SlicerDial

Over-extrusion and its look-alikes: flow ratio, e-steps and filament diameter

Published October 15, 202614 sources

Short answer

Over-extrusion means more plastic leaves the nozzle than the slicer planned, which shows as ridged or uneven top layers, scarred bottom layers, blobs and walls thicker than intended [1, 2]. Check for a leftover firmware flow override, the slicer's filament diameter and the extruder's e-steps or rotation distance, then calibrate the flow ratio per filament [4]. Sagging top layers and a flared first layer look similar but have different causes.

What too much plastic looks like

Prusa describes over-extrusion as flow that is too high: it scars the bottom layers and builds up excess material on the top layers. An over-extruded top surface is uneven enough that two parts glued face to face will not join cleanly [1]. Bambu Lab lists excess material forming blobs or zits, lines that overlap too much and layers that look too thick [2]; on top layers, too high a flow ratio produces uneven lines [3]. OrcaSlicer adds rough surfaces and dimensional inaccuracy [4], which is why over-extrusion also shows up as tight holes and oversized parts in dimensional accuracy checks.

Symptom, likely cause, what to check

Symptom Likely cause What to check
Ridged, rough top surface with excess plastic near the perimeters Flow ratio too high Flow ratio test [1]
Single-wall test cube measures thicker than the line width Flow ratio or e-steps too high Prusa’s precise method [1]
Every filament over-extrudes by a similar amount E-steps or rotation distance, or a firmware flow override M221 value, then extruder calibration [11, 13]
Bulges or blobs where the toolhead speeds up or slows down Pressure advance value wrong Pressure advance calibration [9]
Heavy blobs with foaming or low-density PLA Filament expands as it melts Much lower flow ratio [10]
Sagging lines or holes in the first top layers, worse at low infill Pillowing, not flow More top layers [8]
Only the bottom layers flare outward Elephant’s foot, not flow First layer and bed temperature [7]

Diagnostic order

  1. Clear any firmware flow override. Prusa notes that total flow equals the firmware flow percentage times the slicer multiplier, and the printer’s Tune menu can change it mid-print [1]. On Marlin, M221 S<percent> applies to all extruder moves [11]; Klipper accepts the same command as an extrude factor override [12]. Make sure it is back at 100%.
  2. Check the filament diameter setting. OrcaSlicer uses Diameter to calculate every extrusion and says it must be accurate [6]. The volume error grows with the square of the diameter error (see below).
  3. Verify e-steps or rotation distance. If over-extrusion is similar on every filament, run Klipper’s measure-and-trim test: mark the filament, extrude 50 mm slowly with G1 E50 F60, and scale rotation_distance by the actual over the requested distance [13]. On Marlin, set M92 E<steps> and save with M500 [14].
  4. Calibrate the flow ratio for the filament, as in the table below.
  5. Rule out the look-alikes before lowering flow further.

Why a small diameter error matters

A slicer converts each line’s volume into a length of filament using the cross-section from the Diameter setting [6]. If that setting is smaller than the real filament, every move pushes more plastic than planned. For example, with Diameter set to 1.70 mm and filament that measures 1.75 mm, each move delivers (1.75 / 1.70)² ≈ 1.06 times the planned volume, about 6% over. The same error in the other direction under-extrudes.

Calibrating flow in three slicers

Slicer Setting Built-in test What the docs give
OrcaSlicer Filament Flow ratio (filament_flow_ratio) YOLO: eleven blocks, range −0.05 to +0.05 in 0.01 steps; new value = old ± modifier Recommended range 0.95–1.05 [4, 5]
Bambu Studio Filament Flow ratio Coarse: 80–120% of the current value in 5% steps; fine: 91–100% in 1% steps Pick the smoothest sample without gaps [2]
PrusaSlicer Extrusion multiplier Precise: vase-mode cube, multiplier = 0.45 (default extrusion width for a 0.4 mm nozzle) / average wall thickness; visual: adjust by 1–2% Usually 0.9–1.1 [1]

For Prusa’s precise method, measure the middle of each wall at least three times and average the results; Prusa cautions that cheap digital calipers may not be accurate enough to measure a single-perimeter wall [1]. Its visual method instead lowers the multiplier when plastic piles up near the perimeters and raises it when gaps show between lines.

Bambu Lab notes that a change of ±0.03 to ±0.05 can make a significant difference to the top layer [3]. When lowering flow, stop before gaps appear: Bambu Lab advises against picking a sample that shows even faint under-extrusion, because the gaps grow on larger prints [2]. The flow ratio calculator applies the OrcaSlicer formulas, and flow rate calibration covers the full method.

