How to Calibrate Flow Rate (Extrusion Multiplier)
Inaccurate flow rate causes over-extrusion (blobs, rough surfaces) or under-extrusion (gaps, weak parts). Here's how to calibrate it accurately with a cube test and calipers.
Flow rate (also called extrusion multiplier or extrusion factor) is the slicer setting that controls how much filament is pushed out relative to what the slicer calculates. At 100%, the printer extrudes exactly what the slicer expects. At 95%, it extrudes 5% less. At 105%, it extrudes 5% more.
Factory default is 100%, but real-world variances in filament diameter, hotend geometry, and brand-specific flow characteristics mean your ideal value may be anywhere from 92% to 108%.
When to Calibrate Flow Rate
- When switching to a new filament brand
- When you notice over-extrusion (surfaces lumpy, elephant foot, blobs at seams)
- When you notice under-extrusion (gaps, rough top surfaces, weak parts)
- After changing nozzles
Method 1: Single-Wall Cube (Most Accurate)
Step 1: Print a calibration cube
Print a 20mm cube with:
- 1 perimeter / 1 wall
- 0% infill
- No top or bottom layers (just the walls)
This creates a hollow shell of four single-thickness walls.
Step 2: Measure with calipers
Measure each wall in at least 3 spots. A 0.4mm nozzle should produce 0.4mm walls. A 0.6mm nozzle should produce 0.6mm walls.
Step 3: Calculate the correction
New Flow Rate = (Target Wall Thickness / Measured Wall Thickness) × Current Flow Rate
Example: Target 0.4mm, measuring 0.44mm, current flow 100%:
New Flow Rate = (0.4 / 0.44) × 100 = 90.9%
Step 4: Verify
Print the cube again, measure again. Repeat until within ±0.02mm of target.
Method 2: Extrusion Multiplier by Weight
For even more precision, weigh your cube:
- Calculate the expected weight: volume × density (PLA ≈ 1.24 g/cm³)
- Compare to actual weight
- Adjust flow rate proportionally
Slicer Locations
| Slicer | Where to Find Flow Rate |
|---|---|
| Cura | Material → Flow (%) |
| PrusaSlicer | Filament Settings → Extrusion Multiplier |
| OrcaSlicer | Filament → Flow Ratio |
| Bambu Studio | Filament → Flow Ratio |
Typical Values by Material
| Material | Typical Flow Rate |
|---|---|
| PLA | 95–100% |
| PLA+ | 95–103% |
| PETG | 95–100% |
| ABS | 97–103% |
| TPU | 100–110% |
TPU often needs slightly higher flow because it compresses in the Bowden tube.
E-Steps vs. Flow Rate
E-steps (extruder steps per mm) is a firmware setting calibrated once per extruder. It accounts for the extruder hardware itself.
Flow rate is a per-filament/per-brand slicer setting that accounts for variance in diameter and material properties.
Calibrate in this order:
- E-steps (once when you build/modify the printer)
- Flow rate (each time you switch filament brand)
If your e-steps are wrong, flow rate compensation is a band-aid. Fix e-steps first.
Signs You Need Flow Rate Calibration
| Symptom | Likely Cause |
|---|---|
| Blobs at seam | Over-extrusion or pressure advance needed |
| Rough, bumpy top surface | Over-extrusion |
| Gaps in top surface | Under-extrusion |
| Walls not meeting cleanly | Under-extrusion |
| Elephant foot | Over-extrusion + Z offset too low |