Mob Caps and Chunk Loading Explained

Your sky-platform mob grinder sits empty while the spawner grinder 60 blocks away pours out XP, and neither of them is broken. Minecraft keeps only a limited number of hostile mobs loaded around you, and almost every one that exists inside that radius — spawner mobs, cave mobs, the creeper on a ledge you never lit — spends part of the same budget. Learn the cap, the sphere it applies in, and which chunks are actually running, and you can diagnose a dead farm in two minutes.

One cap, shared by every hostile mob

Every mob belongs to a spawning category, and each category gets its own separate budget:

• Monsters — zombies, skeletons, creepers, spiders, endermen, witches, drowned, husks, slimes, magma cubes, zombified piglins. All of them draw on one shared number.
• Creatures (passive animals), ambient (bats), water creatures (squid, dolphins), water ambient (fish), underground water creatures and axolotls each have their own, much smaller budgets.

That split is why a squid farm or a crop farm never slows a mob grinder, and why village builds are harmless too: villagers sit in the creature category and iron golems are in no spawning category at all, so neither one can touch your hostile budget. Counts are kept per dimension, so Nether mobs do not touch the Overworld's number — though that rarely rescues you, since leaving a dimension stops its chunks ticking anyway.

What does eat your hostile budget:

• Mobs already sitting in your farm. A grinder holding forty zombies in a collection cell has spent most of the budget on mobs that are not being killed. A kill chamber that stalls is the most common self-inflicted farm failure there is.
• Spawner mobs. A spawner ignores the cap when deciding whether to fire — it only checks that six of its own mob are not already nearby and that a player is within 16 blocks — but the mobs it produces absolutely count. Run a zombie spawner grinder beside a dark room farm and the spawner wins.
• Mobs stranded in chunks that are loaded but not ticking. The count covers every non-persistent mob in the dimension, including ones sitting in chunks that are no longer eligible for spawning. Frozen mobs cannot despawn, so they hold their slots until something loads them again.
• Everything in the caves around you, which gets its own section below.

And two things that do not count, despite what plenty of tutorials say:

• Persistent mobs. A mob that has been given a name tag, has picked up an item, or is riding another entity never despawns — but Java skips exactly those mobs when it totals the cap. They still block spawn spaces and clog your build; they do not spend budget.
• Raiders in an active raid. They spawn outside the normal rules, they refuse to despawn while the raid is running, and they are left out of the count for as long as they belong to it. Once a raid ends, ordinary rules resume.

How big the cap actually is

For a single player, the hostile cap is 70 mobs. The global number scales with how many chunks are close enough to a player to be counted:

The 289 is 17 x 17 chunks: eight chunks in every direction plus the one you stand in, the area one player contributes. A lone player is therefore always 289 out of 289 — a flat 70 — and no video setting changes that.

Multiplayer does not simply multiply the number either. Chunks are counted once, so two players AFKing at the same farm barely raise the cap; two players 500 blocks apart each bring a fresh area, and both count. There is a per-player share as well: each non-persistent mob in a chunk whose centre is within 128 blocks horizontally of you counts against your own allowance, so one player cannot quietly hoard the whole server's budget.

The myth worth killing here is that a low simulation distance shrinks that 70. It does not — the counted area is fixed at eight chunks around each player. What a low simulation distance shrinks is the region that actually runs, which hurts in two nastier ways:

• It clips the spawning shell. Mobs only spawn in chunks that are ticking. At simulation distance 4 that is roughly 64 blocks around you instead of 128, so most of the volume your farm was counting on never gets a spawn attempt at all.
• It strands mobs. Anything that ends up outside the ticking area freezes: it cannot move, cannot be killed by your grinder and cannot despawn, yet it still counts. The cap fills with statues.

The spawn sphere: nothing inside 24, nothing beyond 128

Two distances decide where mobs can usefully exist:

• 24 blocks. No mob spawns within 24 blocks of you, measured as a straight line through the air. Straight up counts exactly like sideways, which is why AFK platforms sit at least 24 blocks above the spawning floor.
• 128 blocks. Beyond 128 blocks from the nearest player, hostile mobs despawn immediately. Between 32 and 128 they carry a small rolling chance to despawn each tick. Inside 32 they stay put.

The productive region is therefore a shell: a sphere of radius 128 with a 24-block hole punched in the middle, centred on you and moving with you — and clipped inwards if the simulation distance is under 8. One more rule catches base builders: nothing spawns naturally within 24 blocks of the world spawn point either.

Selection works chunk column by chunk column. The game picks eligible columns by horizontal distance, then a random height inside them, then applies the 24-block rule and checks the spot is a legal spawn space. Height never filters which columns are chosen, only the distance checks afterwards — which is why a cave system 90 blocks straight down competes with your farm even though you have never seen it.

Use that. If your dark room is at y=200 and you AFK 30 blocks above it, the cave-riddled ground at y=50 is more than 128 blocks below you, so anything that does spawn down there despawns on the spot instead of parking in your budget. That is the whole reason mob grinders are floating boxes over an ocean rather than something tucked in behind the base.

Simulation distance is the one that matters

These two settings get confused constantly, and only one of them affects farms.

Render distance controls how many chunks are drawn for you. It is a client-side visual setting, sliding from 2 to 32 chunks, capped by a server's view-distance. It has no effect on spawning, on the cap, or on whether anything ticks — turning it down is free performance, and boosting FPS covers the rest of that list.

