diff --git a/opm/core/utility/RegionMapping.hpp b/opm/core/utility/RegionMapping.hpp
index 9845919ff..151ad07b2 100644
--- a/opm/core/utility/RegionMapping.hpp
+++ b/opm/core/utility/RegionMapping.hpp
@@ -92,6 +92,11 @@ namespace Opm
              */
             typedef CellIter const_iterator;
 
+            /**
+             * Size type for this range.
+             */
+            typedef typename std::vector<CellId>::size_type size_type;
+
             /**
              * Beginning of cell range.
              */
@@ -102,6 +107,11 @@ namespace Opm
              */
             const_iterator end()   const { return e_; }
 
+            /**
+             * Number of elements in the range.
+             */
+            size_type size() const { return e_ - b_; }
+
         private:
             const_iterator b_;
             const_iterator e_;