Minecraft wiki entry for Chunk, associated text: Additionally, the player can know the chunk they are on by this formula:
The X of a chunk is floor(X coordinate / 16)
The Z of a chunk is floor(Z coordinate / 16)
Where floor is the largest previous integer. E.g. Floor( 27.9561 ) is 27
In other words, if X was 27, Z was −15 the chunk is chunk (Floor(27/16), Floor(−15/16)), meaning that the player is on chunk (1, −1).
Also, the coordinates of a block within a chunk can be found by taking the coordinate mod 16.