How the game picks slime chunks
In Java Edition a chunk is a slime chunk when a random number seeded from the world seed and the chunk's position comes out as a multiple of 10, so about one chunk in ten is one. This page runs the game's own sum: the chunk's X and Z are squared and multiplied by fixed numbers in 32-bit integer maths, which wraps around just as the game's does, added to the 64-bit seed, and fed to the same random number generator Java uses. A seed typed as words is turned into a number the way the game does it (Java's String.hashCode), so hello is the seed 99,162,322. Bedrock Edition ignores the seed: a chunk's X and Z alone seed a different generator (a Mersenne Twister), so the same chunks are slime chunks in every Bedrock world. Checked against Java Edition 26.3.
Reading the map
Each square is one chunk, 16 by 16 blocks; the filled ones are slime chunks. East (+X) is to the right and south (+Z) is down, as on the F3 screen. Drag to move around, scroll or pinch to zoom, and point at a chunk (or tap it) to see its chunk number and the blocks it covers. To find your chunk, put your X and Z from F3 into Go to; the map centres on that block. Chunk numbers are the block coordinate divided by 16 and rounded down, so block X −1 is in chunk −1.
Finding a spot for a farm
Slimes spawn in slime chunks below Y 40, at least 24 blocks from you, and vanish once they are more than 128 blocks away. So the best place to stand is where the most slime-chunk ground lies between those distances. Find the best spot checks the middle of every chunk and every corner where four chunks meet, around the middle of the map, and counts the block columns of slime chunk in that ring. It names the top three places, each at least the reach apart, and draws the ring on the map. The distances here are measured flat; the game measures in three dimensions, so standing high above the spawning floors shrinks the ring a little. Lower the reach if you play with a short simulation distance, since slimes only spawn in chunks the game is running.
What it checks against
The Java sum is the game's own (WorldgenRandom.seedSlimeChunk with its salt 987234911), written in exact 64-bit integer maths in the page. It reproduces a published result: for the seed 8354031675596398786, the 16 by 16 chunks starting at chunk −262220, −1206735 hold 67 slime chunks, as the CUDA Slime Finder project reports. The Bedrock rule matches two independent published versions of it (the rbedrock package and Slime Finder PE), and its Mersenne Twister gives the standard reference numbers.
Questions
Why are there no slimes in my slime chunk?
Slimes spawn there only below Y 40, in light or dark alike, on a solid floor with room above it. Mushroom fields and the deep dark spawn none. They also share the hostile mob cap, so light up or fill in the caves nearby and stay within reach; and Peaceful difficulty spawns no slimes at all. Check that you have the right chunk: a block's chunk is its coordinate divided by 16 and rounded down.
Does the seed change slime chunks in Bedrock?
No. Bedrock Edition works them out from the chunk's position alone, so a slime chunk at one place is a slime chunk at that place in every Bedrock world. Pick Bedrock above and the seed box goes away.
Are swamps different?
Yes. Swamps and mangrove swamps spawn slimes between Y 51 and 69 at night, more often near a full moon, whether or not they are slime chunks. This map only shows slime chunks.
My seed has letters in it. Does that work?
Yes. Type it exactly as you typed it when you made the world, including capitals and spaces in the middle; the page turns it into the number the game used. If you are not sure, run /seed in the game and copy the number it shows.
Is my seed sent anywhere?
No. The map is worked out in this page, on your device. The seed and position are kept in the page's address so that Copy link shares the same view.