Flow ratio and extrusion multiplier: calibrating how much plastic leaves the nozzle

What the setting multiplies

A slicer first works out how much plastic each millimeter of path needs, then applies the flow ratio. PrusaSlicer computes the cross-section of a normal extrusion as a rectangle with rounded ends, h × (w − h × (1 − π/4)), where h is layer height and w is line width ^[6]. A 0.45 mm line at 0.20 mm layer height works out to 0.0814 mm³ per millimeter of travel; at a flow ratio of 0.95 the slicer asks for 0.0773 mm³ instead.

The setting has different names but the same job:

The slicer value and the firmware value stack. Prusa’s documentation gives total flow as the firmware flow multiplier (M221) times the PrusaSlicer extrusion multiplier, and changing one does not change the other ^[7]. In OrcaSlicer, an object’s own flow ratio is multiplied by the filament’s flow ratio ^[2].

OrcaSlicer: Archimedean chords with YOLO

OrcaSlicer’s wiki marks this method as recommended ^[1]. The test starts from the flow ratio already stored in the selected filament, so select the right filament first.

1. Select the printer and the filament to calibrate.
2. Open Calibration → Flow ratio → YOLO (Recommended). The Recommended variant spans −0.05 to +0.05 in 0.01 steps; the Perfectionist variant spans −0.04 to +0.035 in 0.005 steps.
3. Slice and print the generated project of eleven blocks, each with a different modifier.
4. Pick the block with the smoothest top, no gaps between the arcs, and little or no visible line where the inner spiral meets the outer arcs. The inner spiral prints last so that excess material shows up at that contact line.
5. Apply new = old ± modifier and save the filament profile. The wiki’s example: 0.98 with a +0.01 block gives 0.99.

All of the above is from the OrcaSlicer flow ratio guide ^[1]. On Bambu Lab printers the same page warns not to tick the printer’s own flow calibration option when sending the test.

The legacy two-pass method

The older Monotonic Line test uses percentage modifiers, so the formula changes to new = old × (100 + modifier) / 100 ^[1].

Bambu Studio’s developer-mode calibration for third-party printers uses the same Pass 1 and Pass 2 structure and formula ^[12]. The common slip is mixing the two formulas: a YOLO modifier of +0.01 is added directly, while a Pass 1 modifier of +5 means plus 5 percent. The flow ratio calculator applies either formula.

Bambu Studio on Bambu Lab printers

Bambu Studio offers automatic and manual flow rate calibration. Automatic calibration uses the Micro-Lidar and is supported only on the X1 series; Bambu Lab warns that transparent, sparkly or highly reflective filaments may give poor results ^[9]. Manual mode has two steps:

• Coarse calibration prints blocks from 80% to 120% of the preset flow ratio in 5% steps.
• Fine calibration prints blocks from 91% to 100% of a chosen value in 1% steps.

Bambu Lab’s advice for reading the fine set is to compare samples under side lighting, look at the middle of each rectangle, and reject any sample with even slight gaps, because that under-extrusion can grow on larger prints ^[9]. For hand tuning outside the wizard, Bambu Lab’s X1 guide recommends small increments and notes that a change of ±0.03 to ±0.05 already makes a visible difference on top layers ^[11].

Flow rate is separate from flow dynamics. Flow dynamics is Bambu’s name for the pressure advance K factor ^[10], covered in the pressure advance guide. Bambu Lab suggests running flow dynamics first and flow rate only if problems remain ^[9].

PrusaSlicer: measuring a wall or reading the top surface

The Prusa Knowledge Base describes two methods that it says give similar results ^[7].

Precise method. Based on the default 0.4 mm nozzle profiles at 0.1 to 0.2 mm layer height, with a 0.45 mm extrusion width and a starting multiplier of 1:

1. Print the Precision Method Cube in Spiral vase mode at your usual layer height.
2. Measure each wall at least three times near the middle and average the readings.
3. Compute extrusion multiplier = 0.45 / average wall thickness.
4. Enter the value in Filament Settings → Filament and reprint to confirm.

An average wall of 0.47 mm gives 0.45 / 0.47 = 0.957. Prusa cautions that cheap calipers may not resolve a single wall reliably, and that other nozzle sizes or layer heights above 0.2 mm need adjusted inputs ^[7].

Visual method. Print the Visual Method Cube with the default PLA preset at 0.20 mm layers and judge the top by eye and fingertip. Lower the multiplier by 1–2% if material piles up near the perimeters; raise it if there are visible gaps between lines. Microscopic gaps near perimeters are acceptable ^[7].

When not to change flow ratio

Flow ratio fine-tunes one filament. It is the wrong place to fix a hardware or process error, and the sources say so in several ways:

• Prusa states the procedure is not necessary on a printer straight from the factory ^[7].
• PrusaSlicer’s tooltip says that if you think you need a value outside 0.9–1.1, check the filament diameter and the firmware E steps instead ^[5]. See e-steps and rotation distance.
• OrcaSlicer’s calibration order puts temperature, max volumetric speed and pressure advance before flow ^[3].
• Bambu Lab asks for dried filament, a clean hotend and extruder gears, and a washed plate, and warns that a partially clogged nozzle skews the result ^[9].

If the top surface still shows scarring after calibration, the over-extrusion and under-extrusion guides cover the other causes.