diff --git a/region.cpp b/region.cpp
index 52ade9b4..d305dea2 100644
--- a/region.cpp
+++ b/region.cpp
@@ -23,6 +23,7 @@
 #include "enable_if.hpp"
 #include "debug.hpp"
 
+#include <cmath>
 #include <type_traits>
 
 using namespace util;
@@ -37,6 +38,13 @@ extent<T>::extent (const T  _width, const T  _height):
     { ; }
 
 
+template <typename T>
+T
+extent<T>::diameter (void) const {
+    return (T)sqrt (width * width + height * height);
+}
+
+
 template <typename T>
 T
 extent<T>::area (void) const
@@ -92,6 +100,13 @@ region<T>::area (void) const
     { return width * height; }
 
 
+template <typename T>
+T 
+region<T>::diameter (void) const {
+    return (T)sqrt (width * width + height * height);
+}
+
+
 template <typename T>
 bool 
 region<T>::empty (void) const
diff --git a/region.hpp b/region.hpp
index 3eb73984..a8e6f46a 100644
--- a/region.hpp
+++ b/region.hpp
@@ -34,6 +34,8 @@ namespace util {
         extent (const T  _width, const T  _height);
 
         T    area  (void) const;
+        T diameter (void) const;
+
         bool empty (void) const;
 
         bool operator ==(const extent& rhs) const;
@@ -54,8 +56,11 @@ namespace util {
 
         region (T _x, T _y, T _width, T _height);
 
-        T    area  (void) const;
+        T area     (void) const;
+        T diameter (void) const;
+
         bool empty (void) const;
+
         point centre (void) const;
 
         bool includes (const point&) const; // inclusive of borders