Slime farm planning
The seed rule behind a slime chunk
A Java chunk covers 16 by 16 blocks. The game mixes the world seed with the chunk X and Z coordinates, then accepts the chunk when a random nextInt(10) draw is zero. Across a large area, about one chunk in ten passes. The result is fixed for that seed and chunk, rather than changing each night.
In Java 26.3, the underground slime rule requires the slime position to be below Y=40. It does not impose a light threshold on that underground branch. Lighting a spawning floor can stop other hostile mobs without disabling the slime-chunk rule. Space, valid spawning surfaces, difficulty and normal spawning checks still determine whether a slime actually appears.
How to use the slime chunk map
- 01
Choose Java and enter the exact seed. Enable the slime layer, then zoom until the 16-block chunk squares are clear.
- 02
Read chunk boundaries before excavating. Divide block X and Z by 16 and round down to obtain chunk coordinates, including for negative positions.
- 03
Choose a floor below Y=40 inside a highlighted chunk. Use the map for the horizontal boundary and plan the spawning room in the world, where obstructions and other mobs can be inspected.
A worked chunk boundary
For seed 8675309 in Java 26.3, the map engine accepts chunk X=3, Z=1. That square covers block X=48 through 63 and Z=16 through 31, inclusive. The repository engine regression checks the same result using the production slime function. A floor at Y=30 is below the required ceiling. The highlight is for the whole chunk; placing the room at X=64 would cross into the next chunk.
Negative coordinates need floor division
Block X=-1 belongs to chunk X=-1, which spans X=-16 through -1. Truncating toward zero would incorrectly call it chunk zero. Block X=-17 belongs to chunk -2. Use both horizontal axes this way and keep the spawning floor within the resulting square. Adjacent highlighted chunks can be combined, but their presence follows the seed draws rather than a regular checkerboard pattern.
Swamp slimes follow another rule
Surface slimes do not require a highlighted slime chunk. In Java 26.3, the surface branch checks the biome tag for surface slime spawning and requires Y greater than 50 and less than 70, meaning integer positions 51-69. It also checks local brightness against a random draw below 8 and uses the surface slime spawn chance supplied by the environment. That is a separate branch from underground slime chunks, so a lit swamp floor is not governed by the underground light rule.
What this layer does not simulate
Bedrock uses a different slime-chunk method. This finder exposes the Java seed calculation and should not be used as a Bedrock slime layout. It also does not simulate mob caps, player positions, caves elsewhere in the area or blocks placed after generation. If a highlighted Java chunk stays empty, inspect the room height, floor, available space and nearby spawning areas before blaming a different seed roll. The map cannot see those live-world conditions.
Slime Chunk FAQ
Must the floor be dark?
The Java underground slime-chunk branch does not test light. The separate surface spawning branch does.
Is Y=40 inside the underground range?
No. The Java 26.3 rule checks Y below 40.
Does one in ten mean one per group of ten?
No. Each chunk receives its own deterministic draw; local groups can contain none or several.
Related farming tools
Other finders
Map and coordinate reports
For a missing or misplaced result, include the full seed, Java or Bedrock, game version, dimension, coordinates, and what appeared in the world. Mention mods, datapacks, add-ons, experiments, server software, or old chunks when relevant.
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