Measuring filament diameter with calipers and knowing whether to adjust the flow

Symptom

Extrusion is uneven: areas with gaps and areas with bulges, odd layer lines, the extruder slips or the filament does not pass smoothly through the tube; or you calibrated the flow and, after changing spools, it goes wrong again.

Likely causes, from most to least likely

1. Diameter out of tolerance: Elegoo gives ±0.02 mm for its PLA and PETG, and Bambu Lab asks for 1.75 ± 0.02 mm when random lines appear.
2. An oval rather than round cross-section: Elegoo detects it by measuring each point twice, the second time with the filament turned 90°.
3. Flow (extrusion multiplier) wrongly set for that spool: too high gives over-extrusion and too low under-extrusion (Bambu Lab). Prusa warns that the value changes with material, colour and even from spool to spool.
4. Calibrating with damp filament, a partly dirty nozzle or debris in the extruder: Bambu Lab warns that the calibration is then unreliable.
5. Calipers not zeroed or held at an angle, giving false readings (Elegoo).

Checks to tell which one it is

• Zero the calipers: clean the jaws with a lint-free cloth, close them with no gap and press ZERO if they do not read 0.00 (Elegoo).
• Measure like Elegoo: pull out 1 m of filament, mark 5-6 points every 15-20 cm and measure each one twice, the second time turning it 90°.
• Compare the readings with the diameter and tolerance on the label.
• Print a test part and look at the top face: build-up next to the walls means too much material; gaps between lines, too little (Prusa).

Before you start

Before taking anything apart, cleaning or adjusting it, switch the printer off and unplug it, unless a step says otherwise, and do not touch the nozzle, the heater block or the bed until they have cooled down. If in any doubt about electricity, heat or moving parts, stop and ask a professional.

Step-by-step fix

1. Measure properly

   With the calipers zeroed, open them slightly wider than the filament, close them slowly until the jaws touch it without squeezing and keep them square to the filament (Elegoo). Measure the 5-6 marked points and write down both readings for each.

   Check: You have a table of 10-12 readings.

   If that does not fix it: Interpret them (next step).

2. Decide from the readings

   If only a short section is out of tolerance, cut it off and keep an eye on it; if several points are, measure another 1-2 m; if many sections vary a lot, stop using that spool. If the reading changes a lot when you turn it 90°, it is oval: measure more points to confirm (Elegoo).

   Check: You know whether the spool is usable.

   If that does not fix it: If it is within tolerance, the problem is something else (next step).

3. Do not try to compensate for random variation

   Elegoo warns that if the diameter varies randomly along the spool, adjusting the diameter or flow in the slicer cannot compensate for it: change spools. If all readings are fine and it still fails, the problem is in the printer: extruder, nozzle or tube.

   Check: With a good spool, extrusion is even.

   If that does not fix it: Calibrate the flow for that spool (next step).

4. Prepare the flow calibration

   Before calibrating, dry the filament, clean the nozzle with several cold pulls or replace it, clean the extruder and wash the plate so the first layer sticks properly (Bambu Lab).

   Check: The filament is dry and the nozzle, extruder and plate are clean.

   If that does not fix it: Calibrate (next step).

5. Calibrate the flow for that spool

   Use the slicer's flow calibration: Bambu Lab prints samples from 80 to 120% in 5% steps and then refines in 1% steps, and you pick the smoothest one without gaps. With calipers you can do it like Prusa: a single-wall cube measured at three or more points, where the multiplier is the profile's line width divided by the average thickness. Prusa warns that cheap calipers may not be accurate enough for a single wall.

   Check: The top face is smooth and the walls measure as intended.

   If that does not fix it: If it does not improve, repeat the calibration with another spool to rule out the filament.

Prevention

• Measure every new spool at a few points before an important job.
• Save each filament's calibrated flow in its profile: Prusa's values usually range from 0.9 to 1.1.
• Keep filament dry: moisture also shifts the calibration result (Bambu Lab).

Still not fixed?

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Printers it applies to

Any FDM (filament) 3D printer, from any brand.

Official sources

This guide summarises the official documentation linked here:

• Unstable FDM Filament Diameter Solution · Elegoo
• Flow Rate Calibration (Bambu Studio) · Bambu Lab
• Troubleshooting and Solutions for 3D Printing Layer Lines (filament quality) · Bambu Lab
• Extrusion multiplier calibration · Prusa Research