diff --git a/interp/tools.py b/interp/tools.py
index 4866f61..2a9a9b4 100644
--- a/interp/tools.py
+++ b/interp/tools.py
@@ -10,11 +10,15 @@ def rms(errors):
   """
     root mean square calculation
   """
-  r = 0.0
-  for i in errors:
-    r += np.power(i, 2)
-  r = np.sqrt(r / len(errors))
-  return r
+
+  # slow pure python way for reference:
+  # r = 0.0
+  # for i in errors:
+  #   r += np.power(i, 2)
+  # r = np.sqrt(r / len(errors))
+  # return r
+
+  return np.sqrt((errors**2).mean())
 
 def baker_exact_2D(X):
   """