Your 20mm calibration cube measures 20.35mm on every axis. The top surface has ridges where excess filament oozed between nozzle passes. Bolt holes meant for M3 hardware won’t accept an M3 bolt because 0.2mm of excess plastic closed the tolerance gap. This is over-extrusion — the printer is pushing more filament than the slicer expects.
The Two Root Causes of Over-Extrusion
Over-extrusion has exactly two root causes. Diagnose which one you’re dealing with before changing settings.
Cause 1: E-Steps Too High (Mechanical)
The extruder’s e-steps (steps per millimeter) tell the firmware how many stepper motor steps push 1mm of filament. If e-steps are set to 100 but the extruder actually pushes 105mm when told to push 100mm, you’re over-extruding by 5% on every move.
Test: Mark the filament 120mm from the extruder entry. Tell the printer to extrude 100mm (G1 E100 F100). Measure the remaining filament from your mark to the extruder entry. If 15mm remains (instead of the expected 20mm), the printer extruded 105mm — 5% over.
Fix: Calculate new e-steps: current_steps × (commanded / actual). If e-steps are 93 and you got 105mm: 93 × (100/105) = 88.57. Set new e-steps (M92 E88.57) and save (M500). Re-test. Repeat until the commanded and actual values match within ±1mm.
Cause 2: Flow Rate Too High (Slicer Setting)
Flow rate (also called extrusion multiplier) tells the slicer how much filament volume to lay down per linear mm. If e-steps are correct but you’re still over-extruding, the flow rate is too high. This varies by filament brand, material, and even color (pigments change flow characteristics).
Test: Print a hollow single-wall cube (vase mode, 0.4mm wall thickness). Measure the actual wall thickness with calipers. If your nozzle is 0.4mm and the wall measures 0.48mm, flow rate is 20% too high. The formula: new_flow = current_flow × (expected_wall / actual_wall).
Fix: Set flow rate in your slicer’s filament profile. For the example above: 100% × (0.40 / 0.48) = 83.3%. Set flow to 83% and re-print the cube. Wall thickness should now measure 0.40mm ±0.02mm.
Step-by-Step Over-Extrusion Fix
1. Calibrate E-Steps First (Hardware)
E-steps are a mechanical constant for your extruder. Calibrate them once per extruder change and don’t touch them for flow rate adjustments. The calibration procedure:
- Heat the hotend to printing temperature (filament must flow freely during the test — cold extrusion fights back and skews results)
- Mark filament 120mm from a fixed reference point (the extruder body or a printed guide)
- Extrude 100mm at a slow speed (F100 — 1.67mm/sec, to avoid skipping)
- Measure the remaining distance
- Calculate and set new e-steps
- Save to EEPROM, re-test
2. Calibrate Flow Rate per Filament (Software)
Each filament gets its own flow rate calibration because each material flows differently:
- PLA: Typically 93-100% flow (PLA flows easily, near 100%)
- PETG: Typically 95-102% (PETG is slightly more viscous)
- TPU: Typically 100-110% (flexible filament benefits from slight over-extrusion for layer adhesion)
- ABS: Typically 95-100% (similar to PLA flow characteristics)
- Silk/metallic PLA: Typically 90-95% (additives make it flow more freely — it swells more)
Save flow rates in your slicer’s filament profiles. When you switch filaments, switch profiles.
3. Verify Dimensional Accuracy
Print a 20mm calibration cube with the corrected flow rate. Measure X, Y, and Z with calipers:
– X and Y should be within ±0.1mm of 20mm
– Z is affected by first layer squish more than flow rate — if Z is off but X/Y are good, the flow is correct
4. Check Top Surface Quality
Over-extrusion is most visible on solid top layers. After calibrating, print a test with 4-5 solid top layers. The surface should be smooth with no ridges between nozzle passes. If you see raised ridges (the nozzle plows through excess material), flow is still too high — drop another 2%.
Over-Extrusion Diagnosis Table
| Symptom | Primary Cause | Secondary Cause | Fix |
|---|---|---|---|
| All dimensions 2-5% oversize | E-steps too high | N/A | Calibrate e-steps |
| X/Y oversize, Z correct | Flow rate too high | E-steps fine | Reduce flow per filament |
| Rough top surface with ridges | Flow rate too high | N/A | Reduce flow 2-3% |
| Bulging corners (not stringing) | Flow rate too high | Pressure/linear advance off | Reduce flow, calibrate advance |
| Good dimensions, visible layer lines | Normal (not over-extrusion) | Slight flow variation | Reduce flow 1% for cosmetic |
| One axis oversize, others fine | Mechanical (not extrusion) | Belt tension, frame square | Check mechanics, not flow |
| Z-axis too tall | First layer squish | Z binding | Fix first layer, check Z movement |
What Most Pilots Get Wrong About Over-Extrusion
Mistake 1: Adjusting flow rate without calibrating e-steps first.
Changing flow rate to compensate for incorrect e-steps means every filament profile is wrong. Fix e-steps once (hardware calibration), then adjust flow per filament (material calibration). Mixing the two means you’ll re-tune every time you change filaments.
Mistake 2: Using the same flow rate for every filament brand.
Two brands of “PLA” can differ by 5% in flow characteristics. eSun PLA+ flows differently than Hatchbox PLA. Even two colors from the same brand can differ (white pigment — titanium dioxide — changes viscosity differently than carbon black). Calibrate flow for every new spool if dimensional accuracy matters (functional prints, drone parts).
Mistake 3: Forgetting to calibrate e-steps with the hotend heated.
Cold extrusion forces the extruder to push against solid filament. E-steps calibrated cold will be 2-5% too low when printing hot (the extruder had to work harder during the test, so it pushed less). Always calibrate with the hotend at printing temperature.
Mistake 4: Mistaking over-extrusion for poor pressure advance/linear advance.
Bulging corners can be either excess flow OR insufficient pressure advance (which doesn’t reduce flow at deceleration). If the cube’s flat faces measure correctly but corners bulge, you need pressure advance calibration, not flow reduction. If the entire cube is oversize, reduce flow. The two problems have distinct signatures.
Mistake 5: Not compensating for filament diameter variation.
Slicers assume 1.75mm filament. If your spool averages 1.73mm (2% undersized), the slicer calculates 2% less volume — and compensates by extruding more linear mm. But volume isn’t linear with diameter (it’s proportional to the square of radius). A 1.73mm filament has 2.2% less cross-sectional area. Measure your filament in 5 places with calipers and average the readings. Set the slicer’s filament diameter to the actual average. Don’t assume 1.75mm.
⚠️ Safety Notice: 3D printers operate at high temperatures. Always ensure thermal runaway protection is enabled. When calibrating e-steps, hot filament extrudes from the nozzle — keep hands clear and use a purge container. Some filaments emit fumes when heated — ensure adequate ventilation. Always power off the printer before handling electrical connections.
For the opposite problem, see our under-extrusion diagnosis guide. Once extrusion is dialed in, our E-step and flow rate calibration guide covers the full calibration workflow including measurement tools. If you’re also fighting stringing — which can look similar to over-extrusion artifacts — our stringing solutions guide covers the retraction side of the equation.
For drone builders printing functional parts, dimensional accuracy is non-negotiable — a camera mount that’s 0.3mm oversize won’t hold the camera. uavmodel’s precision brass nozzles maintain consistent orifice diameter across hundreds of hours, which keeps your calibrated flow rate valid without drift. When every millimeter counts, quality nozzles pay for themselves in avoided reprints.