Save this profile, and start the game. This will start the game with a maximum of 2 GB of memory allocated.
I want more RAM, not only 2GB!
You can adapt the numbers to your needs:
-Xmx is the maximum of RAM wich Minecraft can allocate.
-Xms can be added to allocate RAM at the beginning, but is not required.
If you set it, -Xms must not be greater than -Xmx. It is also not recommended to set -Xms too high
Notice: You can't give Minecraft more RAM than you have in your system! Else, you will receive an error (most likely the same error that made you come here in the first place)!
32-bit system only supports up to 4GB RAM memory. Therefore, most 32-bit based systems only support a maximum of 2GB to be allocated to Java. The limit depends on the system, if you got an error, try less allocation.
If you are not using the native Java that comes with the launcher, be sure to use the latest and correct version of Java (64bit on 64bit-OS-Systems is recommended).
How much RAM do I have?
The easiest way to see how much RAM you have on Windows is to open the Task Manager and change to the Performance Tab. Look how much Physical Memory is available. On MacOS, go to the Apple Menu, select About This Mac. This window shows you your total amount of RAM.
Note that other programs and your system also need RAM to work. For example, if you run TeamSpeak or Firefox in background, you should leave enough memory for this software to run.
Some numbers for reference:
Windows 7: 1 GB RAM (32-Bit) or 2 GB RAM (64-Bit)
Windows 8 / 8.1: 1 GB RAM (32-Bit) or 2 GB RAM (64-Bit)
Windows 10: 2 GB RAM
Linux: 1-2 GB
Skype: 256 MB
Teamspeak: 64 MB
It is recommended to never allocate more than half your system RAM to the game. Also be aware that allocating more RAM does not necessarily make the game faster.
Something doesn't work? You got an error?
First of all check for spelling mistakes. Also important: JVM arguments are case-sensitive
Try to install the non-native version of Java (the download can be found here). If you go that, make sure to set your "Java Executable" in the launcher to the non-native javaw.exe (e.g. if you installed Java 1.8.0_181, set it to C:\Program Files\Java\jre1.8.0_181\bin\javaw.exe). Please note that Java 9 and above are not yet fully supported.
Add -Xms1G and try again
Lower the numbers if your issue still persists.
Check if you've enough free RAM, and make sure you use 64-bit when allocating more than 2GB or RAM.
You found a fault in the post? Something is missing? Please let me know on #minecrafthelp on Esper, or on the official Minecraft Discord.