Overall odds are simply calculated as:
The total number of tickets that have any prize value divided by the total number of tickets printed for a unique game.
For example. If there are 100 tickets printed and 90 of these tickets are not winners, but 5 tickets are $1.00 winners and 3 tickets are $10 winners and 2 tickets are $20 winners, the odds are 10 winning tickets out of 100 printed tickets, or 10 / 100; reduced, this is 1 in 10.
DETAILED odds are explained here.