diff --git a/slic3r.pl b/slic3r.pl
index 651efa483..a593f93c9 100755
--- a/slic3r.pl
+++ b/slic3r.pl
@@ -426,9 +426,9 @@ $j
                         Support material angle in degrees (range: 0-90, default: $config->{support_material_angle})
     --support-material-contact-distance
                         Vertical distance between object and support material (0+, default: $config->{support_material_contact_distance})
-
     --support-material-xy-spacing
-                        horizontal distance between object and support material (0+, default: $config->{support_material_xy_spacing})
+                        "XY separation between an object and its support. If expressed as percentage (for example 50%),
+                        it will be calculated over external perimeter width (default: half of exteral perimeter width)
     --support-material-interface-layers
                         Number of perpendicular layers between support material and object (0+, default: $config->{support_material_interface_layers})
     --support-material-interface-spacing
diff --git a/xs/src/libslic3r/PrintConfig.cpp b/xs/src/libslic3r/PrintConfig.cpp
index c530b30aa..8556783fc 100644
--- a/xs/src/libslic3r/PrintConfig.cpp
+++ b/xs/src/libslic3r/PrintConfig.cpp
@@ -1121,12 +1121,16 @@ PrintConfigDef::PrintConfigDef()
     def->cli = "support-material!";
     def->default_value = new ConfigOptionBool(false);
 
-    def = this->add("support_material_xy_spacing", coFloat);
-    def->label = "Extra separation between part and support (mm)";
+    def = this->add("support_material_xy_spacing", coFloatOrPercent);
+    def->label = "XY separation between an object and its support";
     def->category = "Support material";
-    def->tooltip = "Add offset to support material separation (0 is 1 extrusion width).";
-    def->cli = "support-material-xy-spacing=f";
-    def->default_value = new ConfigOptionFloat(0.0);
+    def->tooltip = "XY separation between an object and its support. If expressed as percentage (for example 50%), it will be calculated over external perimeter width.";
+    def->sidetext = "mm or %";
+    def->cli = "support-material-xy-spacing=s";
+    def->ratio_over = "external_perimeter_extrusion_width";
+    def->min = 0;
+    // Default is half the external perimeter width.
+    def->default_value = new ConfigOptionFloatOrPercent(50, true);
 
     def = this->add("support_material_angle", coInt);
     def->label = "Pattern angle";
diff --git a/xs/src/libslic3r/PrintConfig.hpp b/xs/src/libslic3r/PrintConfig.hpp
index c14889c88..c8933bcd9 100644
--- a/xs/src/libslic3r/PrintConfig.hpp
+++ b/xs/src/libslic3r/PrintConfig.hpp
@@ -164,7 +164,7 @@ class PrintObjectConfig : public virtual StaticPrintConfig
     ConfigOptionBool                support_material_synchronize_layers;
     ConfigOptionInt                 support_material_threshold;
     ConfigOptionBool                support_material_with_sheath;
-    ConfigOptionFloat               support_material_xy_spacing;
+    ConfigOptionFloatOrPercent      support_material_xy_spacing;
     ConfigOptionFloat               xy_size_compensation;
     
     PrintObjectConfig(bool initialize = true) : StaticPrintConfig() {