What is the extrusion multiplier formula?

New multiplier = current multiplier x (expected wall thickness / measured wall thickness). If your single wall measures thicker than the target you are over-extruding, so the multiplier drops below the current value. Thinner means under-extrusion and the multiplier rises.

What expected wall thickness should I use?

Use the extrusion width (line width) you sliced the test at, not the nozzle diameter. Many slicers default to 0.45mm line width on a 0.4mm nozzle. The single-wall test prints exactly one extrusion wide, so the measured thickness should match that width.

Why calibrate flow rate at all?

A correct flow rate gives accurate dimensions, strong layer bonding, and clean top surfaces. Over-extrusion causes blobs, rough tops, and oversized holes; under-extrusion causes gaps, weak parts, and undersized walls. Flow calibration corrects all of these systematically.

Is this the same as E-steps calibration?

They are related but separate. E-steps calibrate the extruder motor so it pushes the requested length of filament. Flow rate is a software multiplier on top of correct E-steps that fine-tunes the deposited volume. Always calibrate E-steps first, then dial in flow.

Should I measure each wall once?

No. Take 3-4 caliper readings around the test wall and average them. Single-point measurements pick up local artifacts and skew the result. The averaged thickness gives a far more reliable correction.

Flow Rate & Extrusion Multiplier Calculator

This tool calibrates your printer’s extrusion multiplier (also called flow rate or flow ratio) by comparing how thick a single-wall test print should be against how thick it actually measures. The result is a precise multiplier that eliminates systematic over- and under-extrusion on any FDM/FFF machine.

How it works

Print a calibration object with a single perimeter (one wall, 0% infill, no top/bottom layers), so exactly one extrusion line forms each wall. In an ideal world that wall is as thick as the extrusion (line) width you sliced at. In practice it comes out thicker (over-extrusion) or thinner (under-extrusion).

The correction is a simple proportional scaling:

New multiplier = current multiplier x (expected thickness / measured thickness)

If you printed at a flow of 1.00 with a 0.45mm target and your calipers read 0.48mm, you are pushing too much plastic. The new multiplier is 1.00 x (0.45 / 0.48) = 0.9375, i.e. 93.75% flow. Re-slice at that value and the wall lands on target.

Worked example

Current multiplier: 0.98 (98%)
Expected wall (line width): 0.40mm
Measured average: 0.43mm

New multiplier = 0.98 x (0.40 / 0.43) = 0.9116, or about 91.2% flow. The drop reflects that you were over-extruding by roughly 7.5%.

Tips

Calibrate E-steps first; flow rate assumes the extruder already pushes the correct length.
Print the test slowly (around 20-30 mm/s) so pressure-advance and flow lag do not distort the wall.
Average several caliper readings; deburr any seam before measuring.
Re-check after changing filament brand or material — different polymers and pigments flow differently.