Pillowing and elephant’s foot are different problems

Pillowing is a top-surface defect caused by bridging. Prusa explains that the first top solid layer is essentially bridging over the infill, so its first lines sag, and lower infill means longer bridges and more sag. Its fix is more top layers (at least 3) or denser infill under the top surface. Thin layers need more of them: Prusa pairs 3 top layers at 0.3 mm with 9 at 0.1 mm [8]. Those fixes change layer count and infill, not flow. See pillowing.

Elephant’s foot is limited to the first layers. OrcaSlicer attributes it to the weight of the layers above, thermal expansion, a bed that is too hot and inaccurate bed height or leveling, and offers Elephant foot compensation, which shrinks only the first layers by a set distance [7]. If the rest of the part measures correctly, the flow ratio is not the problem. See elephant’s foot.

Expanding filaments

Some filaments are designed to grow as they melt. Bambu Lab explains that low-density filaments such as PLA Aero contain voids and gas that expand during printing, and suggests a flow ratio of 0.5–0.7 along with drying and a lower print temperature [10]. Its flow calibration guide calls calibration essential for foaming materials such as LW-PLA, which expand greatly when heated [2]. For these products, a standard flow ratio near 1.0 produces heavy over-extrusion.

Questions

What flow ratio values are normal?

OrcaSlicer recommends a filament Flow ratio between 0.95 and 1.05 [5]. Prusa says adjusted PrusaSlicer Extrusion multiplier values usually fall between 0.9 and 1.1 [1].

Why do Bambu Studio calibration samples look thick at the line ends?

Bambu Lab says slight over-extrusion at the start and end of the printed lines is normal; judge the middle section of each sample and avoid samples that show even faint gaps [2].

Should I change e-steps or the flow ratio?

E-steps or rotation distance describe the extruder and are measured once with a slow test extrusion [13]. The flow ratio belongs to the filament: Prusa notes the ideal multiplier differs by material, color and even spool [1].

Sources

  1. Extrusion multiplier calibration. Prusa Knowledge Base. Manufacturer documentation. Over-extrusion symptoms, precise and visual methods, total flow = M221 x multiplier. Accessed October 12, 2026.
  2. Flow Rate Calibration. Bambu Lab Wiki. Slicer documentation. Over-extrusion signs, coarse and fine calibration ranges, foaming filaments, reading samples. Accessed October 12, 2026.
  3. Manual Flow Rate Tuning for better looking prints. Bambu Lab Wiki. Manufacturer documentation. Uneven top-layer lines when flow is too high; change in 0.03–0.05 increments. Accessed October 12, 2026.
  4. Flow Ratio Calibration. OrcaSlicer Wiki. Slicer documentation. Over-extrusion effects; YOLO and 2-pass calibration math. Accessed October 12, 2026.
  5. Material Flow Ratio and Pressure Advance. OrcaSlicer Wiki. Slicer documentation. Recommended flow ratio range 0.95–1.05; object x filament flow ratio. Accessed October 12, 2026.
  6. Material Basic Information. OrcaSlicer Wiki. Slicer documentation. Filament diameter is used to calculate extrusion and must be accurate. Accessed October 12, 2026.
  7. Precision (Elephant foot compensation). OrcaSlicer Wiki. Slicer documentation. Causes of elephant's foot and the compensation setting. Accessed October 12, 2026.
  8. Layers and perimeters. Prusa Knowledge Base. Slicer documentation. Top solid infill bridges over infill and sags; top layer counts. Accessed October 12, 2026.
  9. Under-Extrusion. Bambu Lab Wiki. Manufacturer documentation. Wrong pressure advance causes too much or too little extrusion at speed changes. Accessed October 12, 2026.
  10. Stringing and Oozing. Bambu Lab Wiki. Manufacturer documentation. Low-density filaments expand; flow ratio 0.5–0.7. Accessed October 12, 2026.
  11. M221 - Set Flow Percentage. Marlin Firmware documentation. Firmware documentation. Flow percentage applies to all E moves. Accessed October 12, 2026.
  12. G-Codes. Klipper documentation. Firmware documentation. M221 sets the extrude factor override percentage. Accessed October 12, 2026.
  13. Rotation distance. Klipper documentation. Firmware documentation. Measure-and-trim calibration of extruder rotation_distance. Accessed October 12, 2026.
  14. M92 - Set Axis Steps-per-unit. Marlin Firmware documentation. Firmware documentation. Setting E steps per mm; saving with M500. Accessed October 12, 2026.