Mega Millions and Powerball 2018: How do the jackpots grow?

Four nights a week, lotto players around the country have the opportunity to become instant millionaires through either the Powerball (on Wednesdays and Saturdays) or the Mega Millions (on Tuesday and Fridays).

Both games are currently poised to join the list of the highest jackpots in history. The Mega Millions prize is at least $1.6 billion -- which will be the largest of any U.S. lottery ever -- and the Powerball is at an impressive $620 million.

Here's a look at how each game's jackpot grows to such a large sum.

How to play

In both games, tickets cost $2 and each ticket has six numbers. Players can either pick out the numbers themselves or have a machine generate random numbers.

To win the Mega Millions jackpot, players must match the white balls (which are numbered 1-70) and the yellow Mega Ball, which can be any number 1-25. Chances of hitting all six numbers are 1 in 302,575,350.

To win the Powerball jackpot, winners must guess the five white ball numbers from a drum of 69 numbers, and one red ball out of a drum of 26 numbers. Chances of hitting all six numbers are 1 in 292,201,338.

Why they both get so big

For both games, each jackpot starts out at $40 million. Each time a drawing is made without a jackpot winner, the pot rolls over to the next drawing. When more tickets are purchased, the jackpot grows.

In 2015, Powerball added more numbers to the drawing, making it easier to win smaller prizes, but also less likely to win the jackpot. The odds of winning the jackpot went from 1 in 175 million to 1 in 292 million.

"More play happens at the higher level so your odds of sharing it are greater at the greater jackpot levels," Powerball chairman Charlie McIntyre told ABC News.

Similarly, in 2017, Mega Millions expanded the possible numbers that could be picked for the jackpot but decreased the possible numbers for the first five. This meant that the odds got better for winning a lower tier prize but worse for winning the jackpot.

For both games, since it's harder to win, the jackpot can grow to large sums as no one wins drawing after drawing. This prompts more people to buy tickets, leading to the ballooning jackpot prizes.