Rollover is the term used to describe the transfer of one lottery jackpot fund over to the jackpot fund of a subsequent lottery. Rollovers take place when no ticket has successful matched enough numbers to win a jackpot, and if several draws pass without a jackpot winner being found, the resulting rollovers from week to week can generate some huge payouts.

Many lottery games have an upper limit on the number of times that jackpots can be rolled over. For example, the Australian Tattslotto does not allow the jackpot to roll over more than four times. This kind of rule prevents the lottery from rolling over so regularly that it starts being viewed as “unwinnable”, which could obviously cause sales of tickets to drop quite markedly.

When an upper limit of rollovers is reached and still no winner is found, the jackpot fund is generally “rolled down” to the next highest prize level that does have winners. For example, in the UK Lotto game, if no player matches six main numbers and the maximum number of rollovers has been reached, the money will be shared between the players matching five main numbers and the bonus ball. The great thing about this arrangement is that players know that the money will be paid out even if no jackpot winner is found, and ticket sales for these “guaranteed payout” draws tend to soar.

Rollovers are always good for attracting more players to a game, because they basically give the potential of landing a win that is much bigger than usual without asking the player to invest any additional funds. If investing $1 in the hope of winning $4 million excites players, then investing the same $1 to buy a chance to win $20 million is even more so. If rollovers are repeated for a few draws, this excitement can build to immense proportions – so much so that queues for lottery tickets can be seen gradually extending on a draw by draw basis.

Interestingly, this behaviour, of buying more tickets when the rollovers generate huge prize pools, is at odds with what players will say publicly. When asked, players always say they'd prefer to see more winners of smaller prizes.