Quick answer
Over-extrusion (blobs, zits, bulging walls, oversized dimensions) means your printer is laying down too much plastic, so lower the flow rate to around 95-97% and drop the temperature by about 5 °C. Under-extrusion (gaps between lines, thin or missing layers, weak infill) means too little plastic, so raise the flow rate by about 5% and check for partial clogs or a slipping extruder. The reliable fix for both is a single-wall flow-rate calibration, but only after your filament diameter setting and extruder e-steps are already correct.
Why extrusion goes wrong
Your slicer calculates exactly how much filament to push for a given line width and layer height, then trusts the printer to deliver it. When the real amount delivered doesn’t match the math, you get over- or under-extrusion. The usual culprits are an inaccurate flow rate (extrusion multiplier) for that specific filament, a filament diameter setting that doesn’t match reality, uncalibrated extruder steps, a printing temperature that’s too low for the chosen speed, or a partial nozzle clog restricting flow. Because flow is material-specific, the same profile can over-extrude one brand of PLA and under-extrude another, which is why calibration per filament matters.
1. Confirm the basics before touching flow
A correct flow rate sits on top of correct hardware settings, so fix those first. Check that the filament diameter in your slicer matches your spool (1.75 mm nominal filament often measures 1.71-1.73 mm, and a slicer set to 1.75 mm will then push slightly too little plastic). On Ender, Prusa and other classic FDM machines, calibrate extruder e-steps once: mark 120 mm of filament, command a 100 mm extrude, measure what actually fed, and adjust e-steps so the two match. E-steps are mechanical and only need redoing after hardware changes. Bambu Lab machines handle this internally, so you go straight to flow calibration.
2. Run a single-wall flow-rate calibration
This is the definitive test for over/under-extrusion. Print a hollow calibration cube with one perimeter, zero top and bottom layers, zero infill, and line width set equal to your nozzle diameter (0.4 mm for a stock nozzle). When the print finishes, measure all four walls at their midpoint with calipers and average the readings. Then calculate: New Flow = Current Flow × (Expected Wall Width ÷ Measured Wall Width). If your 0.4 mm wall measures 0.44 mm you are over-extruding; if it measures 0.37 mm you are under-extruding. Most filaments land within the 90-110% flow range.
3. Use the built-in calibration on Bambu Lab
Bambu Studio and Orca Slicer include guided flow-rate tests. On the X1 series the printer prints calibration lines and uses its onboard lidar to scan them and adjust flow dynamically on the first layer, while the static flow ratio in the filament profile applies to every layer. On the P1 and A1 (no lidar), run the manual flow-rate test in the slicer, pick the cleanest patch, and save that flow ratio into a custom copy of the filament profile. Tune one filament at a time and save each result separately.
4. Don’t confuse flow rate with Flow Dynamics (Pressure Advance)
These are two different calibrations and fixing the wrong one wastes time. Flow rate (the extrusion multiplier or flow ratio) sets how much plastic comes out overall and is what cures steady-state over/under-extrusion. Flow Dynamics calibration, also called Pressure Advance or the K-value, only changes the timing of extrusion during speed changes, so it cleans up corners, blobs at line starts, and gaps after sharp stops. If your walls bulge or your infill is sparse, that’s a flow-rate problem; if only your corners are messy, calibrate Flow Dynamics instead.
5. Raise temperature or slow down for stubborn under-extrusion
If flow is calibrated and you still see gaps, the hot end may not be melting filament fast enough to keep up with your print speed. Raise the nozzle temperature in 5 °C steps (a temperature tower is the clean way to find the sweet spot), or reduce print speed and volumetric flow so the melt zone has time to work. Persistent skipping, clicking from the extruder, or flow that drops mid-print usually points to a partial clog or heat creep rather than a settings issue, so clear the nozzle and check the cooling on the heat-break side.
6. Tame over-extrusion with flow, temperature and slicer tweaks
For blobs, zits and dimensions that print oversized, set the calibrated flow rate first, then lower temperature by about 5 °C to reduce oozing and stringy excess. Enabling or increasing retraction and turning on coasting or wipe can help mop up the small surplus that causes surface zits. If top surfaces look rough and overfilled, reduce the top-layer flow or the infill/top overlap slightly rather than crushing the global flow number.
7. Re-test after every meaningful change
Flow rate is filament-specific, so a new brand, type, or even a different color of the same brand can shift the ideal number. Whenever you swap filament or change nozzle size, re-run the single-wall test (or the slicer’s guided calibration) and save a dedicated profile. Keeping a short note of the flow ratio that worked for each spool saves you from re-diagnosing the same problem twice.
Frequently asked questions
Is over-extrusion or under-extrusion worse?
Under-extrusion is usually worse for part strength because gaps between lines and weak infill create voids that crack under load, while over-extrusion mostly hurts surface finish and dimensional accuracy. Both are worth fixing, but a slightly under-extruded part can fail mechanically.
What flow rate should I start with?
Start at 100% for PLA and PETG and adjust from the single-wall measurement; most well-behaved filaments end up between 95% and 105%. If your calibration calls for a number far outside 90-110%, suspect a wrong filament diameter setting or uncalibrated e-steps rather than a true flow problem.
Do I need to calibrate e-steps on a Bambu Lab printer?
No. Bambu Lab machines manage extruder steps internally, so you skip e-step calibration and go straight to flow-rate (and Flow Dynamics) calibration in Bambu Studio or Orca Slicer. On Ender, Prusa and similar printers, e-steps should be calibrated once after any extruder hardware change.
Can a clog look like under-extrusion?
Yes. A partial nozzle clog restricts flow and produces thin lines, gaps and missing layers that mimic a low flow rate, but no amount of flow increase truly fixes it. If extrusion starts fine and degrades over a print, or the extruder clicks and skips, clear the nozzle before changing flow settings.