diff --git a/src/bespoke-scale.js b/src/bespoke-scale.js
index d0fa78a..1d054dc 100644
--- a/src/bespoke-scale.js
+++ b/src/bespoke-scale.js
@@ -33,6 +33,8 @@
         },
 
       scaleAll = function() {
+        slideHeight = firstSlide.offsetHeight;
+        slideWidth = firstSlide.offsetWidth;
         var xScale = parent.offsetWidth / slideWidth,
           yScale = parent.offsetHeight / slideHeight;