Under-extrusion is when the nozzle outputs less plastic than the slicer ordered. The print looks gappy, walls are weak, lines don't bond, and top surfaces have pinholes. Six different problems all produce the same visible symptom — this page walks each one in the order they're easiest to rule out.
Run a flow-rate calibration print (a single-perimeter cube) and measure the wall thickness with calipers. If walls are thinner than nozzle diameter, raise flow rate by (target / measured). If walls are inconsistent, you have a clog or feeder slip — not a flow problem. Also dry your filament — wet PLA and PETG under-extrude even when everything else is right.
Every filament has a slightly different effective diameter and density. A flow ratio of 1.00 is correct in theory but real-world spools want 0.92–1.05.
Use the flow rate calculator to do the math automatically.
A partial clog reduces flow specifically at high speeds. Symptoms:
The extruder gear (the toothed wheel that pushes filament into the hotend) can slip if:
Check the back of your filament after a print. Healthy filament has small even teeth-marks. Chewed/flat filament means the gear is grinding rather than pushing.
Below a filament's flow temperature the melt zone can't supply enough hot plastic to keep up with the extruder. Try raising 5 °C at a time:
Wet filament foams in the melt zone, dropping effective output by 10–30%. Symptoms overlap with stringing (see Stringing and oozing ): hissing nozzle, bubbly extrusion, gappy walls. Dry it. Full instructions in the filament drying guide .
"E-steps" is the firmware's belief about how many motor steps push 1 mm of filament. If it's wrong, every filament under-extrudes (or over-extrudes) by the same percentage.
The e-steps calculator does the arithmetic.
Use the volumetric flow calculator to verify you're not asking the hotend for more than it can deliver.