diff --git a/lib/less/functions.js b/lib/less/functions.js
index f93a5c4b..b83361c3 100644
--- a/lib/less/functions.js
+++ b/lib/less/functions.js
@@ -187,6 +187,18 @@ tree.functions = {
     'is-keyword': function (n) {
         return this._isa(n, tree.Keyword);
     },
+    'is-url': function (n) {
+        return this._isa(n, tree.URL);
+    },
+    'is-pixel': function (n) {
+        return (n instanceof tree.Dimension) && n.unit === 'px' ? tree.True : tree.False;
+    },
+    'is-percentage': function (n) {
+        return (n instanceof tree.Dimension) && n.unit === '%' ? tree.True : tree.False;
+    },
+    'is-em': function (n) {
+        return (n instanceof tree.Dimension) && n.unit === 'em' ? tree.True : tree.False;
+    },
     _isa: function (n, Type) {
         return (n instanceof Type) ? tree.True : tree.False;
     }
diff --git a/test/css/functions.css b/test/css/functions.css
index f823a992..82328145 100644
--- a/test/css/functions.css
+++ b/test/css/functions.css
@@ -34,6 +34,9 @@
   keyword: true;
   number: true;
   string: true;
+  pixel: true;
+  percent: true;
+  em: true;
 }
 #alpha {
   alpha: rgba(153, 94, 51, 0.6);
diff --git a/test/less/functions.less b/test/less/functions.less
index 44f4242c..dcbec22f 100644
--- a/test/less/functions.less
+++ b/test/less/functions.less
@@ -38,6 +38,9 @@
     keyword: is-keyword(hello);
     number: is-number(32);
     string: is-string("hello");
+    pixel: is-pixel(32px);
+    percent: is-percentage(32%);
+    em: is-em(32em);
   }
 }