There is a 1 in 13,983,816 (or about 0.00000715%) chance for a win, assuming you need to pick 6 numbers out of the set of 49 possibilities. However if 5 of 49 satisfies the winning condition; the odds change to 1 in 1,906,884 (or about 0.0000524%).
To estimate such probabilities, divide one over the combination of the corresponding values, e.g., 1/C(49,6) in the first case. Read more