diff --git a/.travis.yml b/.travis.yml
index 8bcbf70e33..1f3d2d033f 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -117,10 +117,10 @@ install:
 
 script:
   - if [[ $TEST_TARGET == 'default' ]]; then
-      python -m iris.tests.runner --default-tests --system-tests --print-failed-images --num-processors=8;
+      python -m iris.tests.runner --default-tests --system-tests --print-failed-images --num-processors=3;
     fi
   - if [[ $TEST_TARGET == 'example' ]]; then
-      python -m iris.tests.runner --example-tests --print-failed-images --num-processors=8;
+      python -m iris.tests.runner --example-tests --print-failed-images --num-processors=3;
     fi
 
   # Capture install-dir: As a test command must be last for get Travis to check
diff --git a/lib/iris/tests/runner/_runner.py b/lib/iris/tests/runner/_runner.py
index c9406dd64c..340923f370 100644
--- a/lib/iris/tests/runner/_runner.py
+++ b/lib/iris/tests/runner/_runner.py
@@ -1,4 +1,4 @@
-# (C) British Crown Copyright 2010 - 2016, Met Office
+# (C) British Crown Copyright 2010 - 2017, Met Office
 #
 # This file is part of Iris.
 #
@@ -127,7 +127,9 @@ def finalize_options(self):
         if self.stop:
             print('Stopping tests after the first error or failure')
         if self.num_processors is None:
-            self.num_processors = multiprocessing.cpu_count() - 1
+            # Choose a magic number that works reasonably well for the default
+            # number of processes.
+            self.num_processors = (multiprocessing.cpu_count() + 1) // 4 + 1
         else:
             self.num_processors = int(self.num_processors)