diff --git a/js/reveal.js b/js/reveal.js
index 65ac29ff..915f8bb5 100644
--- a/js/reveal.js
+++ b/js/reveal.js
@@ -1566,7 +1566,7 @@
 		};
 
 		// Reduce available space by margin
-		size.presentationWidth -= ( size.presentationHeight * config.margin );
+		size.presentationWidth -= ( size.presentationWidth * config.margin );
 		size.presentationHeight -= ( size.presentationHeight * config.margin );
 
 		// Slide width may be a percentage of available width