What is positive EV?

Every bet has an expected value (EV) based on the odds and the implied probability of hitting. When this value is positive for a bet, it's said to be "+EV". The formula for calculating EV is:

(Amount won per bet × probability of winning) – (Amount lost per bet × probability of losing)

+EV bets = good

-EV bets = bad

Coin flip example

Consider a coin flip where you can bet a dollar on heads or tails at +100. You win $1 if you're right and lose $1 if you're wrong. The expected value of this bet is $0, because over an infinite amount of bets, your wins and losses will exactly cancel out. +100 is the fair value of a bet that hits 50% of the time.

Now imagine the same bet at +110. You win $1.10 if you guess correctly, but still only lose $1 if wrong. Your wins are going to make up for your losses, and then some. The expected value of this bet is 0.05. You can plug these numbers into the devig calculator to see it's a 5% EV bet.

Using the formula:

EV = (1.1 × 0.5) - (1 × 0.5)

EV = 0.55 - 0.5

EV = 0.05

Across many bets, you can expect to win $0.05 (or 5%) for every $1 wagered.

Conversely, imagine the odds were instead -110. You risk $1 to win only $0.90. The expected value of this bet is negative, and thus is a bad bet. The payout is not high enough for how often it wins. Over time you would expect to lose 5 cents for every $1 bet you made. This is how sportsbooks make their money, and why most people are losing bettors.