Maximum heap size for 32 bit or 64 bit JVM looks easy to determine by looking at addressable memory space like 2^32 (4GB) for 32 bit JVM and 2^64 for 64 bit JVM. The confusion starts here because you can not really set 4GB as the maximum heap size for 32 bit JVM using -Xmx JVM heap options. You will get could not create the Java virtual machine Invalid maximum heap size: -Xmx error. There could be much different reasons why maximum heap space for JVM is less than its theoretical limit and varies from one operating system to other e.g. different in Windows, Linux, and Solaris.