Target
Target is a 256-bit number that a block’s hash must be equal to or less than for that block to be considered valid by the Bitcoin network. It is the numerical representation of the current mining difficulty.
Understanding the Target
Section titled “Understanding the Target”Every time a miner hashes a block header, the resulting 256-bit number is compared against the target. If the hash is numerically less than or equal to the target, the block is valid. If it is greater, the miner must try a different nonce and hash again.
Think of the target as a limbo bar. The hash is a person trying to walk under it. A high target (easy difficulty) means the bar is raised high and most people can pass. A low target (hard difficulty) means the bar is very close to the ground, and only a few can make it through. As difficulty increases, the target gets lower, making it harder for any given hash to qualify.
The target is stored in a compact form called “nBits” in the block header, which uses a custom floating-point encoding. The full 256-bit target is derived from this compact representation. At difficulty 1 (the minimum), the target is a very large number. At current network difficulty levels, the target is extraordinarily small, meaning valid hashes must begin with many leading zeros.
Practical Example
Section titled “Practical Example”If the current target in hexadecimal starts with many leading zeros like 00000000000000000004a1b..., then any SHA-256 hash of a block header must produce a value smaller than this number. The probability of any single hash meeting this criterion is extremely low, which is why miners must make trillions of attempts per second.
Mining pools use a similar concept with “share targets” — these are much higher (easier) targets that allow individual miners to submit proofs of work frequently, even though those shares would not satisfy the network target.