diff --git a/src/Mod/Fem/femsolver/elmer/writer.py b/src/Mod/Fem/femsolver/elmer/writer.py
index 8fee32f351..e2077a3b81 100644
--- a/src/Mod/Fem/femsolver/elmer/writer.py
+++ b/src/Mod/Fem/femsolver/elmer/writer.py
@@ -147,7 +147,7 @@ class Writer(object):
             )
             self.unit_system = {
                 "L": "m",
-                "M": "t",
+                "M": "kg",
                 "T": "s",
                 "I": "A",
                 "O": "K",