The extrusion multiplier is a setting in your 3D printer’s slicer software that controls how much plastic comes out of the nozzle. It works as a simple scaling factor applied to the calculated flow rate: a value of 1.0 means 100% of the theoretical amount, 0.95 means 95%, and 1.05 means 105%. Getting this number right is one of the most effective ways to improve print quality, dimensional accuracy, and layer strength.
How the Extrusion Multiplier Works
Your slicer calculates exactly how much filament needs to be pushed through the nozzle for every movement the print head makes. It bases this on your nozzle diameter, layer height, print speed, and the expected diameter of your filament. The extrusion multiplier then scales that calculated amount up or down by whatever percentage you set.
Different slicers use different names for this setting. In PrusaSlicer and Bambu Studio, it’s called “Extrusion Multiplier” and expressed as a decimal (1.0, 0.95, etc.). In Cura, it’s called “Flow” and expressed as a percentage. They control the same thing. Your printer’s firmware also has its own flow rate setting, sometimes called “flow factor,” which can be adjusted with the M221 command. These two settings multiply together, so if your firmware is set to 95% and your slicer multiplier is 1.0, your actual output is only 95%. Many default printer profiles include an M221 command in the start G-code that quietly overrides what you see in the slicer, which can cause confusion if you’re not aware of it.
Signs Your Multiplier Is Too Low
Under-extrusion happens when not enough plastic comes out during printing. The results are easy to spot: visible gaps between lines on top surfaces, thin or translucent walls, missing sections in the print, and poor bonding between layers. Prints feel brittle and weak because the lines of plastic aren’t pressing firmly enough into each other to fuse properly. If you can pull layers apart with your fingers, under-extrusion is a likely culprit.
Signs Your Multiplier Is Too High
Over-extrusion pushes out more plastic than the space allows. The excess material has to go somewhere, so it creates blobs on outer surfaces, stringing between parts of the print, and a bumpy or ridged top layer. Dimensions tend to run larger than the model because the extra plastic bulges outward from walls. On top surfaces, look for raised ridges where lines of plastic overlap and pile up instead of sitting flat.
Calibrating With a Test Print
The most reliable way to dial in your extrusion multiplier is to print a single-wall test object and measure the result with digital calipers. Here’s the process:
- Print a hollow cube with a single perimeter wall and zero infill. A 20mm cube works well. Set your extrusion multiplier to 1.0 as a starting point.
- Measure the wall thickness at several points using digital calipers (not the cheap plastic kind). Take multiple readings on different walls and average them.
- Calculate the correct multiplier. Divide the expected wall thickness (your nozzle diameter or configured line width) by the measured wall thickness. For example, if your line width is set to 0.45mm and you measure 0.48mm, your new multiplier is 0.45 / 0.48 = 0.9375, which you’d round to about 0.94.
- Print again and verify. Repeat the test with your new value. The measured wall should now match the expected thickness closely.
This method works because a single wall isolates the extrusion variable. With multiple walls or infill, other factors like overlap settings can mask what’s really happening with flow.
Typical Values by Material
PLA is the baseline material for most printers, and a well-calibrated machine typically lands between 0.93 and 1.0 for PLA. PETG tends to need slightly less flow than PLA, with many users settling around 0.92 to 0.95. PETG is notorious for producing blobs and stringing when even slightly over-extruded, so erring on the lower side often helps.
Flexible filaments like TPU are the exception. Because the soft material compresses in the extruder before reaching the nozzle, you often need to increase the multiplier to 1.05 to 1.15 to compensate for the lag and squish. Every material and even every brand within a material type can behave differently, so calibrating per spool gives the best results.
Why Filament Diameter Matters
Your slicer assumes a specific filament diameter, usually 1.75mm. But filament manufacturing isn’t perfect, and actual diameter can vary by 0.02 to 0.05mm or more between brands and even along the same spool. A filament that actually measures 1.72mm instead of 1.75mm contains less plastic per millimeter of length, which means the printer pushes through less material than the slicer expects. The extrusion multiplier compensates for this kind of variance. Measuring your filament at several points with calipers and entering the average diameter in your slicer’s filament settings reduces how much correction the multiplier needs to do.
Per-Feature Flow Adjustments
Most modern slicers let you adjust flow rates separately for different parts of a print. Cura, for instance, offers individual settings for wall flow, top/bottom flow, infill flow, and support flow. This gives you finer control than a single global multiplier. A common approach is to keep walls at your calibrated value for dimensional accuracy but bump up top and bottom flow by a few percent to eliminate gaps on flat surfaces. Infill can sometimes tolerate slight under-extrusion without any real penalty, since it’s hidden inside the part and structural strength comes more from pattern choice than perfect line bonding.
These per-feature adjustments are secondary to getting the base multiplier right. Calibrate the global value first with the single-wall test, then fine-tune individual features if specific areas of your prints still show problems.
Firmware Flow vs. Slicer Multiplier
Because the firmware flow rate and the slicer extrusion multiplier multiply together, changing both at once creates confusion. If your firmware is set to 95% (M221 S95) and your slicer multiplier is 0.95, your actual flow rate is 0.95 × 0.95 = 0.9025, or about 90%. That’s a significant under-extrusion that might not be obvious from looking at either setting alone.
The cleanest approach is to remove any M221 commands from your start G-code (or set them to S100) and control everything through the slicer’s extrusion multiplier. This way, what you see in your filament profile is what you actually get. Check your start G-code if your prints seem consistently under-extruded despite the multiplier looking correct, as a hidden M221 line is one of the most common causes of unexpected flow issues.