Here's a table I got from bitcoin wiki.
A special case, leading numbers 1 (one) is especially difficult.
An address has 1 + 20 + 4 bytes, which has a total of 200 bits. Each of the Base58 characters contain log_2 58 bits of information, which is approximately 5.858. If one created an address that has the public key hash 0000...00, the result would be a 5-6 character address, due to the checksum. To prevent this, Satoshi decided to add "1"s for each 0x00 byte in front of the address. Those "1"s each contain 8 bits of information instead of 5.85... As they carry more information, it's harder to find an address starting with those 1s.