diff --git a/sample.cfg b/sample.cfg
index 06c4d50..34d5047 100644
--- a/sample.cfg
+++ b/sample.cfg
@@ -50,6 +50,7 @@ power_spectrum = planck15
 sim_name = mini-SURFS
 volume = 144703.125
 lbox = 210
+particle_mass = 2.21e8 # in Msun/h
 tot_n_subvolumes = 64
 min_snapshot = 51
 max_snapshot = 199
diff --git a/sample_lagos23.cfg b/sample_lagos23.cfg
index 3dfee36..03e90b2 100644
--- a/sample_lagos23.cfg
+++ b/sample_lagos23.cfg
@@ -65,6 +65,7 @@ spin_mass_dependence = true
 sim_name = mini-SURFS
 volume = 144703.125
 lbox = 210
+particle_mass = 2.21e8 # in Msun/h
 tot_n_subvolumes = 64
 min_snapshot = 51
 max_snapshot = 199