diff --git a/sympyextensions.py b/sympyextensions.py
index ad52a6ca1b5c545a769be10d93905a54dcada36d..261438c641cb8fb8fd182bf32fd96763d66cd4ab 100644
--- a/sympyextensions.py
+++ b/sympyextensions.py
@@ -326,3 +326,7 @@ def matrixFromColumnVectors(columnVectors):
     c = columnVectors
     return sp.Matrix([list(c[i]) for i in range(len(c))]).transpose()
 
+
+def commonDenominator(expr):
+    denominators = [r.q for r in expr.atoms(sp.Rational)]
+    return sp.lcm(denominators)