diff --git a/timestring.js b/timestring.js
index 750a6ab..ea11302 100644
--- a/timestring.js
+++ b/timestring.js
@@ -75,6 +75,7 @@
 
         // split string into groups and get total seconds for each group
         var groups = string
+                        .toLowerCase() // convert words to lower case
                         .replace(/[^\w+-]+/g, '') // remove white space
                         .match(/[-+]?[0-9]+[a-z]+/g); // match time groups (digit followed by time unit - i.e 5d 15m = 2 time groups)