Background

Apart from spawning in swamps (added in 1.4), slimes can only spawn in every 10th chunk in average ("Slime Chunks"). Chunks are 16x16 blocks wide (x and z axis) and 256 blocks high (y axis). Slime Chunks are predetermined by your savegame seed, and are always the same for a specific seed, just like the generated terrain. The only way to find Slime Chunks ingame without using third party apps or mods is to observe the slime spawns.

Slime Chunks in Slime Finder and ingame

Does Slime Finder work with Bedrock Edition?

Yes! Make sure to select Bedrock as the version.

Does it work in superflat worlds?

Yes :)

No slimes are spawning in my slime chunk!

First of all, even in slime chunks, slimes spawn far less frequently than most other hostiles, maybe comparable to endermen. So make sure you wait long enough. Also, double check the seed you used as well as the coordinates. Here are a few more things to consider:

In slime chunks, slimes only spawn on layer 39 and below (feet pos). The spawn rate is equal on those layers.

Mobs can't spawn closer than 24m to you (but despawn when farther away than 128m), so don't stand too close to where you expect them to spawn

It's a good idea to hollow out a 16x16 wide (= 1 chunk) and at least 3 blocks high room inside a slime chunk to maximize the spawn rate. Waiting in naturally generated caves and hallways will take much longer.

Use lighting to prevent other mobs from spawning (light has no effect on slime spawning in slime chunks). But don't use torches on the ground or glowstone, as these blocks will reduce the spawn rate. You can safely use Jack-O-Lanterns or Redstone Lamps + Redstone Blocks.

Sometimes, hostile mob spawning may be jammed by mobs that don't despawn automatically. If that happens, you should walk about 200m in any direction and then come back in order to force them to despawn.

Mobs cannot spawn on bedrock (see Block Compendium)

Slimes don't spawn on peaceful

Slimes don't spawn in mushroom island biomes

Also have a look at my Slime Farming Tutorial which explains some details about slime spawning as well.

Requirements

Seed

For the reason mentioned above, you need to know the seed of your world to use Slime Finder, unless, of course, you want to find a seed for a new world. If you're playing SSP, the app is able to fetch the seed from your savegame. Alternatively, you can use the /seed command ingame. In SMP, you can use the same command if you have sufficient rights. Otherwise, however, you're dependent on the server owner, who started the world and has access to the savegame and config files.

Technical

This app uses some relatively new web technologies. As a result, some features may be disabled for older browsers. I recommend using the latest version of a major browser such as Chrome, Firefox, Edge or Safari.

Usage

Seed Selection

The first thing you should do is select a seed and version. You can either type it in manually, or you can load it from your savegame. The latter can be done by clicking on "Load from Save..." and selecting your level.dat, or by drag&dropping the level.dat file into your browser window. Level.dat is a small file located in the folder of every Minecraft savegame. You can find the savegames in the saves folder of your Minecraft installation. On Windows you can use %appdata%\.minecraft\saves to get to that folder.

Levels allow you to store and reuse seeds on this website, without having to load your savegame every time. When loading a seed from a level, the seed will automatically be stored as a level.

You should also know that a seed is always a number (up to around 20 digits). If you type in anything else (like letters), it will be converted to a number. The app does this the same way Minecraft does, so it's safe to use letters (and other characters) as well.

Navigation

Once you entered your seed and version you can start using the map. To scroll, use your arrow keys while your mouse cursor points at the map, or move your mouse while holding down the left mouse button. You can use the slider below the map or your mousewheel for zooming. The lower inputs allow you to go to a specific point (e.g., your base) of the map and set a marker there. You can always remove and add the marker by double clicking on the map. The "Save Map" button allows you to save the currently shown map as png image file.

Touchscreen

When using a touch-enabled device, an extra option for enabling/disabling touchscreen control will appear below the map. With that option enabled, you can drag the map with your finger to navigate, you can pinch to zoom in and out, and you can tap and hold to set a marker on the map. By double tapping on the map, you can quickly enable/disable the functionality as well.

Credits

Thanks to @protolambda and @jocopa3 for figuring out the slime chunk algorithm for pocket/bedrock edition, and depressed-pho for porting it to JavaScript.