Message Entropy (H)= -∑_i(p_iLog₂(p_i)).
where p_i is the frequency of each character.

For Example:
Message: AC
P(A) = 0.5
P(C) = 0.5
H(AC) = - log(P(A)) * P(A) / log(2) - log(P(C)) * P(C) / log(2)
H(AC) = 1