Minecraft RAM calculator
Pick your PC's memory and what you run, and get a memory setting that leaves room for Windows.
How it works
This calculator uses common rules of thumb: about 2 to 3 GB for vanilla or a few mods, around 4 GB for Fabric with performance mods, 6 GB for large mod lists and 8 GB for very large modpacks. It adds 1 GB for shaders and 2 GB if you host a server on the same PC, then keeps the result at or below half of your total RAM so Windows and other programs have room. Treat the number as a starting point. Watch your frame rate, and if the game stutters or reports out-of-memory errors, change it in small steps.
Questions
Does more RAM make Minecraft faster?
Not always. Too little memory causes stutters and crashes, but giving the game far more than it needs leaves less for Windows and can make pauses longer.
Where do I set the memory?
In your launcher's memory setting. FireClient has a memory setting for Java, and the RAM guide shows where to find it.
What does -Xmx mean?
It is the Java argument that sets the maximum memory the game can use. -Xmx4G means 4 GB.