If your prints have bulging corners, blobby seams, or thin gaps right after a fast move, you don't have an extrusion problem — you have a pressure problem. The fix is pressure advance (Klipper) or linear advance (Marlin). This guide covers what it actually does, how to calibrate it with a tuning tower, and the small bit of math that turns the test into a final number.

What pressure advance actually does

When the nozzle moves fast, pressure builds up in the melt zone. When it slows for a corner, that pressure keeps pushing plastic out for a moment — so the corner bulges. When it speeds back up, there's a brief under-extrusion gap. The extruder simply can't start and stop the flow instantly.

Pressure advance tells the firmware to advance (or relax) the extruder ahead of speed changes to cancel that lag. It does not change how much filament is extruded overall — that's what rotation_distance (Klipper) or E-steps (Marlin) and flow rate control. So calibrate those first; pressure advance is the layer on top.

The tuning tower method

Klipper has a neat trick: the TUNING_TOWER command sweeps a parameter as the print gets taller, so a single object shows every value at once.

1. Slice a tall test model (the notched-cube PA test from the Klipper docs is ideal).
2. Start the print, then run this in the console:

   TUNING_TOWER COMMAND=SET_PRESSURE_ADVANCE PARAMETER=ADVANCE START=0 FACTOR=.005

1. That starts pressure_advance at 0 and adds 0.005 for every 1 mm of height.
2. Let it print, then look for the height where corners are sharpest — no bulge before them, no gap after.

The math: turn a height into a value

The value at any height is just:

pressure_advance = START + measured_height × FACTOR

So if you used START=0 and FACTOR=0.005 and the cleanest band is at 35 mm:

0 + 35 × 0.005 = 0.175

Your pressure_advance is 0.175. If you'd rather not do it by hand (or want a suggested FACTOR for a tighter second tower when the best band lands near the top or bottom), this free browser tool does exactly this: Pressure Advance calculator.

Apply the value in printer.cfg, then RESTART and reprint the test to confirm:

[extruder]
pressure_advance: 0.175

Reading the tower (the part people get wrong)

You're not looking for "smooth walls" — you're looking at corners and direction changes. Too little PA: corners bulge outward. Too much: corners get rounded and under-filled, and you see gaps. The sweet spot is the lowest band where corners are crisp and consistent. Don't judge the seam or the first few layers — they're not representative.

If the best result is at the very top or bottom of the tower, your range was off. Widen it, or use the suggested-factor approach to center a tighter second tower.

Direct drive vs Bowden

The right value depends almost entirely on how far the filament has to travel:

• Direct drive: small, often 0.02–0.08.
• Bowden: much larger because of the tube, often 0.3–1.0.

Tube length, extruder gearing, temperature, and even the filament shift it, so never just copy someone else's number — run the tower.

Marlin users: linear advance

Same idea, different name and command. Marlin calls it linear advance, set with M900 K<value> and saved with M500. If you run a height-based tower, the formula above applies directly. If you print the classic labeled K-factor pattern instead, just read K off the cleanest line.

M900 K0.05
M500

Order of operations

Pressure advance only makes sense once extrusion is already accurate:

1. rotation_distance / E-steps — correct length of filament fed.
2. Flow rate — correct amount in the wall.
3. Pressure advance — correct pressure at corners. ← you are here.
4. Retraction — kill stringing on travel moves.

Do it in that order, or you'll be chasing one error with another tool. If you want the math done for you, there are small browser calculators for rotation_distance, flow rate, and retraction.

TL;DR

• Bulging corners = pressure, not extrusion.
• Run TUNING_TOWER COMMAND=SET_PRESSURE_ADVANCE PARAMETER=ADVANCE START=0 FACTOR=.005.
• pressure_advance = START + height × FACTOR (e.g. 35 mm → 0.175).
• Direct drive ≈ 0.02–0.08, Bowden ≈ 0.3–1.0 — but always test.
• Calibrate extruder + flow first, retraction after.

Happy tuning — sharp corners await.