Get your flow ratio wrong and everything downstream suffers: weak layers, gaps on top surfaces, blobby corners and parts that measure a fraction off. If you print functional parts, this is the single calibration that pays back fastest. This guide walks through flow rate calibration in OrcaSlicer the way the tool actually works today — both built-in methods, the post-v2.3.0 pattern change that trips people up, the exact ranges and formulas, and a more objective single-wall approach for when eyeballing test tiles isn’t good enough.
What flow rate (flow ratio) actually does
Flow rate — also called the extrusion multiplier — determines how much filament your nozzle pushes out. In OrcaSlicer the parameter is labelled Flow ratio in the filament settings, and the default is 1.0 (100%). A value of 1.0 means the nominal amount; higher values extrude more, lower values extrude less.
A properly calibrated flow ratio gives you consistent layer adhesion and accurate dimensions. Get it too low and you under-extrude: gaps, weak layers, poor structural integrity. Too high and you over-extrude: excess material, rough top surfaces and dimensional inaccuracy. If you’re already fighting gaps, our guide on how to fix under-extrusion in the right order pairs well with this — flow ratio is usually step one.
The two built-in methods in OrcaSlicer
Both live under Calibration menu → Flow Rate. Since v2.3.0 there are two approaches:
- YOLO (Recommended) — a simplified single-pass method using the formula OldFlowRatio ± modifier.
- 2-Pass Calibration (legacy) — the older two-pass method using OldFlowRatio × (100 + modifier) / 100.
Important version note: after v2.3.0 the top pattern for the recommended method changed from Monotonic Line to Archimedean chords. This new pattern deliberately prints the inner spiral last, so you can check for material accumulation on the contact line at the very end of the print. If you’re following an older tutorial that shows straight monotonic lines, you’re likely on a pre-2.3.0 workflow — the assessment cues are different.
YOLO method: single-pass flow rate calibration
This is the fastest route and the one most makers should use. It builds on your filament’s current flow ratio, so select the correct printer and filament first — the maths only works if the starting value is right.
- Open Calibration → Flow Rate → YOLO (Recommended).
- OrcaSlicer creates a new project with eleven blocks, each carrying a different flow ratio modifier.
- Slice and print. Run our gcode pre-flight checklist first if you want to catch obvious slicing mistakes before wasting filament.
- Assess and pick the best block.
- Update your filament profile and save.
YOLO ranges and steps:
- Recommended: calibration range [-0.05, +0.05], flow ratio step 0.01.
- Perfectionist: calibration range [-0.04, +0.035], flow ratio step 0.005.
What to look for on the Archimedean chords pattern: the smoothest top surface, no visible gaps between the pattern arcs, and minimal or no visible line between the inner spiral and the outer arcs.
Applying the result: use OldFlowRatio ± modifier. If your previous flow ratio was 0.98 and the best block had a modifier of +0.01, your new value is 0.98 + 0.01 = 0.99. Save the filament profile — nothing sticks until you do.
2-Pass method: the legacy workflow
The legacy method uses the Monotonic Line pattern and refines flow ratio in two rounds. It’s more involved but some makers still prefer it for fine control.
Pass 1
Select Calibration → Flow Rate → Pass 1. OrcaSlicer generates a project with nine numbered blocks, each with a different modifier spanning roughly +20 to −20. Print it, then find the block with the smoothest top surface.
Calculate the new flow ratio with OldFlowRatio × (100 + modifier) / 100. Example: a previous ratio of 0.98 and a chosen modifier of +5 gives 0.98 × (100 + 5) / 100 = 1.029.
Pass 2
Run Pass 2 with your new value entered. This generates ten blocks with modifiers ranging from −9 to 0 for finer resolution. Assess as before, then apply the same formula. Example: 1.029 with a chosen modifier of −6 gives 1.029 × (100 − 6) / 100 = 0.96726. Save the profile.
Tie-break rule: if two blocks look equally good, choose the one with the higher flow rate. A whisker of over-extrusion is easier to live with than gaps in your top layers.
How to assess the test tiles properly
Whichever method you use, judge each tile against the same criteria:
- No visible gaps between lines — that’s under-extrusion.
- No excessive buildup or roughness — that’s over-extrusion.
- Hold the tile up to the light to spot tiny gaps you’d miss otherwise.
A quick practical trick: run a fingernail across the top surface. The smoothest tile — no catching, no ridges — usually indicates balanced flow. Eyeballing works most of the time, but it’s subjective. If you can’t decide between two tiles, use the objective method below.
The objective alternative: single-wall cube measurement
Test tiles rely on your eyes. A single-wall cube relies on your calipers, which is far more repeatable — especially for dimensional accuracy on functional parts.
Set-up:
- Wall loops / perimeters: 1 — verify this in the slice preview, don’t just trust the input.
- Top layers: 0.
- Infill: 0%.
- Vase (spiralize) mode: often enabled; usually works fine in OrcaSlicer.
- Flow ratio: set to 1.0 (100%) for this test print.
Measuring: use accurate digital calipers or a micrometer. Take multiple readings (e.g. three per wall, all four walls) towards the centre of each wall. Avoid corners and the first/last layers — they’re unreliable. Average the lot.
The maths: New Flow Rate = Current Flow × (Target width / Measured width). Worked example: target 0.4 mm, measured 0.42 mm, current flow 1.0 → 1.0 × (0.40 / 0.42) = 0.952. Enter that, save, and reprint to confirm.
Where this sits in your calibration order
Flow rate isn’t a one-off cure. Do it after your first layer is dialled in and before you chase surface finish or pressure advance. Our calibration order guide lays out the full sequence so you stop wasting spools re-testing in the wrong order, and the OrcaSlicer first-layer adhesion settings piece covers the step you should nail first.
If a print still fails after calibrating, don’t guess. Upload a photo to our Diagnose tool — it identifies the defect and returns slicer-specific settings, including downloadable .ini patches for PrusaSlicer and OrcaSlicer, so you’re not left staring at a bad top surface wondering whether it’s flow, temperature or something else entirely.
Related: dialling in flow ratio matters most when you’re printing functional parts — see how 3D printing fits the custom race engine components workflow in the UK.
FAQ
Should I use YOLO or the 2-pass method?
Use YOLO (Recommended) for most work — it’s a single pass, uses the newer Archimedean chords pattern, and lands you within 0.01 (or 0.005 in Perfectionist mode). Reach for the 2-pass legacy method only if you specifically want its wider first-pass range and staged refinement.
What flow ratio should I calibrate for each filament?
Every material and, realistically, every spool can differ. Calibrate per filament profile: PLA, PETG and ABS pull noticeably different values. Start from the default 1.0, or from the manufacturer’s profile value, and refine from there.
Do I need to recalibrate flow rate when I change nozzle size?
Yes. Flow behaves differently at 0.4 mm versus 0.6 mm, so recalibrate after changing nozzle diameter — and after a nozzle swap in general, since wear and bore tolerances vary. Save each combination as its own profile.
My top surface still shows gaps after calibration — now what?
Nudge flow slightly higher (remember the tie-break rule favours the higher value), and check that it isn’t actually a temperature or top-infill issue. If you’re stuck, run the real causes and exact fixes checklist or send a photo to Diagnose for a targeted answer.