Simulation distance controls how many chunk columns around you are ticked: mobs move and spawn, redstone fires, hoppers move items, furnaces smelt, crops grow, water flows. In singleplayer it is a slider with a minimum of 5 chunks. On a server it is the simulation-distance line in server.properties, defaulting to 10, and the server's value always wins — you cannot raise it from the client.

Two things follow:

• Raising simulation distance above 8 does nothing for spawn rates. Spawning only ever considers chunks within eight chunks of a player, so past that you pay for extra ticking and get no extra mobs.
• Lowering it below 8 costs you spawning volume, not cap. The 70 stays; the shell it has to fill gets smaller, and mobs that drift out of the ticking area jam in place while still counting.

Simulation distance is measured horizontally, in whole chunk columns; height is irrelevant to it. AFKing 150 blocks directly above your farm keeps the redstone ticking perfectly — but a mob farm that far below you is outside the 128-block sphere, so it will still hand you nothing. Walking 150 blocks sideways stops everything dead.

Which chunks are actually loaded

Four tiers, one of which history has since closed:

1. 1.Ticking chunks — columns within simulation distance of a player. Everything runs here, and this is the only tier a farm cares about.
2. 2.Loaded but not ticking — columns between simulation distance and render distance. You see the terrain and nothing in it is happening. Mobs left in this ring are frozen solid: they cannot move, cannot despawn, and still count against the hostile cap.
3. 3.Force-loaded chunks — columns pinned with the /forceload add command, which needs cheats switched on (operator permission on a server) and keeps them ticking with no player nearby.
4. 4.Spawn chunks — an always-loaded patch around world spawn, which existed up to Java 1.21.8 and is covered below.

Everything else is unloaded, which means paused rather than deleted. Mobs, dropped items, hopper contents and furnace progress are saved with the chunk and resume where they left off, and item despawn timers freeze too. It is also why a farm 400 blocks from your AFK spot yields nothing. It is not running slowly. It is not running.

What the spawn chunks did, and where they went

For most of Minecraft's history a patch of the Overworld around the world spawn point stayed loaded and ticking even with no player in the dimension. Two changes have all but erased that:

• 1.20.5 added the spawnChunkRadius game rule and dropped the default from 10 to 2, leaving roughly a 3 x 3 chunk patch ticking instead of the enormous area older tutorials describe.
• 1.21.9 removed spawn chunks and that game rule outright. On any version from 1.21.9 onward there is no free always-loaded patch at all.

If you are on 1.21.8 or earlier, the rule still buys you the classic trick: redstone clocks keep ticking, furnace arrays keep smelting, hoppers and item sorters keep moving items, and crops keep growing while you are off in the Nether. Set it to 0 to switch spawn chunks off, or raise it for a bigger always-on area. If you are on anything newer, /forceload is the replacement, and it needs cheats or operator permission.

What neither ever buys you is mobs. Spawning needs a player in range, so a mob farm parked at world spawn produces exactly nothing while you are away, and the 24-block rule around world spawn makes it worse. The redstone half of the build runs; the half that needs mobs does not.

Why lighting nearby caves helps so much

Caves hurt a farm in two separate ways, and only understanding both explains the size of the effect.

They hold cap slots. Every zombie standing in a dark cavern inside your 128-block sphere is a slot your farm wanted. A properly cave-riddled area can hold the cap full on its own, at which point your farm spawns nothing whatsoever.

They steal spawn attempts. Even well below the cap, the game runs spawn attempts across the eligible chunks and picks random positions inside them. Every valid dark block in range is a candidate, so your farm's share of the output is roughly its share of the legal spawn spaces in range. One platform against thousands of cave blocks is a rounding error.

The two fixes you always see recommended should now make sense:

• Build where there is no competition — high in the sky, or over deep ocean. Nothing else in range can spawn, so the whole budget is yours.
• Light or slab the caves in range if you must build near terrain. Hostile mobs need a block light level of exactly 0 in current Java Edition, so a single torch shuts down a large area. The full rules are in how mob spawning works, and the practical version in mob-proofing your base.

Two warnings. Lighting a 128-block sphere by hand is an enormous job, which is why sky and ocean builds win on effort alone. And light does not save you everywhere: Nether mobs such as zombified piglins spawn regardless of light level, so a gold farm is protected by covering or removing nearby spawnable surfaces, never by torching them. The End is the friendly extreme — an enderman farm over the void has no competition at all.

Fixing a farm that produces nothing

Work down this list in order:

1. 1.Is the farm inside simulation distance of where you stand, measured horizontally? If not, nothing else matters.
2. 2.Are you at least 24 blocks from the spawning surface, and is it within 128 blocks of you?
3. 3.Is another hostile farm — spawner grinder, raid farm, second dark room — running inside the same sphere? Separate hostile farms by more than 128 blocks and AFK at one of them.
4. 4.Is your kill chamber actually killing? Mobs stalled on a ledge or backed up in a holding cell hold the cap open indefinitely.
5. 5.Are there unlit caves within 128 blocks, including straight down?
6. 6.On a server, what is the simulation distance? Below 8 it clips your spawning shell and can freeze mobs into the cap, and no amount of rebuilding fixes a setting.

Nine failures out of ten are number three or number five, and both come back to one sentence: nearly everything hostile inside that sphere is spending your budget.