Minecraft Server Chunk Pre-Generation Guide
If the server only lags when someone explores new terrain, this is the fix — generate it before they get there instead of while they're standing in it.
For lag that isn't tied to exploration, see the general lag fix guide or the TPS/tick-lag explainer instead.
Build-heavy, sprawling packs like Create: Above and Beyond are a good example of where pregeneration pays off most — players spread out fast once factory builds start sprawling.
The symptom: lag that follows explorers, not population
If your server runs smoothly with everyone in the same built-up area but dips hard the moment someone rides or flies into terrain nobody's visited yet, that's chunk generation lag, not a general performance problem. Generating a brand-new chunk — running worldgen, placing structures, populating biomes — is CPU-expensive, and it happens exactly once per chunk, synchronously, the first time anything needs it.
Why pre-generation fixes it
A chunk that already exists on disk loads near-instantly — it's a disk read, not a generation pass. Pre-generation means running that expensive generation step for a large radius *before* anyone plays, during a maintenance window or right after world creation, so every later visit to that area is a cheap load instead of an expensive generate.
Use Chunky — don't walk/fly the area manually
Chunky (available as both a Paper/Spigot plugin and a Fabric/Forge mod, same name, same author) is the standard tool. Manually flying around to "force" generation is slow, leaves gaps, and still lags the server live while you do it. Chunky runs generation in background-friendly batches and reports progress, and critically can run with the server otherwise idle so it doesn't compete with player-facing ticks.
Basic Chunky commands
After installing: `/chunky radius 5000` sets a 5000-block pregeneration radius from spawn (adjust to taste — see sizing below), `/chunky center 0 0` sets the center point if you want something other than world spawn, then `/chunky start` begins the run. `/chunky pause` and `/chunky continue` let you stop and resume across restarts — it's not an all-or-nothing operation.
Sizing the radius: balance against disk space
A 5000-block radius is roughly 625,000 chunks and can use tens of gigabytes depending on world type and version (Nether/End dimensions compress smaller than the Overworld). Don't pregenerate further than players will realistically reach in the pack's lifetime — oversized pregen just burns disk and install time for areas nobody visits. For a small survival SMP, 3000-4000 blocks from spawn covers most early-to-mid-game exploration; for a large public server, check your actual player-count-driven spread before committing to a huge radius.
Run it when nobody's playing, and expect it to take time
Pregeneration is still CPU work — running it with players online competes with them for the same ticks it's trying to save you from later. Run it during setup before launch, or during a scheduled low-traffic window. A multi-thousand-block radius can take anywhere from tens of minutes to several hours depending on your server's CPU and the world's complexity — this is expected, not a sign something's wrong.
Modded packs: pregenerate after confirming the pack boots cleanly
For heavily modded packs, pregenerate only after you've confirmed the modpack itself loads correctly and plays without crashing — modded worldgen (custom dimensions, structure mods, biome mods) is itself a common crash source, and you don't want to spend hours pregenerating against a world that then needs to be regenerated anyway after a worldgen-affecting mod config change.
FAQ
Does pre-generating chunks actually help, or is it placebo?+
It genuinely helps — generating a chunk (worldgen, structures, biome population) is measurably more expensive than loading an already-generated chunk from disk. Pregeneration front-loads that cost to a maintenance window instead of players experiencing it live as a lag spike the moment they explore somewhere new.
How big a radius should I pregenerate?+
Match it to how far players will realistically explore during the pack's lifetime — 3000-4000 blocks from spawn covers most small-to-mid survival servers. Going much larger mostly burns disk space and generation time on terrain nobody will visit; you can always run Chunky again later to extend the radius if players approach the edge.
Will pregeneration lag the server while it runs?+
Yes, it's real CPU work, so it competes with any players currently online. Run it before launch or during a scheduled downtime window rather than live during normal play, unless the server is otherwise idle.
How much disk space does pregenerating use?+
It varies by world type, dimension, and Minecraft version, but a 5000-block Overworld radius can run into the tens of gigabytes. Check your remaining disk space before starting a large run, and consider pregenerating the Overworld only first if space is tight — Nether and End dimensions are usually lower priority for most servers.
Can I pause and resume a Chunky pregeneration run?+
Yes — /chunky pause stops the current run, and /chunky continue resumes it from where it left off, including across server restarts. You don't need to run it in one uninterrupted session.
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