Under-Extrusion vs. Over-Extrusion: How to Tell the Difference

These two show up looking almost like opposites, but they get confused constantly because both leave a print looking "off" in a way that's hard to describe until you know what you're looking at. Here's how to tell them apart at a glance, and what actually fixes each one.

What Each One Looks Like

Under-extrusion is not enough plastic coming out of the nozzle. Walls look thin or have visible gaps between lines, top surfaces have pinholes or a see-through, wispy look, and layer lines don't quite fuse together — you can sometimes see daylight through a wall that should be solid.

Over-extrusion is too much plastic coming out. Walls look slightly swollen or lumpy, fine details blob together and lose sharpness, layer lines bulge outward instead of sitting flat, and dimensional accuracy suffers — parts print slightly larger than designed.

Under-Extrusion: Causes and Fixes

• Partial nozzle clog — the most common cause. Try a cold pull or a nozzle needle to clear debris.
• Extruder tension too loose — the gear can't grip the filament firmly enough to push it consistently. Check and adjust extruder tension.
• Flow rate set too low — check your slicer's flow/extrusion multiplier; 100% is standard, but some filaments need slight adjustment.
• Filament diameter inconsistent — cheap filament varies in thickness, causing inconsistent flow. Try a known-good spool to rule it out.
• Wet filament — moisture creates steam pockets that interrupt smooth flow. See our filament drying guide if this print used to come out clean and suddenly doesn't.
• Nozzle temperature too low — plastic isn't melting fully or flowing fast enough. Try increasing in 5°C increments.

Over-Extrusion: Causes and Fixes

• Flow rate set too high — check your slicer's flow/extrusion multiplier; drop it in small increments (2–5%) and reprint a test.
• E-steps miscalibrated — if your printer is consistently over-extruding across multiple filaments, the extruder's steps-per-mm may need recalibration.
• Nozzle temperature too high — plastic flows more freely than it should. Try lowering in 5°C increments.
• Wrong filament diameter setting — if your slicer profile says 1.75mm but you're loaded with 3mm filament (or a miscalibrated batch), flow calculations will be off.
• Print speed too slow for the flow rate — more plastic gets deposited per distance traveled than intended. Try increasing print speed slightly instead of just cutting flow.

How to Tell Them Apart When You're Not Sure

If you're staring at a print and can't decide which one you're looking at, a flow calibration tower or a simple single-wall test print will make it obvious — under-extrusion shows visible gaps and inconsistent width, over-extrusion shows excess width and rounded, bulging corners. Measuring a single printed wall with calipers against what the slicer says it should be is the most reliable diagnostic: thinner than expected means under-extrusion, thicker means over-extrusion.

Quick Reference

• Walls too thin, gaps visible, pinholes on top surfaces → under-extrusion
• Walls swollen, details blobbed, oversized dimensions → over-extrusion
• Both can be caused by flow rate or temperature — check those first before assuming a mechanical issue
• If the problem appeared suddenly on a print that used to come out fine, check filament moisture before anything else

Once flow rate, temperature, and extruder tension are dialed in together, most prints land right in the middle — solid walls, sharp details, accurate dimensions.

Still not sure which one you're dealing with? Tell us your printer, filament, and a photo of the print in the comments and we'll help you figure it out.