Over-extrusion causes rough surfaces and dimensional creep. Under-extrusion causes weak parts and gaps. Both have the same fix: a correct flow rate (extrusion multiplier). Here's how to find it for every filament you own.
Flow rate (also called extrusion multiplier or volumetric flow multiplier) is the percentage by which your slicer scales the amount of filament extruded. A value of 1.00 (or 100%) means the slicer trusts its own calculations exactly. Values above 1.00 push more filament; values below 1.00 push less.
Why does this need calibrating? Because the slicer calculates extrusion amounts based on the theoretical diameter of your filament — usually 1.75mm — and an assumed density. Real filament deviates from both. A spool labelled 1.75mm might measure 1.72mm or 1.78mm on average, and densities vary by colour, manufacturer, and even batch. The result is a systematic over- or under-extrusion that compounds across every layer of every print.
This is separate from first-layer squish (which is a Z-offset issue) and separate from pressure advance (which compensates for start/stop lag). Flow rate is the baseline that all other extrusion settings sit on top of. If your flow rate is wrong, calibrating everything else is pointless.
Before calibrating, it helps to know which direction you're off in. The symptoms are distinct once you know what to look for.
Over-extrusion pushes excess material into each layer, causing it to squeeze outward and upward rather than staying where it belongs. What you'll see: surface texture that looks rough or bumpy, elephant foot on the base layers, walls that measure thicker than they should, gaps between perimeters that look "squashed", and a slight ridge or seam at the top of tall prints where material has nowhere to go. Dimensional accuracy suffers — printed holes are smaller than designed, and parts that should fit together don't.
Under-extrusion starves each layer of material, leaving gaps and weak bonding. Symptoms include: visible gaps between perimeter lines, weak layer adhesion that allows delamination under stress, a rough "grid" texture on top surfaces where infill shows through, prints that break easily along layer lines, and walls that measure thinner than designed. In severe cases you'll see missing segments or "zits" where the extruder caught up momentarily.
There are several methods. The wall-thickness method is the most practical and gives reliable results without specialist equipment. OrcaSlicer's built-in flow calibration automates much of the process — use that if available.
OrcaSlicer includes a built-in calibration tool that prints a series of small cubes or lines at different flow rates, then lets you pick the one that looks best. This is the fastest route for Bambu Lab owners.
Print a 20mm calibration cube with these slicer settings: perimeters = 1, no top surface, no bottom, 0% infill, layer height 0.2mm, print speed ≤40 mm/s. The result is a hollow open-top single-wall box.
Once printed and cooled, use digital calipers to measure the wall thickness at several points around the cube. Your target is exactly your nozzle diameter — so 0.40mm for a 0.4mm nozzle. Take the average of four or more measurements.
If you have digital calipers accurate to 0.01mm, measure your filament diameter at 10 points along a 50cm length, rotating 90° at each measurement to catch oval cross-sections. Take the average. Enter this value in your slicer's filament profile (Filament Settings → Filament → Diameter). This pre-corrects a significant portion of flow error before any prints, and the wall-thickness method then fine-tunes the rest.
This extra step matters most with budget filament from lesser-known brands. Premium filaments like eSUN and Bambu Lab's own spools typically hold diameter to ±0.03mm and diameter entry is largely optional — but it still doesn't hurt.
| Step | What to Do | Expected Outcome |
|---|---|---|
| 1. Measure filament | Caliper readings at 10 points, enter average diameter | Coarse pre-correction of flow error |
| 2. Print calibration cube | Single-wall 20mm cube, 0% infill, 1 perimeter, 40 mm/s | Wall to measure against nozzle diameter |
| 3. Measure walls | 4+ measurements with calipers; average them | Deviation from 0.40mm reveals correction factor |
| 4. Calculate new flow | New flow = current flow × (0.40 / measured) | Proportional correction to current setting |
| 5. Apply and verify | Update filament profile; print another cube | Walls within 0.01–0.02mm of 0.40mm |
| 6. Save to profile | Store flow rate per filament in slicer | No recalibration needed unless you change brand/colour |
The location varies slightly between slicers. Here's where each one hides the flow rate setting:
The per-filament flow ratio is under Filament Settings (the spool icon) → Basic → Flow ratio. This is a multiplier — 1.00 is the default. Values above 1.00 increase extrusion; below 1.00 reduce it. OrcaSlicer also has a Calibration → Flow Rate menu at the top that automates the print-and-compare workflow described above. The calibration wizard saves directly to your active filament profile when you accept a result.
Note: OrcaSlicer separates flow ratio (long-term per-filament correction) from first layer flow ratio (single-layer override). Only the base flow ratio needs calibration — don't adjust the first-layer override to compensate for flow rate errors; it masks the real problem.
In Bambu Studio, the flow ratio is under Filament Settings → Filament → Flow ratio. Bambu's own filament profiles come pre-calibrated for Bambu printers — if you're printing exclusively on a Bambu filament, the out-of-box profiles are usually accurate enough that flow calibration is optional. For third-party filaments, always calibrate.
| Material | Typical Flow Rate Start | Common Drift Direction | Notes |
|---|---|---|---|
| PLA | 0.96–1.00 | Slightly high on budget brands | Most forgiving; easiest to calibrate |
| PETG | 0.95–1.00 | Often under 1.00 | Viscous at print temps; less flow needed |
| ASA / ABS | 0.96–1.04 | Varies by brand | Shrinkage makes measurement slightly tricky — measure hot-off-print |
| TPU / Flexible | 0.90–0.98 | High — needs less than rigid filaments | Compression in extruder can inflate apparent extrusion |
| Nylon (PA) | 0.97–1.03 | Varies; humidity changes diameter | Calibrate on a freshly-opened dry spool |
| PLA-CF / PETG-CF | 0.97–1.05 | Can drift high with nozzle wear | Recalibrate more often; worn nozzles under-extrude |
One mistake that trips up Bambu Lab owners specifically: the lidar-assisted flow calibration that runs automatically before each print (on X1C and P1S) already compensates for per-print variation. However, it calibrates flow relative to your current filament profile's base flow rate. If your base flow rate is wrong, the lidar correction has less range to work with and may not fully compensate. Set the base flow rate correctly in your profile first, and then Bambu's auto-calibration handles the minor session-to-session variation on top of that.
For AMS multicolor prints, this matters even more: the lidar calibrates the active extruder before the print, but during colour changes the AMS has to purge and re-establish flow. A correct base flow rate per filament profile is what makes AMS colour transitions clean and waste-efficient.
Tight dimensional tolerance makes flow calibration faster and the final result more stable across a spool. These filaments consistently measure close to nominal diameter and respond predictably to flow rate adjustments.
| Brand | Diameter Tolerance | Best For | Where to Buy |
|---|---|---|---|
| Bambu Lab | ±0.03mm | Bambu printer owners, AMS use | bambulab.com → |
| eSUN | ±0.05mm | Value for money, wide material range | esun3d.com → |
| 3DJake house brands | ±0.05mm | EU buyers, fast shipping, bundled deals | 3djake.de → |
| Prusament | ±0.02mm | Highest precision; premium price | via 3DJake → |
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