Flow rate (sometimes called extrusion multiplier) scales how much filament the slicer commands on top of an already-correct E-steps value. Where E-steps corrects a mechanical property of the extruder, flow rate corrects everything else that affects actual output: nozzle wear, real filament diameter versus the diameter typed into the slicer, and small thermal effects on flow.
Why a printed test beats a formula
E-steps calibration measures the motor directly. Flow rate calibration measures the result — the actual printed wall thickness — because that’s the only way to catch errors that happen after the motor turns, like a nozzle that’s worn slightly oversize or filament that measures 1.78 mm instead of the 1.75 mm typed into the slicer profile.
The single-wall test
- Model or download a simple hollow box or cylinder, roughly 20 mm tall, with no top layers, no bottom layers, and a single perimeter wall — you want pure wall extrusion with nothing else affecting the result.
- Slice it with 100% flow (the default) and print it.
- Once printed, measure the wall thickness with calipers at several points around the shape and average the readings.
- Compare to your nozzle diameter. For a 0.4 mm nozzle, a single wall printed at 100% flow should measure close to 0.4–0.45 mm.
- Calculate the correction:
new flow % = old flow % × (target wall thickness / measured wall thickness).
For example, if you measured 0.46 mm on a 0.4 mm nozzle at 100% flow: 100 × (0.40 / 0.46) ≈ 87%. Re-slice and print the same test at the new percentage to confirm it lands closer to target.
What a correct flow rate actually fixes
- Solid top layers that are fully closed instead of showing pinholes or gaps
- Vertical walls that measure at the dimension you modeled, not systematically oversize or undersize
- Consistent layer adhesion, since correctly-sized extrusion lines bond to their neighbors more predictably than over- or under-sized ones