Flow Ratio Calibration: Why and How

Flow ratio is a single multiplier applied to every extrusion move in a print, correcting for the fact that no two filaments — even two spools of the "same" material from different brands — extrude at quite the same rate for a given amount of filament fed in. Getting it right fixes over- and under-extrusion at the source, instead of papering over it with temperature or speed changes.

What flow ratio actually corrects

Filament changes volume slightly between its solid, room-temperature state and its molten, printing state, and that change isn't identical across brands, colors or humidity levels. Flow ratio is Bambu Studio's (and every OrcaSlicer-family fork's) way of scaling all extrusion to compensate: a ratio below 1.0 tells the printer to extrude less material than the geometry math suggests; above 1.0, more.

Bambu's stock values

None of these are far from 1.0 — Bambu's own filament is well characterized, which is exactly why generic and off-brand spools (with no vendor-specific calibration behind them) are the ones that usually need a manual flow pass.

The calibration test

1. Use Bambu Studio's built-in flow rate calibration (Calibration tab) for any filament that isn't a first-party Bambu spool. It prints a small pattern and reports a suggested flow ratio directly — faster and more repeatable than eyeballing a test cube.
2. If tuning manually, print a single-wall calibration cube and measure wall thickness with calipers against the slicer's expected line width. Thicker-than-expected walls mean lower the flow ratio; thinner means raise it.
3. Adjust in small steps (0.01–0.02) and reprint. Flow ratio is sensitive — the recommended working range for most materials is roughly 0.95–1.05; values outside that window usually point to a different problem (wrong filament diameter setting, a partial clog, or genuinely unusual filament) rather than a flow-ratio fix.

Reading the symptoms correctly

• Blobs, over-full walls, or infill fusing into a solid mass: flow ratio too high, or check filament diameter is set correctly first. See the over-extrusion guide.
• Gaps between infill lines, weak layer bonding despite correct temperature: flow ratio too low, or a genuine max-volumetric-speed ceiling being exceeded. See the under-extrusion guide.
• A generic or "no-name" filament that behaves oddly across many symptoms at once is the single strongest signal that flow calibration — not any one symptom fix — is the missing step.

Field notes: reading real tiles

Three signs cover almost every plate, and they read the same on Bambu’s Pass 1/Pass 2 tiles as on any other flow test:

• Ridges and a rough, torn top — over-extrusion. The lines crowd each other and pile up. Typical for the +10/+15/+20 tiles.
• Glossy, glassy patches — under-extrusion. The lines are stretched thin and reflect light instead of fusing matte. On dark or satin filament this sheen is the first thing to appear — judge under one lamp in raking light and tilt the plate so every candidate gets the same angle.
• A smooth, even, matte weave — the winner. If two tiles tie by eye, run a fingernail across the lines: smoothest wins. Still tied? Pick the value closest to 0 — an adjustment you cannot see adds noise, not precision.

And the classic trap: the result saves into whichever filament preset is selected when you start the test. Select your custom filament first and verify the flow ratio field changed afterwards — our custom filament guide walks the whole flow, click by click.

Try it yourself: three real tiles from a Pass 1 plate. Which one has the right flow?

A B C