Slime Chunks Explained
Slime chunks are the 1-in-10 chunks where slimes spawn underground. Here is exactly how the game decides which chunks qualify on Java and Bedrock, how slimes spawn, and a checker for any chunk.
How it works
Java Edition: seed-dependent
For each chunk, Java seeds a fresh random generator from the world seed and the chunk coordinates, then checks whether one draw out of ten comes up zero:
Random rnd = new Random(
seed
+ (int)(chunkX * chunkX * 0x4c1906)
+ (int)(chunkX * 0x5ac0db)
+ (int)(chunkZ * chunkZ) * 0x4307a7L
+ (int)(chunkZ * 0x5f24f)
^ 0x3ad8025fL
);
boolean slimeChunk = rnd.nextInt(10) == 0;The result depends only on the seed and the chunk position, so a seed map can list every slime chunk instantly — and changing the seed changes all of them. The checker on this page runs this exact formula (verified against the cubiomes library used by the seed map).
Bedrock Edition: seed-independent
Bedrock seeds a 32-bit Mersenne Twister (MT19937) with (chunkX × 0x1f1f1f1f) XOR chunkZ and tests whether the first output is divisible by 10. The world seed never enters the calculation, which is why every Bedrock world shares the same slime chunks and why Java slime finders give wrong answers for Bedrock.
Slime spawning rules
- Slime chunks: below Y 40, at any light level (Java), on any difficulty except Peaceful. On Java each spawn attempt additionally passes only 1 time in 10, so farms need many spawnable blocks and other mobs cleared from the mob cap.
- Swamps and mangrove swamps: between Y 51 and 69 at light level ≤ 7, with a chance scaled by the moon’s brightness — most on a full moon, none on a new moon — and a further 50% roll.
- Slimes need space matching their size (a big slime needs about 3×3×3 blocks of room) and, like other mobs, spawn 24 or more blocks from the player and despawn beyond 128.
Building a slime farm
Pick a slime chunk (two adjacent ones are even better), dig out the whole 16×16 chunk from Y 39 down as far as practical in 3-block-tall layers, keep it lit to stop other mobs or leave it dark and light up surrounding caves, then push slimes with water or iron golems into a killing chamber. The Slime Finder shows every candidate chunk on the map.
FAQ
What is a slime chunk?
A slime chunk is one of the roughly 10% of chunks where slimes can spawn underground, below Y 40, regardless of light level. On Java the set of slime chunks depends on the world seed; on Bedrock it is fixed and identical in every world.
How do I find slime chunks?
Use the Slime Finder on the seed map: enter your seed (Java) or just the edition (Bedrock) and every slime chunk is highlighted. The checker on this page tells you whether one specific chunk qualifies and shows the 11×11 chunks around it. In-game, dig out a large dark area below Y 40 and wait — if slimes appear over a few nights, you are in a slime chunk.
Why do slimes not spawn in my slime chunk?
Check that the spawning floor is below Y 40, that you have at least a 3-block-tall, 3×3 clear space for large slimes, that difficulty is not Peaceful, and that you are within 24–128 blocks of the spawn spots. On Java every spawn attempt in a slime chunk also has only a 1-in-10 chance to succeed, so spawning is slow — light up nearby caves so other mobs don't fill the mob cap.
Do slimes spawn in swamps?
Yes. In swamp and mangrove swamp biomes slimes spawn on the surface between Y 51 and 69 at light level 7 or lower, with a chance that follows the moon phase: most on a full moon, never on a new moon. Slime chunks are not needed for swamp spawning.
Are Bedrock slime chunks the same as Java?
No. Java derives slime chunks from the world seed using java.util.Random; Bedrock seeds a Mersenne Twister with only the chunk coordinates, so its slime chunks are the same in every world. A Java and a Bedrock world with the same seed therefore have completely different slime chunks.