Flow Rate and Extrusion Multiplier Calibration
Flow ratio, called Extrusion Multiplier in PrusaSlicer, scales how much plastic the slicer asks for. Print a set of blocks at stepped flow values, pick the one with the smoothest top and no gaps, and save the corrected value to the filament profile. Calibrate temperature first, and in OrcaSlicer's order pressure advance too; most good filament profiles only need a small correction.
What the test measures
The slicer works out how much filament each line needs from its width, height and length. Flow ratio multiplies that amount. OrcaSlicer: too low causes under-extrusion (gaps, weak layers), too high causes over-extrusion (excess material, rough surfaces, dimensional errors). Prusa adds that over-extrusion scars bottom layers and builds up material on top layers.
The same thing can be changed in two places. Prusa: the Extrusion Multiplier in PrusaSlicer and the Flow setting in printer firmware adjust the same thing without changing each other.
If you turned Flow up on the printer's screen during an earlier print, reset it before testing, or the test measures both at once.
Prerequisites
- Extruder calibrated in firmware. OrcaSlicer assumes E-steps or rotation distance are already right and says most modern printers ship that way. Flow ratio corrects small filament-to-filament differences, not a mis-set extruder. Changing rotation distance is a firmware configuration change that alters every extrusion move; only do it on a printer whose firmware you maintain, using the method in Before You Calibrate.
- Temperature tuned: Temperature Tower.
- Pressure advance, in OrcaSlicer's and Bambu Lab's order. OrcaSlicer runs it before flow ratio because it is more often wrong and a tuned value makes the flow test easier to read; Bambu Lab says to try Flow Dynamics calibration first. See Pressure Advance.
- A reason to calibrate. Prusa says this is not necessary on a new printer from the factory; Bambu Lab says many defects on its printers with its own filament come from other settings, not flow.
OrcaSlicer warns Bambu Lab owners not to tick the printer's own 'Flow calibration' option when sending its flow or pressure advance tests.
How to run it
OrcaSlicer: Archimedean chords with YOLO (recommended)
Select the printer and the exact filament profile (the test starts from that profile's flow ratio), then Calibration, Flow ratio, YOLO (Recommended). OrcaSlicer creates eleven blocks at different modifiers. Its recommended range is -0.05 to +0.05 in steps of 0.01 Starting point, and a Perfectionist option uses -0.04 to +0.035 in steps of 0.005.
OrcaSlicer's example: 0.98 with the +0.01 block chosen gives 0.99.
OrcaSlicer and Bambu Studio: two-pass method
Pass 1 prints nine blocks at coarse modifiers; Pass 2 prints ten blocks from -9 to 0. Bambu Studio's manual flow rate calibration also uses two passes, and applies the sample you pick for you: Bambu Lab describes the coarse pass as 80 to 120 percent of the profile value in 5 percent steps, and the fine pass as 91 to 100 percent in 1 percent steps. On X1 series printers Bambu Studio also offers automatic flow rate calibration with the Micro-Lidar; Bambu Lab warns it may be unreliable on transparent, sparkly or very reflective filament.
Example from OrcaSlicer: 0.98 with the +5 block gives 1.029 after Pass 1; then 1.029 with the -6 block gives 0.96726 after Pass 2.
PrusaSlicer: Prusa's two methods
Visual: print Prusa's visual cube with its PLA preset at 0.20 mm, look at the top and feel it with a fingertip, and change the Extrusion Multiplier by 1 to 2 percent at a time Starting point. Too much material near the perimeters: decrease. Visible gaps between lines: increase. Prusa says microscopic gaps near the perimeters are fine.
Precise: print Prusa's cube in Spiral vase mode, measure the single wall three or more times in the middle of each side, average, and calculate.
Prusa's figure of 0.45 mm for extrusion width belongs to its default 0.4 mm nozzle profiles at 0.1 to 0.2 mm layers; use your own profile's width for other nozzles. Prusa warns that cheap digital calipers may not be accurate enough to measure one perimeter reliably.
Reading the result
OrcaSlicer's checklist for the chords pattern: the smoothest top surface, no visible gaps between the arcs, and little or no line between the inner spiral and the outer arcs. For the line pattern and Bambu Studio's blocks, pick the smoothest top. When two blocks look equal, Bambu Lab's example rules out the ones with overlapping lines (too high) and the ones with gaps (too low), and compares only those in between.
What to change
The flow ratio (Extrusion Multiplier) in the filament profile, then save it. Prusa says usual values fall between 0.9 and 1.1 Rule of thumb for its profiles, and that the right value can differ by material, color and even spool. If your answer is far outside that, remember Bambu Lab's point that not every defect comes from flow rate, and check the other causes in Under-Extrusion and Over-Extrusion.
OrcaSlicer notes flow ratio and pressure advance affect each other slightly; if flow changed a lot, run a quick pressure advance test again.
Leave these alone during the test
- Nozzle temperature, speed and layer height: the blocks must differ only in flow.
- Line width. The precise method depends on the width the slicer was told to print.
- The printer's on-screen Flow or M221 override.
- XY compensation. Fix flow first, then dimensional accuracy.
- Flow Ratio Calibration OrcaSlicer
- Calibration Guide OrcaSlicer
- Flow Rate Calibration Bambu Lab
- Extrusion multiplier calibration Prusa Research
- Rotation distance Klipper
Last reviewed 2026-11-05