diff --git a/src/physical_constants.c b/src/physical_constants.c
index 3936d07f4207263a4c391715ab0a8dd9ded6fa6d..ed25ff1b2be09a04400f98476c78a537e6d8e2bf 100644
--- a/src/physical_constants.c
+++ b/src/physical_constants.c
@@ -126,6 +126,11 @@ void phys_const_init(const struct unit_system *us, struct swift_params *params,
   internal_const->const_T_CMB_0 =
       const_T_CMB_0_cgs /
       units_general_cgs_conversion_factor(us, dimension_temperature);
+
+  const float dimension_Yp[5] = {0, 0, 0, 0, 0}; /* [ - ] */
+  internal_const->const_primordial_He_fraction =
+      const_primordial_He_fraction_cgs /
+      units_general_cgs_conversion_factor(us, dimension_Yp);
 }
 
 /**
diff --git a/src/physical_constants.h b/src/physical_constants.h
index 16628bfd6894699608e167d4b309fa5636209219..a1b5e9c243c54bfd97179364170e8b3bbc36fe4f 100644
--- a/src/physical_constants.h
+++ b/src/physical_constants.h
@@ -93,6 +93,9 @@ struct phys_const {
 
   /*! Temperature of the CMB at present day */
   double const_T_CMB_0;
+
+  /*! Primordial Helium fraction */
+  double const_primordial_He_fraction;
 };
 
 void phys_const_init(const struct unit_system* us, struct swift_params* params,
diff --git a/src/physical_constants_cgs.h b/src/physical_constants_cgs.h
index 40eef2c992e819e01980cbcbd7ea7f05721e93cf..59e36e288e70dabecbdccd86891853b1daa0c5bd 100644
--- a/src/physical_constants_cgs.h
+++ b/src/physical_constants_cgs.h
@@ -95,4 +95,7 @@ const double const_earth_mass_cgs = 5.9724e27;
 /*! Temperature of the CMB at present day [K] */
 const double const_T_CMB_0_cgs = 2.7255;
 
+/*! Primordial Helium fraction */
+const double const_primordial_He_fraction_cgs = 0.245;
+
 #endif /* SWIFT_PHYSICAL_CONSTANTS_CGS_H */