As Wikipedia tells us, Hartley's formula determines the amount of information contained in a message of length n.
That is, we have an alphabet A of which letters we make up a message:
The number of possible variants of different messages:
where N — the possible number of different messages; m — number of letters in the alphabet; n — the number of letters in a message.
Then the Hartley's formula is defined as:
where I — the amount of information, bits.
The Hartley's formula was proposed by Ralph Hartley in 1928 as one of the scientific approaches for message evaluation.