Pressure Advance K Factor Calculator (Klipper/Marlin)

What pressure advance actually solves

Filament is not rigid. The 30-50 cm of plastic between your extruder gears and your nozzle behaves like a long spring: when the extruder pushes, the plastic compresses; when it stops, the compressed plastic keeps oozing out the nozzle for tens of milliseconds. The classic visible result is a bulge at every external corner of a print, where the head slows down but the trapped pressure keeps pumping plastic.

Pressure Advance (Klipper terminology) and Linear Advance (Marlin terminology) are the same idea: tell the firmware ahead of time how much to back off the extruder before a deceleration, and how much to push forward before an acceleration. The firmware looks at the planned velocity profile and pre-compensates.

The formula

E_advance = K × v_print

Where:

• E_advance = extra mm of filament added (or retracted) ahead of the move
• v_print = current head velocity (mm/s)
• K = the pressure advance constant (units: seconds, but treated as dimensionless in firmware config)

K captures the total compliance of your specific filament path: how much the plastic compresses, how long the bowden tube is, how stiff the filament is at the current melt temperature.

Typical starting K values

These are starting points. The right K depends on the specific filament, hotend, extruder, and even the day’s humidity. Calibrate per filament.

How to calibrate

1. Print a calibration tower (Klipper has a built-in TUNING_TOWER macro; for Marlin, there are gcode generators online).
2. The tower prints lines of varying speed with K stepping from 0 to a maximum. K = 0.0 leaves you with the original corner-bulge problem; the right K is where the line corners look squared.
3. Read the K value off the layer where the corners look cleanest, neither bulging (K too low) nor under-extruded (K too high).
4. Set the value with one config line.

Setting K in firmware

Klipper config in printer.cfg: [extruder] pressure_advance: 0.04 pressure_advance_smooth_time: 0.04

Klipper runtime (no restart): SET_PRESSURE_ADVANCE ADVANCE=0.04

Marlin (M900 inline gcode, or persistent in firmware): M900 K0.04

After setting via gcode, save to EEPROM with M500 on Marlin.

Why K differs by filament

K depends on three things:

1. Melt viscosity at print temperature. Hotter or softer melts compress more, requiring higher K.
2. Filament path length. Bowden setups have 30-50 cm of compressible plastic; direct drives have under 5 cm.
3. Filament rigidity above the melt zone. TPU is much softer than PLA even cold, so much more of the filament is compliant.

A bowden ABS at 240°C compresses far more than a direct-drive PLA at 200°C. Hence the 10x range between typical K values.

What good calibration looks like

A correctly-tuned pressure advance gives you:

• Sharp external corners with no bulge
• Clean overhangs (no ooze hanging off the underside)
• Consistent extrusion across speed changes
• Less stringing in some cases

What it does NOT fix:

• Z-banding (mechanical, not extrusion)
• Layer adhesion (more about temperature)
• General under-extrusion (calibrate E-steps and flow rate first)

Print-speed dependence in the calculator

The formula E_advance = K × v_print means the advance distance grows linearly with print speed. At 30 mm/s the advance is small (1.2 mm for K=0.04); at 200 mm/s it is much larger (8 mm for K=0.04). This calculator shows you the advance amount at multiple speeds so you can sanity-check whether your extruder can move the required filament fast enough during transitions.

A practical note

Klipper’s input shaper and pressure advance work in concert: input shaper handles mechanical ringing, pressure advance handles extrusion lag. Calibrate input shaper first (it changes the velocity profile), then pressure advance. Doing them in the wrong order means re-calibrating pressure advance after every input shaper change.