The Kelly Criterion is a formula used to determine the optimal size of a series of bets. In its simplest form, it suggests that you should bet a percentage of your bankroll equal to your edge divided by the odds. However, in sports betting, since it's hard to quantify the exact edge, using the full Kelly Criterion can be risky. Many bettors use a fraction of the Kelly Criterion, such as a quarter or half, to determine their bet size.