From block to chunk, section and region
Take the block at X −100, Y 70, Z 250:
| Value | Calculation | Result |
|---|---|---|
| Chunk X, Z | −100 ÷ 16 = −6.25 → −7, 250 ÷ 16 = 15.625 → 15 | Chunk −7, 15 |
| Position inside the chunk | −100 − (−7 × 16) = 12, 250 − (15 × 16) = 10 | 12, 10 (always 0 to 15) |
| Chunk borders | −7 × 16 = −112 to −97, 15 × 16 = 240 to 255 | X −112 to −97, Z 240 to 255 |
| Section | 70 ÷ 16 = 4.375 → 4 | Section 4 (Y 64 to 79) |
| Region file | −7 ÷ 32 → −1, 15 ÷ 32 → 0 | r.-1.0.mca |
Every step rounds down. With negative numbers that moves further from zero, which is why block −100 is in chunk −7, not −6. The chunk and region coordinate reference lists the formulas with more examples.
How tall a chunk is
A chunk covers the full height of its dimension. In the Overworld that is Y −64 to 319, 384 blocks; in the Nether and the End it is 256 blocks. The game splits that height into sections of 16 blocks, which matter for lighting and some technical builds.
Why chunk borders matter
- Loading: the game loads and unloads whole chunks. A farm or redstone machine that crosses a border can have one half running while the other half is unloaded, which breaks it.
- Slime chunks: in Java Edition, one in ten chunks lets slimes spawn below Y 40 regardless of light. Which chunks those are depends on the world seed.
- Storage: chunks are saved in groups of 32 × 32 per region file, so a region file name tells you which area of the world it holds.
To see borders while building, press F3 + G. How to see chunk borders has more on building around them.
What keeps chunks loaded
- Players: chunks within the server’s
simulation-distanceof a player are fully processed. Further out, up to the view distance, chunks are sent to the player but mostly not updated. - Forceloading: an operator can keep chunks loaded with
/forceload add, using block coordinates of the area. - No spawn chunks: Java Edition 1.21.9 removed the always-loaded spawn chunks, so a farm at spawn no longer keeps running without a player or forceloading.
Where chunks are stored
A Java Edition world saves chunks in .mca region files. Block data is in the region folder, entities in entities and points of interest, such as beds and workstations, in poi. Each file is named r.X.Z.mca after its region coordinates.