diff --git a/shell/android/custom_rules.xml b/shell/android/custom_rules.xml
index 26c710374..27f8fc668 100644
--- a/shell/android/custom_rules.xml
+++ b/shell/android/custom_rules.xml
@@ -1,12 +1,18 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <project name="reicast-touchpad" default="help">
-    <property name="ndkbuildopt" value=""/>
+    <property environment="env" />
+    <condition property="ndkbuildopt" value="-j${env.NUMBER_OF_PROCESSORS}" else="-j4">
+        <isset property="env.NUMBER_OF_PROCESSORS" />
+    </condition>
+    <condition property="ndkbuildopt" value="${env.NDKBUILDOPT}">
+        <isset property="env.NDKBUILDOPT" />
+    </condition>
     <target name="-pre-build">
         <exec executable="${ndk.dir}/ndk-build.cmd" failonerror="true" osfamily="windows">
-            <arg line="-j8 ${ndkbuildopt}"/>
+            <arg line="${ndkbuildopt}"/>
         </exec>
         <exec executable="${ndk.dir}/ndk-build" failonerror="true" osfamily="unix">
-            <arg line="-j8 ${ndkbuildopt}"/>
+            <arg line="${ndkbuildopt}"/>
         </exec>
     </target>
     <target name="clean" depends="android_rules.clean">