Index: src/core/qgsscalecalculator.cpp
===================================================================
--- src/core/qgsscalecalculator.cpp	(revision 9268)
+++ src/core/qgsscalecalculator.cpp	(working copy)
@@ -81,7 +81,7 @@
     QgsDebugMsg( "Can't calculate scale from the input values" );
     return 0;
   }
-  double scale = ( delta * conversionFactor ) / ( canvasWidth / mDpi );
+  double scale = ( delta * conversionFactor ) / ( (double) canvasWidth / (double) mDpi );
   return scale;
 }