diff --git a/algorithms/distance/rmsdiff.py b/algorithms/distance/rmsdiff.py
new file mode 100644
index 0000000..c30a1a8
--- /dev/null
+++ b/algorithms/distance/rmsdiff.py
@@ -0,0 +1,14 @@
+from PIL import ImageChops
+import math
+import operator
+import functools
+
+def rmsdiff(im1, im2):
+    "Calculate the root-mean-square difference between two images"
+
+    h = ImageChops.difference(im1, im2).histogram()
+
+    # calculate rms
+    return math.sqrt(functools.reduce(operator.add,
+        map(lambda h, i: h*(i**2), h, range(256))
+    ) / (float(im1.size[0]) * im1.size[1]))