At the poker table, you need to know the chances of hitting a winning card, if you want to have any chance of making a profit. Counting your ‘outs’, as it is known amongst professional poker players, is a way to work out the percentage chance that you will hit one of the cards you need to scoop the pot. 

For example, if you are chasing a flush draw, and there are two cards left to be dealt, you will have around a 36% chance of making your hand. While the mathematics involved in this are far from complicated, they are too tricky to perform at the table, which is why you need to apply the rule of “two and four”.

This rule requires you to count the number of cards that will make your winning hand, multiply them by two, then four. 

Thus, if there were nine cards of your suit left in the deck, you would multiply this number by two, to find out what percentage of the time you will make a flush on the next card.

In this instance, it is around 18%. However, because you are on the flop, and there are two cards to come, you multiply your nine outs by four (which is essentially 18x2), which gives you 36%. 

Once you get to the turn, and you have not made your flush, you will already know that the chance of making your hand on the final card will be 18%. 

In conjunction with understanding your outs and chances of winning, you need to assess the size of the pot. Thus, if you have a 2/1 chance of making your hand by the river, and the bet you need to call vs. the size of the pot creates a ratio of 4/1, then you would be right to call. However, if the price to call were less than 2/1, it would be mathematically correct to fold.