The extrusion multiplier, sometimes called flow rate or flow multiplier depending on the slicer, is the setting that corrects the gap between how much plastic your slicer thinks it is pushing out and how much actually comes out of the nozzle. It is a small correction, usually a few percent in either direction, and it is worth understanding before you touch it, since flow problems and machine problems can look identical. If you have not gone through a full calibration pass yet, the calibration order guide on this site explains where flow fits relative to e-steps, temperature and the rest.
What the extrusion multiplier controls
Every slicer calculates how much filament a print needs based on the nozzle diameter, line width and layer height you set, then tells the firmware how far to turn the extruder motor to push that volume through. The extrusion multiplier (PrusaSlicer’s term) or flow ratio (OrcaSlicer, Bambu Studio) scales that calculation up or down. A value of 1.0 or 100 percent means print exactly what was calculated. Set it to 0.95 and the printer pushes 5 percent less plastic than the math says it should; set it to 1.05 and it pushes 5 percent more. Most well-behaved filaments land somewhere between 0.90 and 1.10 once calibrated, according to Prusa’s own calibration documentation, and a correction outside that band usually points to a mechanical issue rather than a filament quirk.
Rule out other causes before you touch the number
Flow problems and extrusion multiplier problems produce the same symptoms, so it is easy to chase the wrong fix. Before adjusting flow, confirm the nozzle is not partially clogged, the extruder gear is not slipping or clicking under normal pressure, and the spool is feeding freely without snagging on the holder. A worn nozzle, a loose idler tension screw, or filament that has swollen from moisture will all mimic a flow miscalibration, and no amount of multiplier tweaking fixes a mechanical fault. If you are already seeing the symptoms below and want to work through the mechanical causes first, this site’s guides on fixing under-extrusion and fixing over-extrusion and bulging walls cover that ground in more depth, and the under-extrusion vs over-extrusion guide helps confirm which direction the problem is actually going before you adjust anything.
Signs your flow actually needs adjusting
Under-extrusion from low flow shows up as visible gaps between adjacent wall lines, top surfaces that never quite close up and look holey no matter how many top layers you add, and layers that pull apart more easily than they should. Over-extrusion from high flow looks almost the opposite: rippled or bulging vertical walls where lines are fighting each other for space, scarring or blobs on the bottom of overhangs and bridges, and top surfaces that look lumpy instead of flat. If you only ever see one of these, on one specific filament, across otherwise normal prints, flow calibration is a reasonable next step.
The single-wall cube test, step by step
Print a small cube, 20 to 30 mm on a side, sliced with one wall and no top or bottom layers and no infill, either with spiral vase mode turned on or with walls set to 1 and top/bottom layers and infill set to zero. Set your line width to match your nozzle diameter (0.4 mm for a 0.4 mm nozzle, for example) and start flow at 100 percent. Once it prints, measure the wall thickness with digital calipers at three or more points around the cube, avoiding the very top and bottom where the print tends to be uneven, and average the readings. Cheap calipers can struggle with a wall this thin, so a proper micrometer or a well-calibrated digital caliper will give you a more trustworthy number.
Compare the average measured thickness to your slicer’s target wall width, which is usually listed under external perimeter width in the print settings (commonly around 0.45 mm for a 0.4 mm nozzle, though this varies by slicer default). New flow value equals your current flow value multiplied by target thickness divided by measured thickness. If the wall measured thicker than target, the printer is over-extruding and the new value comes out lower than 1.0; if it measured thinner, the new value comes out higher.
Applying the correction and knowing when to stop
Enter the new value in the filament’s extrusion multiplier or flow ratio field, not in a one-off tweak inside a single project, so it carries over to future prints with that spool. Reprint the cube and remeasure. Most filaments settle within one or two rounds. If you find yourself making a third or fourth correction and the number keeps moving by more than a percent or two each time, stop and suspect a measurement error or a mechanical cause rather than chasing an ever-shifting target. Large corrections, anything approaching or beyond the 0.90 to 1.10 band, are a signal that something else, like e-steps, a partially blocked nozzle, or wet filament, needs attention first.
Checklist
- Confirm e-steps, nozzle condition and filament path are fine before adjusting flow.
- Print a single-wall cube with line width matched to your nozzle diameter.
- Measure at three or more points and average, using calipers precise enough for a sub-millimeter wall.
- New flow = current flow x (target wall thickness / measured wall thickness).
- Adjust in small steps and expect the value to settle within one or two rounds.
- Treat a correction outside roughly 0.90 to 1.10 as a sign of a mechanical problem, not a filament quirk.