Tool
How much RAM does your server need?
A quick estimate from the server type, how many people play at once, and how many mods or plugins you run. It also tells you how much to leave the machine.
Allocate to the server
3.5 GBof heap
Set -Xms and -Xmx both to 3.5G, and give the machine about 4.5 GBtotal so the operating system and the JVM’s own overhead have room. More heap than this is usually worse, not better: a larger heap makes garbage-collection pauses longer.
java -Xms3.5G -Xmx3.5G -jar server.jar nogui
This is an estimate. The biggest lever it cannot see is view distance: dropping it from 12 to 8 saves more than several gigabytes of allocation would.
Frequently asked
Does more RAM make the server faster?
Only up to the point where it stops running out. Past that, a larger heap makes garbage-collection pauses longer, so a server smooth on 6 GB can be worse on 16. Allocate enough, not the maximum.
Should Xms and Xmx be the same?
On a server, yes. Setting the starting and maximum heap equal stops the heap being resized while the game runs, which is a pause you can simply avoid.
Why does the estimate jump so much for modded?
Because mods load classes and world data before a single player joins, and each player loads far more with a heavy modpack. A hundred-mod pack uses several gigabytes at idle.
The full reasoning is in how much RAM a Minecraft server needs.