Target (Mining)

The target is the 256-bit number that defines what counts as a winning hash in Bitcoin mining. For a block to be valid, the double-SHA-256 hash of its header must be numerically equal to or below the current target. Miners cannot predict hashes, so they try enormous numbers of header variations until one lands under the threshold, which is the essence of proof of work.

nBits: the compact encoding

Storing a full 256-bit number in every block header would be wasteful, so the header carries the target in a 4-byte compressed form called nBits. It works like scientific notation but in base 256: a one-byte exponent and a three-byte coefficient expand back into the full 256-bit target. Every node decodes nBits to verify that a submitted block actually meets the required threshold.

Target, difficulty, and adjustment

Target and difficulty are inverse expressions of the same thing. A lower target leaves fewer valid hash values, so it is harder to hit and corresponds to higher difficulty. Every 2,016 blocks the network recalculates the target to keep average block time near 10 minutes: if blocks came too fast, the target drops and difficulty rises; if too slow, the target loosens. This automatic feedback keeps issuance on schedule no matter how much hashrate joins or leaves.

The target is read directly from the nBits field of the Block Header. When a miner exhausts the header nonce without beating the target, it rolls the Extranonce to generate a fresh batch of candidate hashes.