Flow Rate Calculator (Extrusion Multiplier)

The formula

The slicer assumes each wall line comes out exactly as wide as the line width setting. If the wall is thicker than that, the printer is pushing too much plastic and the flow comes down in proportion; thinner, and it goes up. Because it's a ratio, it works whether your slicer shows flow as 100% or 1.00.

Printing the test cube

1. Slice a 20 × 20 × 20 mm cube (any calibration cube model works, or add a plain cube in the slicer).
2. Set 1 wall, 0 top layers, 0% infill. In Cura enable Spiralize Outer Contour; in PrusaSlicer/Orca/Bambu enable Spiral vase. Keep bottom layers — you want the base for stability.
3. Use the same line width, layer height, temperature and speed you normally print with. Flow is partly a temperature effect, so calibrate the profile you'll actually use.
4. Let it cool completely. Measure each wall with digital calipers about halfway up, squeezing gently — a plastic wall compresses. Take two readings per side if they disagree.
5. Enter the four readings above. Apply the new flow, reprint, and expect the walls to land within ±0.02 mm of the set width.

Where flow lives in each slicer

Set it in the filament profile, not the print profile. Flow is a property of the material and the temperature, and different spools of the same material can differ by a few percent.

Order of operations

Flow interacts with everything upstream of it. One order that works:

1. E-steps / rotation distance — once per extruder, so the right length of filament is delivered.
2. Flow — this page, once per material.
3. Temperature and retraction — with a test tower, per material.
4. Dimensional accuracy — scale and hole compensation, per material.

Worth knowing that OrcaSlicer's own guide uses a different order — temperature, then max volumetric speed, then pressure advance, then flow, then retraction, then tolerance. The disagreement is genuine: flow depends on temperature, so tuning it first means redoing it if the temperature moves; but over-extrusion reads like printing too hot, so a temperature tower with flow badly out can mislead. Both converge if you go round twice. If you use Orca, follow Orca's order, because its tests are built for that sequence. The tower generator discusses this in more detail.

How much to trust this number

Worth being straight about: the leading community reference disagrees with this method. Ellis' Print Tuning Guide lists "tuning extrusion multiplier by measuring 1–2 cube walls" as the first entry on its Misconceptions page. The reasoning is sound and worth understanding rather than ignoring:

• The error ratio is brutal. A 0.02 mm slip with calipers — well within normal for squeezing a plastic wall — is about 4% of a 0.48 mm wall. That is larger than the correction most people are trying to make. The calculator now shows this figure live, from your own numbers, so you can see how much of the result is noise.
• Measurements read high. A single unsupported wall bulges, and layer wobble and filament diameter variation both add width. The measurement captures all of that alongside the flow you are trying to isolate.
• Small objects amplify everything. Dimensional work is more reliable on larger parts, where a fixed error is a smaller proportion.

None of that makes the arithmetic wrong — the formula is exact. It means the input carries more uncertainty than the output's decimal places suggest. Two runs landing within 1–2% of each other is as good as this method gets, and that is roughly the diameter tolerance of the filament itself.

What to do instead, or as well

• Tune by top surface. Ellis' preferred approach: print test blocks at 1–2% flow increments and pick the one whose top surface is smoothest. Too much flow looks and feels rough; too little leaves gaps and valleys between lines. It is a visual judgement rather than a measurement, which is precisely why it sidesteps the caliper problem. Look at the widest flat areas of the print.
• Use your slicer's built-in test. OrcaSlicer's Calibration → Flow Rate runs a two-pass version of the same visual method and applies the result for you.
• Fix dimensions separately. If parts come out the wrong size, that is a job for shrinkage compensation and hole offsets, not for flow. Using flow to chase dimensional accuracy trades a good top surface for a number on the calipers.

Use this page when you want a fast numerical starting point, or to sanity-check a value you arrived at visually. If the two disagree, trust the top surface.

Sources

• Ellis' Print Tuning Guide — Extrusion Multiplier and Misconceptions & Bad Advice. Source of the top-surface method and of the criticism above.
• Slicer documentation for where the setting lives and whether it is expressed as a percentage or a ratio.

The formula and the sensitivity figure on this page are exact arithmetic. Everything else — which method is better, how much to trust a reading — is judgement, sourced above so you can weigh it yourself.