Prusa Knowledge Base
  • Čeština
  • Español
  • Italiano
  • Deutsch
  • Polski
  • Français
Last updated 2 months ago
This article is also available in following languages: 

Macros allow you to create conditions and evaluate math equations in the Custom G-code fields.

PrusaSlicer macro language is not compatible with Slic3r conditional expressions.


Conditional evaluation

{if <condition_1>}<GCode_condition_1>[][{else}<GCode_else>]{endif}

Expression evaluation


Legacy Placeholder syntax


Inside either construct, PrusaSlicer configuration variables can be accessed by their names


and elements of vector variables can be accessed by using square braces


refers to the temperature of the first extruder.

Multidimensional vectors can currently be only accessed as a simple vector variable resulting in a string value (e.g. if the extruder_variable[1] resulting in [n,0] string value -- can be posted in g-code but can not be used in arithmetic operations).

Strings are identified by double quotes

and regular expressions by slashes
 Strings are not parsed recursively, i.e. curly and square brackets inside strings will appear in the output. This allows constructs such as
{"[text in square brackets]"}


The following operators are allowed:

  1. Comparison
    <, >, ==, !=, <>, <=, >=
    • Example:
      {if layer_height == 0.2};Do something{endif}
  2. Boolean logic
    and, or, not
    or, equivalently,
    &&, ||, !
    • Example
      {if layer_height > 0.1 and first_layer_temperature[0] > 220};Do something{endif}
  3. Arithmetics
    +, -, , /
    • Example:
      M104 S{first_layer_temperature[0] * 2/3}
      (Note that first_layer_temperature is a vector)
  4. Ternary operator
    (<condition>? <cond_true>:<cond_false>)
    This needs to be enclosed in parentheses to work.
    • Example:
      M104 S{(first_layer_temperature[0]>220 ? 230 : 200)}
      This expression sets the extruder temperature to either 230 or 200 depending on whether the first layer needs to be above 220°.
  5. Regular expression matching
    (=~ (matching), !~ (not matching))
  6. Regular expressions are enclosed in slashes /
    • Example:
      {if printer_notes=~/.PRINTER_VENDOR_PRUSA3D./};Printer is Prusa{endif}






Casting to integer


Rounding to an integer


Rounding / padding with spaces: rounds fractional part to num_decimals digits (adding trailing zeros if needed), then left fills with spaces so the number has num_digits characters in total (including a decimal point if present). Last argument can be omitted, defaults to zero.

digits(a, num_digits, num_decimals=0)

Rounding / padding with zeros: same as above, only left filled with zeros

zdigits(a, num_digits, num_decimals=0)


Variables (placeholders)

You can use variables inside customs scripts and in the output file name template.

Check the List of all PrusaSlicer placeholders, grouped according to their scope.


Scalar variables

These values are scalar and can be directly referenced.

  • printer_notes (string)
  • layer_z (only available in layer change GCode)
  • layer_num (only available in layer change GCode)
  • toolchange_z (available in custom toolchange GCode since version 2.4.0)

Some variables are defined by complex formulas. For example, the perimeter_extrusion_width if left to zero takes the value of extrusion_width if non zero, otherwise a default perimeter_extrusion_width is calculated for the current layer_height. These substitutions are only done for the new syntax expressions (enclosed in curly braces {}), while the expressions written using the old placeholder syntax (enclosed in square braces []) are interpreted verbatim.

Vector (array variables)

These variables are arrays and need to be accessed as such (e.g. temperature[0]).

  • temperature
  • first_layer_temperature
  • bed_temperature (note that this is a vector, even though only one value makes sense: bed_temperature[0])
  • first_layer_bed_temperature (same as above!)

Multidimensional vector

These variables can only be accessed as simple vectors and can not be used in arithmetic expressions.

  • extruder_offset
  • bed_shape


Temp tower

You can use the "Before layer change" custom G-code to slowly decrease hot end temperatures. First, one may use the if/elseif/else expression:

{if layer_z < 10}M104 S265
{elsif layer_z < 17}M104 S260
{elsif layer_z < 24}M104 S255
{elsif layer_z < 31}M104 S250
{elsif layer_z < 38}M104 S245
{elsif layer_z < 45}M104 S240

 The same result may be achieved by a shorter if/else/endif expression together with a linear interpolation:

M104 S{if layer_z < 10}265{elsif layer_z > 45}240{else}{265+(240-265)*(layer_z-10.0)/(45-10)}{endif}

Or one may use the ternary operator:

M104 S{((layer_z < 10) ? 265 : ((layer_z > 45) ? 240 : 265+(240-265)*(layer_z-10.0)/(45-10)))}

Faster heat up with weak bed heaters before print

If you have a bed that takes a very long time to get up to temperature you can shave off a little bit of waiting time by using "Start G-code" to heat up the bed to the destination temperature minus 5 degrees and then starting to warm up the hot end while the bed continues to heat to the target value:

M190 S{first_layer_bed_temperature[0] - 5} ; wait for bed temperature - 5
M140 S[first_layer_bed_temperature] ; continue bed heating using legacy [<variable>] syntax
M109 S[first_layer_temperature] ; wait for nozzle temperature



Please sign in before adding a comment.
Still have questions?

Still have questions?

If you have a question about something that isn't covered here, check out our additional resources.
And if that doesn't do the trick, you can send an inquiry to or through the button below.