diff --git a/rtengine/improcfun.cc b/rtengine/improcfun.cc
index 5aea884fb..3451bb0be 100644
--- a/rtengine/improcfun.cc
+++ b/rtengine/improcfun.cc
@@ -5740,10 +5740,11 @@ void ImProcFunctions::lab2rgb (const LabImage &src, Imagefloat &dst, const Glib:
 void ImProcFunctions::colorToningLabGrid(LabImage *lab, int xstart, int xend, int ystart, int yend, bool MultiThread)
 {
     const float factor = ColorToningParams::LABGRID_CORR_MAX * 3.f;
-    float a_scale = (params->colorToning.labgridAHigh - params->colorToning.labgridALow) / factor;
-    float a_base = params->colorToning.labgridALow;
-    float b_scale = (params->colorToning.labgridBHigh - params->colorToning.labgridBLow) / factor;
-    float b_base = params->colorToning.labgridBLow;
+    const float scaling = ColorToningParams::LABGRID_CORR_SCALE;
+    float a_scale = (params->colorToning.labgridAHigh - params->colorToning.labgridALow) / factor / scaling;
+    float a_base = params->colorToning.labgridALow / scaling;
+    float b_scale = (params->colorToning.labgridBHigh - params->colorToning.labgridBLow) / factor / scaling;
+    float b_base = params->colorToning.labgridBLow / scaling;
 
 #ifdef _OPENMP
     #pragma omp parallel for if (multiThread)
diff --git a/rtengine/procparams.cc b/rtengine/procparams.cc
index 36fa4aa22..344804f99 100644
--- a/rtengine/procparams.cc
+++ b/rtengine/procparams.cc
@@ -606,6 +606,7 @@ bool LocalContrastParams::operator!=(const LocalContrastParams &other) const
 
 
 const double ColorToningParams::LABGRID_CORR_MAX = 12000.f;
+const double ColorToningParams::LABGRID_CORR_SCALE = 3.f;
 
 ColorToningParams::ColorToningParams() :
     enabled(false),
@@ -4591,6 +4592,13 @@ int ProcParams::load(const Glib::ustring& fname, ParamsEdited* pedited)
             assignFromKeyfile(keyFile, "ColorToning", "LabGridBLow", pedited, colorToning.labgridBLow, pedited->colorToning.labgridBLow);
             assignFromKeyfile(keyFile, "ColorToning", "LabGridAHigh", pedited, colorToning.labgridAHigh, pedited->colorToning.labgridAHigh);
             assignFromKeyfile(keyFile, "ColorToning", "LabGridBHigh", pedited, colorToning.labgridBHigh, pedited->colorToning.labgridBHigh);
+            if (ppVersion < 337) {
+                const double scale = ColorToningParams::LABGRID_CORR_SCALE;
+                colorToning.labgridALow *= scale;
+                colorToning.labgridAHigh *= scale;
+                colorToning.labgridBLow *= scale;
+                colorToning.labgridBHigh *= scale;
+            }            
         }
 
         if (keyFile.has_group ("RAW")) {
diff --git a/rtengine/procparams.h b/rtengine/procparams.h
index 121fb8afa..54e8b2d56 100644
--- a/rtengine/procparams.h
+++ b/rtengine/procparams.h
@@ -454,6 +454,7 @@ struct ColorToningParams {
     double labgridAHigh;
     double labgridBHigh;
     static const double LABGRID_CORR_MAX;
+    static const double LABGRID_CORR_SCALE;
 
     ColorToningParams();
 
diff --git a/rtgui/ppversion.h b/rtgui/ppversion.h
index 1fb34ca0a..051dc6e2c 100644
--- a/rtgui/ppversion.h
+++ b/rtgui/ppversion.h
@@ -1,11 +1,13 @@
 #pragma once
 
 // This number has to be incremented whenever the PP3 file format is modified or the behaviour of a tool changes
-#define PPVERSION 336
+#define PPVERSION 337
 #define PPVERSION_AEXP 301 //value of PPVERSION when auto exposure algorithm was modified
 
 /*
   Log of version changes
+   337  2018-06-13
+        new scales for the LabGrid color toning parameters
    336  2018-06-01
         new demosaic method combobox for pixelshift
    335  2018-05-30