diff --git a/autoninja.py b/autoninja.py
index 2ba7c864aa..0e5f5a3797 100755
--- a/autoninja.py
+++ b/autoninja.py
@@ -15,6 +15,7 @@ from __future__ import print_function
 
 import multiprocessing
 import os
+import platform
 import re
 import subprocess
 import sys
@@ -167,17 +168,21 @@ def main(args):
   if not j_specified and not t_specified:
     if use_goma or use_remoteexec:
       args.append('-j')
-      core_multiplier = int(os.environ.get('NINJA_CORE_MULTIPLIER', '40'))
+      default_core_multiplier = 80
+      if platform.machine() in ('x86_64', 'AMD64'):
+        # Assume simultaneous multithreading and therefore half as many cores as
+        # logical processors.
+        num_cores //= 2
+
+      core_multiplier = int(
+          os.environ.get('NINJA_CORE_MULTIPLIER', default_core_multiplier))
+
       j_value = num_cores * core_multiplier
 
       if sys.platform.startswith('win'):
         # On windows, j value higher than 1000 does not improve build
         # performance.
         j_value = min(j_value, 1000)
-      elif sys.platform == 'darwin':
-        # On Mac, j value higher than 500 causes 'Too many open files' error
-        # (crbug.com/936864).
-        j_value = min(j_value, 500)
 
       args.append('%d' % j_value)
     else: