diff --git a/gprMax/gprMax.py b/gprMax/gprMax.py
index e29e09bd..e87912f0 100644
--- a/gprMax/gprMax.py
+++ b/gprMax/gprMax.py
@@ -257,7 +257,7 @@ def run_mpi_sim(args, inputfile, usernamespace, optparams=None):
     
     # Number of workers and command line flag to indicate a spawned worker
     worker = '--mpi-worker'
-    numberworkers = args.mpi
+    numberworkers = args.mpi - 1
 
     # Master process
     if worker not in sys.argv: