diff --git a/Elwig/Helpers/Validator.cs b/Elwig/Helpers/Validator.cs
index 1ea8042..599e7e9 100644
--- a/Elwig/Helpers/Validator.cs
+++ b/Elwig/Helpers/Validator.cs
@@ -246,9 +246,16 @@ namespace Elwig.Helpers {
                 return new(false, "Betriebsnummer zu kurz");
             }
 
-            // TODO
+            // https://statistik.at/fileadmin/shared/QM/Standarddokumentationen/RW/std_r_land-forstw_register.pdf#page=41
+            int s = 0;
+            for (int i = 0; i < 6; i++)
+                s += (input.Text[i] - '0') * (7 - i);
+            s = (11 - (s % 11)) % 10;
 
-            return new(true, "Not implemented yet");
+            if (s != (input.Text[6] - '0'))
+                return new(false, "Prüfsumme der Betriebsnummer ist falsch");
+
+            return new(true, null);
         }
 
         public static ValidationResult CheckUstId(TextBox input, bool required) {