diff --git a/flowtype.js b/flowtype.js
index 4a918b0..7829874 100644
--- a/flowtype.js
+++ b/flowtype.js
@@ -23,22 +23,30 @@
          maxFont   : 9999,
          minFont   : 1,
          fontRatio : 35,
-         lineRatio : 1.45
+         lineRatio : 1.45,
+         useEm: false
       }, options),
 
 // Do the magic math
 // =================
       changes = function(el) {
          var $el = $(el),
-            elw = $el.width(),
-            width = elw > settings.maximum ? settings.maximum : elw < settings.minimum ? settings.minimum : elw,
-            fontBase = width / settings.fontRatio,
-            fontSize = fontBase > settings.maxFont ? settings.maxFont : fontBase < settings.minFont ? settings.minFont : fontBase;
+            unit = 'px',
+            elw = $el.width(); 
 
-         $el.css({
-            'font-size'   : fontSize + 'px',
-            'line-height' : fontSize * settings.lineRatio + 'px'
-         });
+            if (settings.useEm) {
+               unit = 'em';
+               elw = elw / parseFloat($('body').css('font-size'));
+            }
+
+            var width = elw > settings.maximum ? settings.maximum : elw < settings.minimum ? settings.minimum : elw,
+               fontBase = width / settings.fontRatio,
+               fontSize = fontBase > settings.maxFont ? settings.maxFont : fontBase < settings.minFont ? settings.minFont : fontBase;
+
+            $el.css({
+               'font-size'   : fontSize + unit,
+               'line-height' : settings.useEm ? settings.lineRatio + unit : fontSize * settings.lineRatio + unit
+            });
       };
 
 // Make the magic visible