diff --git a/src/EOS.hpp b/src/EOS.hpp
index 54e71d39b..952d12eca 100644
--- a/src/EOS.hpp
+++ b/src/EOS.hpp
@@ -27,7 +27,7 @@
 namespace quokka
 {
 static constexpr double boltzmann_constant_cgs = C::k_B;   // cgs
-static constexpr double hydrogen_mass_cgs = 1.6726231e-24; // cgs
+static constexpr double hydrogen_mass_cgs = C::m_p + C::m_e; // cgs
 
 // specify default values for ideal gamma-law EOS
 //