How to Tune Marlin Linear Advance

You’ve calibrated your e-steps, tuned your flow rate, and dialed in your temperatures. Your first layers look like glass. But the moment you try to speed up your print, your corners start bulging, your sharp edges look like rounded blobs, and layer seams become painfully obvious.

Sound familiar? It’s one of the most frustrating walls to hit in 3D printing.

The good news is that you don't need expensive new hardware to fix this. You just need to teach your printer's brain how to handle the physics of melting plastic. This is exactly what Marlin Linear Advance does. It is easily one of the most powerful—and most misunderstood—tools for optimizing your print quality.

Let's demystify Linear Advance, calibrate your K-factor, and get you printing faster with razor-sharp precision.

What Actually is Linear Advance?

To understand Linear Advance, you have to understand the "spring" effect.

When your extruder gear pushes filament into the hotend, the plastic doesn't melt and flow out of the nozzle instantaneously. Solid filament is slightly flexible. As the extruder quickly pushes it, the filament compresses momentarily against the pressure of the nozzle before it actually begins to flow.

The opposite happens when your printer slows down for a corner. The extruder gear might stop pushing, but because the filament inside the tube is compressed, it acts like a loaded spring. It wants to decompress, so plastic keeps oozing out of the nozzle even though the extruder motor has stopped.

This lag means your X and Y motors are perfectly in sync, but your plastic flow is constantly a step behind. The result? Under-extrusion when accelerating, and over-extrusion (bulging corners) when slowing down.

Linear advance fixes this by decoupling the extruder motor from the X/Y axes. It calculates exactly how much pressure is building up and intentionally speeds up or slows down the extruder motor to maintain perfectly even nozzle pressure at all times.

Direct Drive vs. Bowden Setups

Linear advance can be used on almost any machine, but the results you see will depend heavily on your hardware.

Because Bowden setups use a long PTFE tube to guide the filament, there is a massive amount of room for the filament to bend, flex, and compress. Therefore, Bowden printers see a dramatic night-and-day improvement from Linear Advance. Direct Drive machines have a very short path to the melt zone, so their "spring" effect is much smaller—but tuning Linear Advance will still provide meaningful improvements.

Step-by-Step: Calibrating Your K-Factor

The number your printer uses to calculate this pressure compensation is called the K-Factor. Because every filament is different (PLA is stiff, TPU is basically a rubber band), you need to find the specific K-Factor for the different material types you print with.

1. Generate the Test G-Code

To determine your K-factor, head over to the official Marlin K-factor Calibration Pattern Generator.

1. Input your standard print settings (Nozzle temperature, bed temperature, layer height, and retraction). Use the exact settings you normally slice with.
2. For the Starting Value for K, use 0.
3. For the Ending Value for K, use 0.2 (for Bowden) or 0.1 (for Direct Drive).
4. Set the K-factor Stepping to 0.01 to ensure you get a highly accurate result.
5. Click "Generate G-code" and save the file to your SD card.

2. Print and Read the Pattern

Run the G-code on your printer. It will print a series of straight lines.

Notice how the printer behaves: it will start printing a line slowly, suddenly speed up in the middle, and then rapidly slow down at the end. The only difference between each line is the K-factor applied to it. The numbers printed next to the lines tell you which K-factor was used.

How to pick the right line:
Look at the points where the print speed changes (the transition from slow to fast, and fast to slow).

• If the K-factor is too low, you will see thick blobs where the speed changes.
• If the K-factor is too high, you will see severe thinning or even empty gaps where the extruder pulled back too aggressively.
• The best line will look uniform in width from start to finish.

If none of them look perfect, pick the one that looks the least terrible, write down that number, and you can run a second test with a tighter range around that number.

Saving Your Settings

Once you have your golden number (let's say it was 0.06), you need to tell your printer to use it. You have two ways to do this:

Method 1: Slicer Start G-Code (Recommended)
Because PLA, PETG, and ABS all have different flexibilities, they need different K-factors. The easiest way to manage this is to save the value in your slicer's material profile.

Add the M900 command to your material's starting G-code:

M900 K0.06

Method 2: Save to EEPROM (Best if you only print one material)
If you exclusively print with one brand of PLA, you can lock it into your printer's memory via a terminal (like Pronterface or OctoPrint) using these commands:

M900 K0.06
M500