Expected value, or EV, is how much a bet would win or lose on average if you could place it again and again. It compares the chance of the bet landing with the odds you are getting: when the odds pay more than that chance deserves, the bet has positive EV, and when they pay less, negative EV.
Every bet has an EV, whether or not anyone works it out, and most bets carry negative EV because the bookmaker's margin is built into the odds.
With decimal odds, the expected value of every £1 staked is the chance of the bet winning multiplied by the odds, minus 1.
EV is often given as a percentage of the stake: the first bet is +12.5% EV, and the second loses 10%.
The sum is the easy part. The odds are there for anyone to see; the chance of the bet landing has to come from somewhere, and that is where data comes in.
Every price carries its own chance, the implied probability: 1 divided by the decimal odds. Odds of 2.50 imply 40%, odds of 2.00 imply 50% and odds of 1.50 imply 67%.
A bet has positive EV when its real chance is higher than the chance its odds imply. At 2.50 the price says 40%; if the real chance is 45%, the price is better than it should be.
Bookmakers build their margin into every market, so the implied chances add up to more than 100%. A both teams to score market priced at 1.90 for Yes and 1.90 for No implies 52.6% for each, 105.3% in all, and that extra 5.3% is the margin a bettor has to overcome.
A value bet is a bet with positive EV: one where the odds are bigger than the real chance deserves. It is not the same as a likely winner. A team at 1.20 is poor value if it only wins 80% of the time, while a 6.00 outsider is good value if it wins one game in five.
EV is an average over many bets, not a promise about the next one. A bet with a 45% chance loses 55% of the time, so long losing runs happen even when every price was a good one.
That is why the size of each stake matters as much as the price: staking the same small share of your bankroll every time lets the average come through. The Kelly calculator sizes a stake from your edge and the odds.
Positive EV is a long-run idea and guarantees nothing. Only bet what you can afford to lose.
Our betting tables set the model's chance beside the market's. Proj is the chance our model gives a selection, Market is the chance the best available price implies, and Edge is the gap between them: green where the model rates a bet more likely than the odds suggest, red where it rates it less likely.
You will find them on Player Props and Fixture Projections, and the Prop Pricer prices any selection against the model.
Positive EV in betting means the odds pay more than the chance of the bet landing deserves, so the bet would make money on average if it could be placed over and over. A 45% chance at odds of 2.50 is +12.5% EV.
Negative EV in betting means the odds pay less than the chance of the bet landing deserves, so the bet loses money on average over many repeats. Most bets are negative EV, because the bookmaker's margin is built into every price.
EV percentage is the average profit or loss of a bet as a share of the stake: the chance of winning times the decimal odds, minus 1. A 45% chance at 2.50 is +12.5%, so £10 staked returns £1.25 profit on average.
Betting only at positive EV is the one approach that can come out ahead over the long run, but it depends on your chances being more accurate than the bookmaker's, and results over a few weeks swing widely either way.
Every bet is a price on a chance, and expected value is simply the question of whether that price is fair, asked before the bet rather than after it.
For more on reading odds, see common betting terms, line moves and our football betting tips for beginners.