Under-extrusion — the complete diagnosis & fix list

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.

What under-extrusion looks like

• Pinholes on top surfaces (especially first top layer).
• Gaps between perimeters — you can see the infill through the wall.
• Walls peel off in layers when you flex the print.
• Inconsistent line widths — one line full, the next thin or missing.
• Skipping/clicking noises from the extruder (filament slipping).

Cause 1: Flow rate is wrong

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.

1. Print a single-wall calibration cube (20×20×10 mm, 0 perimeters, 0 infill, 1 wall). Use the same filament you'll print with.
2. Measure 4 sides with calipers. Average them.
3. Compare to your line width (default 0.42 mm on a 0.4 nozzle).
4. New flow ratio = old flow ratio × (target line width / measured wall thickness).
5. Re-print and verify. Walls should be within ±0.02 mm of target.

Use the flow rate calculator to do the math automatically.

Cause 2: Partial nozzle clog

A partial clog reduces flow specifically at high speeds. Symptoms:

• Walls look fine at slow speeds but become gappy at fast travel speeds.
• Visible "spurt" pattern as pressure builds and releases.
• An extrusion test (push 100 mm of filament at 5 mm/s through a hot, idle nozzle) outputs noticeably less than 100 mm of bead.

Fix:

• Do an atomic / cold pull (see Clogged nozzle and hotend).
• Swap the nozzle — nozzles are consumable. Brass wears in ~500 hours of PLA; abrasives kill them in tens of hours.

Cause 3: Feeder slipping or under-tensioned

The extruder gear (the toothed wheel that pushes filament into the hotend) can slip if:

• Idler tension is too loose — filament has horizontal play.
• Filament is too tight on the spool and the motor can't pull it — check the spool for tangles.
• The gear teeth are clogged with shavings ("filament dust") — clean with a small brass brush.
• Idler tension is too tight, deforming the filament into a chewed mess.

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.

Cause 4: Hotend temperature too low

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:

• PLA: if under-extruding at 200 °C, try 205–215.
• PETG: if under-extruding at 230 °C, try 235–245.
• PA-CF / PC-CF: these need a high-flow hotend, not just higher temperature. A 0.4 standard hotend tops out around 12 mm³/s; carbon-filled blends often need 20 mm³/s.

Cause 5: Wet filament

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.

Cause 6: E-steps out of calibration

"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.

1. Heat hotend to printing temperature, load filament.
2. Mark filament with a sharpie exactly 100 mm above the extruder body.
3. Send a G-code command to extrude 100 mm at 100 mm/min: G1 E100 F100
4. Measure remaining filament above the extruder. The gap to the mark is what was actually extruded.
5. New E-steps = (current E-steps × 100) / measured.
6. Save with M92 E<new value> and M500.

The e-steps calculator does the arithmetic.

Cause 7: Slicer settings

• Layer height too thick. A 0.4 nozzle should never exceed 0.32 mm layer height — above that, line bonding fails.
• Line width too narrow. Line width below ~85% of nozzle diameter under-extrudes by design.
• Speed too high for the hotend. Most 0.4 hotends max out at ~12 mm³/s volumetric. Anything beyond that under-extrudes regardless of e-steps.

Use the volumetric flow calculator to verify you're not asking the hotend for more than it can deliver.

Related articles