Do bitcoin miners calculate the hash value of current block or next block?
After reading your question, you are a little confused, so i will try to explain:
Miners do double round of SHA256
What they hash?
They hash the blockheader...
What's the blockheader?
The blockheader is build from data
What data?
It's Build from:
Version: The version of the block.
Previous block hash: The Block Hash of the block that this block is being built on top of. This is what “chains” the blocks together.
Merkle root: All of the transactions in this block, hashed together. Basically provides a single-line summary of all the transactions in this block.
Time: When a miner is trying to mine this block, the Unix time at which this block header is being hashed is noted within the block header itself.
Bits: A shortened version of the Target.
Nonce: The field that miners change in order to try and get a hash of the block header (a Block Hash) that is below the Target.
All this is in little endian