diff --git a/Marlin/src/Marlin.cpp b/Marlin/src/Marlin.cpp
index 9dcc90b75..1204f944f 100644
--- a/Marlin/src/Marlin.cpp
+++ b/Marlin/src/Marlin.cpp
@@ -359,10 +359,6 @@ void quickstop_stepper() {
   SYNC_PLAN_POSITION_KINEMATIC();
 }
 
-#if ENABLED(LIN_ADVANCE)
-  #include "gcode/feature/advance/M900.h"
-#endif
-
 #if ENABLED(HAVE_TMC2130)
   #include "feature/tmc2130.h"
   #include "gcode/feature/trinamic/M906.h"
diff --git a/Marlin/src/gcode/feature/advance/M900.h b/Marlin/src/gcode/feature/advance/M900.cpp
similarity index 86%
rename from Marlin/src/gcode/feature/advance/M900.h
rename to Marlin/src/gcode/feature/advance/M900.cpp
index c5734943c..074dc506a 100644
--- a/Marlin/src/gcode/feature/advance/M900.h
+++ b/Marlin/src/gcode/feature/advance/M900.cpp
@@ -20,6 +20,14 @@
  *
  */
 
+#include "../../../inc/MarlinConfig.h"
+
+#if ENABLED(LIN_ADVANCE)
+
+#include "../../gcode.h"
+#include "../../../module/planner.h"
+#include "../../../module/stepper.h"
+
 /**
  * M900: Set and/or Get advance K factor and WH/D ratio
  *
@@ -27,7 +35,7 @@
  *  R<ratio>                   Set ratio directly (overrides WH/D)
  *  W<width> H<height> D<diam> Set ratio from WH/D
  */
-void gcode_M900() {
+void GcodeSuite::M900() {
   stepper.synchronize();
 
   const float newK = parser.floatval('K', -1);
@@ -39,7 +47,7 @@ void gcode_M900() {
                 newW = parser.floatval('W', -1),
                 newH = parser.floatval('H', -1);
     if (newD >= 0 && newW >= 0 && newH >= 0)
-      newR = newD ? (newW * newH) / (sq(newD * 0.5) * M_PI) : 0;
+      newR = newD ? (newW * newH) / CIRCLE_AREA(newD * 0.5) : 0;
   }
   if (newR >= 0) planner.advance_ed_ratio = newR;
 
@@ -50,3 +58,5 @@ void gcode_M900() {
   if (ratio) SERIAL_ECHO(ratio); else SERIAL_ECHOPGM("Auto");
   SERIAL_EOL();
 }
+
+#endif // LIN_ADVANCE
diff --git a/Marlin/src/gcode/gcode.cpp b/Marlin/src/gcode/gcode.cpp
index f2b701722..3df717908 100644
--- a/Marlin/src/gcode/gcode.cpp
+++ b/Marlin/src/gcode/gcode.cpp
@@ -122,7 +122,6 @@ extern void gcode_M165();
 extern void gcode_M350();
 extern void gcode_M351();
 extern void gcode_M355();
-extern void gcode_M900();
 extern void gcode_M906();
 extern void gcode_M911();
 extern void gcode_M912();
@@ -658,9 +657,7 @@ void GcodeSuite::process_next_command() {
       #endif
 
       #if ENABLED(LIN_ADVANCE)
-        case 900: // M900: Set advance K factor.
-          gcode_M900();
-          break;
+        case 900: M900(); break;  // M900: Set advance K factor.
       #endif
 
       #if ENABLED(HAVE_TMC2130)