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08/04/2026

How to run a Minecraft modpack server (ATM11, 2026)

How to run a Minecraft modpack server (ATM11, 2026)

A Minecraft modpack server is not a vanilla server with a few mods dropped in. You download the modpack's server pack, run the Java version its Minecraft version demands, and give it serious RAM — and those are exactly the three things people get wrong. Here's how I set up a modpack server, with numbers that hold as of August 2026.

Which modpack — and what it costs you in RAM

Pick the pack first, because the pack dictates everything else: loader, Java and RAM. These five cover almost everything people ask for:

  • All the Mods 10 (ATM10) — Minecraft 1.21.1, NeoForge, Java 21. The classic kitchen-sink pack, around 500 mods and a quest line running all the way to the ATM Star. The official server guidance is 10 GB for 2–5 players, 12–14 GB for 5–15, and 16 GB upward beyond that. This is the pack I recommend when someone goes modded for the first time — it's stable, and every problem you'll hit already has an answer somewhere.
  • All the Mods 11 (ATM11) — Minecraft 26.1.2, NeoForge, Java 25. The successor to ATM10. But watch out: the newest build at the time of writing is 0.4.0-beta, dated 3 August 2026. It's still beta and it behaves like it. If the server is for a group that plays every night, stay on ATM10 until ATM11 leaves beta.
  • Better MC BMC5 (NeoForge) — Minecraft 1.21.1, NeoForge, Java 21. A vanilla+ pack: new biomes, structures, dungeons and mobs, but the game still looks and feels like Minecraft. Far lighter than ATM — 6–8 GB is plenty. This is my pick for a group that wants "the same Minecraft, just richer".
  • Cobblemon Official Modpack (Fabric) — Minecraft 1.21.1, Fabric, Java 21. Pokémon in Minecraft, version 1.7.3 from February 2026. The lightest pack on this list — 4–6 GB carries a small group without trouble. If you play with kids or younger siblings, this is the one they'll ask for.
  • Prominence II: Hasturian Era — Minecraft 1.20.1, Fabric, Java 17. The RPG pack with the cleanest progression of anything here; latest is v3.9.27 from May 2026, with a matching server pack. The only one on this list still on 1.20.1 — which also means older Java, more on that next. Budget 6–8 GB.

Java follows the modpack, not the newest Minecraft

This is mistake number one and I see it constantly. You install the newest Java because it's newest, the server won't boot, and you go hunting for a broken mod. You don't pick Java by what's new — you pick it by which Minecraft version your pack runs:

  • Minecraft 1.20.1 (Prominence II) — Java 17.
  • Minecraft 1.21.1 (ATM10, Better MC, Cobblemon) — Java 21. ATM lists this as mandatory, not a suggestion.
  • Minecraft 26.1 and newer (ATM11) — Java 25.

If your server dies immediately on startup with a message about "class file version", that's this — wrong Java, not a broken mod. I wrote up why Minecraft suddenly numbers versions by year, and why 26.1 wants Java 25, in a separate post.

Practical consequence: with only one Java on the machine you can't run Prominence II and ATM11 side by side. I keep several Java versions installed and point each server at its own.

Upload the server pack — never your client folder

Every serious modpack has two downloads: the pack itself, which goes into your launcher, and the server pack, a separate file under Files or Additional Files on the pack's page. Only the server pack belongs on the server.

The difference isn't cosmetic. The client pack ships mods that have no business on a server and will take it down on startup — shaders, Sodium or Embeddium, ReplayMod, anything that draws pixels. The server pack simply doesn't contain them. Upload your .minecraft instance folder and the server usually never reaches "Done".

Modrinth packs come as a .mrpack file. For those use mrpack-install — it pulls the loader and every mod onto the server for you, no manual unpacking.

And if a pack ships no server pack at all — it happens with smaller packs — you have to strip the client-only mods out by hand. Doable, but tedious, and you repeat it on every pack update. That alone is enough for me to skip such a pack when I'm choosing for someone else.

First boot, step by step

  • Extract the server pack into an empty folder. No spaces and no accented characters in the path — Java will thank you.
  • Run the start script: startserver.sh on ATM packs, run.sh on a plain NeoForge server. The first run stops at the EULA — open eula.txt and set eula=true.
  • RAM does not live in the start script. On NeoForge it's in user_jvm_args.txt; on ATM packs it's in variables.txt. This is what people hunt for longest, because opening the script is the obvious first move.
  • First boot takes several minutes — every mod is writing its configs. Let it run all the way to "Done".
  • Stop it and start it once more before letting anyone in. Only after that first boot do the configs you want to edit actually exist.

How much RAM — and why "give it everything" is wrong

The numbers above are real, not marketing: light packs 4–8 GB, ATM class 10–16 GB depending on player count. A modpack server eats far more than vanilla, and not because of players — it's the sheer number of mods loading and the chunks being generated with extra world gen on top.

But the myth that more RAM is always better costs people performance. Two things:

  • Leave the operating system room. If the machine has 8 GB, give the server about 6.5 GB, not 8. That's Paper's own guidance — 1 to 1.5 GB belongs to the OS and everything around it. Hand over all of it and stability is the first thing to go.
  • A big heap means longer GC pauses. Java has to clean up memory periodically, and the bigger the heap, the longer that takes. 32 GB doesn't mean smoother play — it means rarer but more noticeable stalls when the flags aren't tuned.

Realistically: give the pack what it asks for plus a little headroom, and spend the rest on a faster CPU. Modded Minecraft is still largely single-threaded, and your TPS will break at the CPU long before RAM is the problem.

JVM flags for modded

Aikar's flags are still the standard starting point and work with Vanilla, Paper, Purpur and Forge/NeoForge alike. Two things worth taking from them:

  • Set -Xms and -Xmx to the same value. Don't let Java grow and shrink the heap; reserve what you need up front.
  • For heavy modpacks, raise the young generation. Mods allocate a lot of medium-lived objects — tile entities, mod caches, custom world gen — so -XX:G1NewSizePercent=40 and -XX:G1MaxNewSizePercent=50 are commonly recommended over the default 30 and 40.

And to stay honest: that second one is tuning, not law. Before copying someone else's flags, put Spark on the server and look at where the time actually goes. I've fixed more servers by lowering view distance than by any flag.

What to skip

  • Don't run a modpack on 4 GB "because my client runs on 4". The client draws the chunks around you; the server holds the whole world, every base and every chunk loaded by every player.
  • Don't mix loaders. A Forge mod won't run on a NeoForge server or the other way round, however similar the names look. If the pack says NeoForge, everything you add has to be NeoForge.
  • Leave OptiFine and shaders off the server. They're client-side and do nothing but harm server-side.
  • Don't update a pack on a live world without a backup. An update can drop a mod whose blocks are already placed in your world — and then half your base is gone. Back up before every update, no exceptions.

Where to run all this

All of the above is free and you can set it up yourself — but a modpack server is exactly where free hosts fall over, because nobody hands out 10–16 GB and a decent CPU for nothing.

On mcbalkan you pick the modpack straight from the panel: search CurseForge and Modrinth, click the pack, and I set it up — no server pack downloads, no juggling Java versions. The machines sit in Germany, you get full access to mods and files, and when something breaks you talk directly to me — no ticket queue, no bot. See the Minecraft hosting.