# Question regarding mining difficulty

I understand that the Bitcoin mining problem is to find a string `s` (hash of previous block + transaction hashes + nonce) such that `sha256(s)` has `n` leading zeros, where `n` determines the mining difficulty.

The result of `sha256(s)` is a 64-length hexadecimal string. Each position of 64 has 36 possibilities (a-z, 0-9). So, the expected number of guesses would be `36^n`.

The hash of block #485891 is

``````0000000000000000007ea3c67381ecfcf8e44e9941dcef554e8b029068857b55
``````

which has 18 leading zeros. Thus, the expected number of guesses is

``````36^18 = 10314424798490535546171949056.
``````

This hash was found by Antpool which has a hash power of 1284.32 PH/s. Therefore, the expected time for Antpool to find the hash would be

``````36^18 / (1284.32 * 10^15) = 8.03104*10^9 s = 255 years.
``````

This time is obviously incorrect. Is there anything I missed out?

As you say, the result of `sha256` is a hexadecimal string. Hexadecimal only uses the digits 0-9 and a-f, which is 16 possibilities, not 36.