diff --git a/Source/Plugins/BuilderModes/ClassicModes/DrawGridMode.cs b/Source/Plugins/BuilderModes/ClassicModes/DrawGridMode.cs
index da4a4300aef2ff126cd107340e15d9ed61b7c702..8f3388067590184241d56ae46be2c5b1aa4850e7 100755
--- a/Source/Plugins/BuilderModes/ClassicModes/DrawGridMode.cs
+++ b/Source/Plugins/BuilderModes/ClassicModes/DrawGridMode.cs
@@ -360,12 +360,12 @@ namespace CodeImp.DoomBuilder.BuilderModes
 
 				case GridLockMode.VERTICAL:
 					slicesH = horizontalslices;
-					slicesV = height / General.Map.Grid.GridSize;
+					slicesV = Math.Abs(height / General.Map.Grid.GridSize);
 					break;
 
 				case GridLockMode.BOTH:
-					slicesH = width / General.Map.Grid.GridSize;
-					slicesV = height / General.Map.Grid.GridSize;
+					slicesH = Math.Abs(width / General.Map.Grid.GridSize);
+					slicesV = Math.Abs(height / General.Map.Grid.GridSize);
 					break;
 			}