Definition

At 95 percent flow, a following 10 mm E command is treated as 9.5 mm of commanded filament movement on firmware that applies M221 directly. This can fine-tune bead volume after the extruder's mechanical steps and filament profile are correct.

Flow percentage is not the same as volumetric flow rate. The percentage is a dimensionless multiplier; volumetric flow is material volume per time, usually mm³/s, and also depends on line width, layer height and speed. Raising percentage to fix a mechanical or temperature problem can hide the cause and create inconsistent dimensions.

What matters in practice

Scales E commands

M221 changes later commanded extrusion without changing the XY path.

Not mm³/s

A multiplier and a rate describe different things even though both are called flow in interfaces.

Tune after mechanics

Drive calibration, filament diameter, nozzle condition and temperature should be credible first.

Example in G-code

Apply a 95 percent factor

M221 S95
G1 X100 E10 F1800

The firmware scales the following E motion. The physical amount can still differ if filament slips or the hotend cannot melt fast enough.

What this viewer can show

In this viewer

The viewer applies file-written M221 factors from their source line to filament totals and flow diagnostics. It cannot know a front-panel override that was never saved in the file.

Primary sources

This explanation uses firmware documentation and details visible in G-code. Your printer's settings still determine what the machine does.