printer calibration guide after firmware updates or big hardware changes.
Frequently asked questions
What does over-extrusion look like?
Over-extruded prints have rough, ridged or bumpy top surfaces, walls that bulge or look blobby, corners that are rounded off and parts that measure slightly too big while holes come out too small. Moving parts and snap fits often fuse or bind.
How do I fix over-extrusion?
Work in this order: measure the filament with calipers and make sure the slicer's filament diameter and nozzle size match reality, calibrate e-steps (Marlin) or rotation_distance (Klipper) if your printer lets you, then run a flow ratio calibration for each filament and lower the nozzle temperature 5 to 10 °C if surfaces look glossy and saggy.
Should I change e-steps or flow rate to fix over-extrusion?
Calibrate e-steps (or Klipper's rotation_distance) once for the machine, because it is a mechanical constant of the extruder. Then use the slicer's flow ratio or extrusion multiplier for per-filament tuning. On Bambu Lab and recent Prusa printers the extruder is factory set, so you only tune flow ratio.
Is it better to over-extrude or under-extrude?
Neither, but a print that is 1 to 2% over is more forgiving than one that is under: it stays strong and watertight, while under-extrusion leaves gaps and weak layers. For parts that must fit or look good, aim for a flow ratio where top surfaces are smooth with no gaps and no ridges.
Can wet filament cause over-extrusion?
Wet filament mimics over-extrusion with bubbly, rough, zit-covered surfaces, but it actually reduces how much solid plastic comes out. If the nozzle pops or hisses, dry the filament before you change any flow settings, or you will calibrate around the moisture.
Related articles
Sources & further reading
- OrcaSlicer Wiki — Flow ratio calibration
- Bambu Lab Wiki — Flow rate calibration
- Prusa Knowledge Base — Extrusion multiplier calibration
- Klipper Docs — Rotation distance
- Marlin Firmware — M92: Set axis steps-per-unit
- Ellis' Print Tuning Guide — Extrusion multiplier
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