diff --git a/include/Rivet/Math/MathHeader.hh b/include/Rivet/Math/MathHeader.hh
--- a/include/Rivet/Math/MathHeader.hh
+++ b/include/Rivet/Math/MathHeader.hh
@@ -1,45 +1,45 @@
 #ifndef RIVET_Math_MathHeader
 #define RIVET_Math_MathHeader
 
 #include "Rivet/Tools/Exceptions.hh"
 #include "Rivet/Tools/Utils.hh"
 #include <cmath>
 
 namespace Rivet {
 
 
   /// Pre-defined numeric type limits
   /// @deprecated Prefer the standard DBL/INT_MAX
   static const double MAXDOUBLE = DBL_MAX; // was std::numeric_limits<double>::max(); -- warns in GCC5
   static const double MAXINT = INT_MAX; // was std::numeric_limits<int>::max(); -- warns in GCC5
 
   /// A pre-defined value of \f$ \pi \f$.
   static const double PI = M_PI;
 
   /// A pre-defined value of \f$ 2\pi \f$.
   static const double TWOPI = 2*M_PI;
 
   /// A pre-defined value of \f$ \pi/2 \f$.
   static const double HALFPI = M_PI_2;
 
   /// A pre-defined value of \f$ \sqrt{2} \f$.
   static const double SQRT2 = M_SQRT2;
 
   /// A pre-defined value of \f$ \sqrt{\pi} \f$.
-  static constexpr double SQRTPI = sqrt(M_PI);
+  static const double SQRTPI = 2 / M_2_SQRTPI;
 
-  /// A pre-defined value of \f$ \sqrt{2\pi} \f$.
-  static constexpr double SQRT2PI = sqrt(2*M_PI);
+  // /// A pre-defined value of \f$ \sqrt{2\pi} \f$.
+  // static const double SQRT2PI = SQRT2 * SQRTPI;
 
   /// Enum for signs of numbers.
   enum Sign { MINUS = -1, ZERO = 0, PLUS = 1 };
 
   /// Enum for rapidity variable to be used in calculating \f$ R \f$, applying rapidity cuts, etc.
   enum RapScheme { PSEUDORAPIDITY = 0, ETARAP = 0, RAPIDITY = 1, YRAP = 1 };
 
   /// Enum for range of \f$ \phi \f$ to be mapped into
   enum PhiMapping { MINUSPI_PLUSPI, ZERO_2PI, ZERO_PI };
 
 }
 
 #endif