diff --git a/AppKit/CPCollectionView.j b/AppKit/CPCollectionView.j
index 2b6505682..0bd92dc1c 100644
--- a/AppKit/CPCollectionView.j
+++ b/AppKit/CPCollectionView.j
@@ -623,7 +623,7 @@ var HORIZONTAL_MARGIN = 2;
 
     height = MAX(height, numberOfRows * (_minItemSize.height + _verticalMargin));
 
-    var itemSizeHeight = itemSize.height;
+    var itemSizeHeight = FLOOR(height / numberOfRows) - _verticalMargin;
 
     if (maxItemSizeHeight > 0)
         itemSizeHeight = MIN(itemSizeHeight, maxItemSizeHeight);