There are plenty of sources explaining how to calibrate the extruder of a Bowden extruder with the Marlin firmware, but they are usually a bit lengthy for my taste and I’m way too impatient for all that, so here’s the short version. If you are already familiar with the process and just need the formula go straight to formula.

Calibrating the Extruder

Marlin will not allow you to control the extruder motor if it is not heated by default, or at least with the firmware configured like I have. So go to Prepare > Preheat PLA and wait for it to heat up to 180 °C.

Next go to Prepare > Move Axis > Move 1mm > Extruder.

Extrude a certain amount of filament (I used 120mm) and measure the length of the filament coming out. I now got 131mm which is not the 120mm I expected. The problem is my configuration. Here’s an excerpt from my configuration.h in Marlin:

//
// MOVEMENT SETTINGS
// @section motion
//

// default settings
#define DEFAULT_AXIS_STEPS_PER_UNIT   {80,80,4000,107}

The latter value is of 107 needs to be adjusted. We will call this value E0 and calulate the right value using the formula following now.

The Formula

E0 = (desired length / actual length) * E0.

So that is the desired length of 120mm divided by the actual length of 131mm and multiplying the result of that with the configuration value E0, which is 107. In this example you end up with (120/131) * 107 = 98. These values are just an example. The real value I calculated ended up being 102, but I don’t remember the values I was working with.

Replace the value in the config file and flash Marlin again and it should be feeding the desired length of filament. Measure again and repeat if needed.