How to increase the memory heap size on IntelliJ IDEA?

I want to allocate around 1GB of heap size, but I can't seem to figure it out.

How to do this?

Use Help | Edit Custom VM Options…

An editor will open automatically for the right .vmoptions file, adjust the value of -Xmx, save and restart IntelliJ IDEA:

Check these documents from IntelliJ IDEA knowledge base for more details:

• Configuring JVM options and platform properties
• The JVM could not be started. The main method may have thrown an exception.

Answers below suggest to edit .vmoptions file directly inside the application installation directory. Please note that it's not recommended since it will cause conflicts during patch updates. The method above creates a copy of the file in the CONFIG directory and your IDE installation remains intact.

Also be aware of the 32-bit address space limit on Windows which makes it hard to use heap sizes higher than 750m. Should you need to use larger heap, make sure to switch to the 64-bit JVM first, otherwise IDE may crash on start or start to crash randomly during work.