Expected value betting is the idea behind almost every serious wager, even if you have never heard the term. Expected value, or EV, is the average amount a bet would win or lose if you could place it over and over again under the same conditions. It is not a prediction of one result. It is a measure of whether the price you are getting is generous or mean compared with the real chance of the thing happening. Get comfortable with it and you start to see odds the way a careful bettor does: as a number to judge, not just a payout to chase. This guide defines EV, gives the formula in plain words, and walks through worked +EV and -EV examples. If you want a refresher first, our beginner betting guide sets the scene.
Quick answer
Expected value is the average profit or loss a bet returns over the long run, found by weighing each outcome by how likely it is. A bet is positive expected value (+EV) when your estimated chance of winning is higher than the odds imply. It is negative (-EV) when it is lower. EV describes the long-run average, not any single result.
What expected value actually means
Every bet has two parts: what you can win and how likely you are to win it. Expected value puts those two together into a single number. It answers one question. If you placed this exact bet a thousand times, what would your average outcome per bet be?
That long-run framing is the part people miss. A +EV bet can lose. A -EV bet can win. EV says nothing about the next result. It tells you what direction the average drifts once the sample gets large. Think of it like a slightly weighted coin. On any flip you cannot say which way it lands, but flip it ten thousand times and the bias shows up clearly.
The reason this matters is that bookmakers price odds with a built-in edge, called the margin. Understanding how the bookmaker margin inflates prices is what separates a bet that looks fair from one that quietly works against you. EV is the tool that exposes the difference.
The expected value formula in plain words
You do not need heavy maths. The formula has two halves: the part where you win and the part where you lose.
In plain words: expected value equals (your chance of winning, multiplied by the profit if you win) minus (your chance of losing, multiplied by the stake you lose).
Written shorter:
EV = (probability of winning x profit) – (probability of losing x stake)
Two things feed into it. The first is the profit and loss, which the odds and your stake give you directly. The second is your own estimate of the true probability. That second number is the hard part, because the bookmaker does not hand it to you. Working out a fair probability is a skill in itself, and it leans on the same groundwork covered in our piece on sports betting math.
A quick note on converting odds to an implied probability, since you will need it. Divide 1 by the decimal odds. Odds of 2.00 imply a 50% chance (1 / 2.00). Odds of 4.00 imply 25% (1 / 4.00). That implied figure is what the price is telling you. Your job is to decide whether the real chance is higher or lower. The same conversion works whatever format you read prices in, including American lines, which we unpack in our explainer on reading moneyline odds.
A worked +EV example
Say a tennis player is priced at 2.10 to win a match. You stake 10. The implied probability from the price is 1 / 2.10, which is about 47.6%. After studying form, surface and the head-to-head, you estimate the player’s true chance of winning is 52%, a fair bit higher than the price suggests. That gap is where value lives.
If they win, your profit is 11 (a 21.00 return minus your 10 stake). If they lose, you lose your 10 stake.
| Outcome | Your probability | Profit or loss | Contribution to EV |
|---|---|---|---|
| Player wins | 0.52 | +11.00 | +5.72 |
| Player loses | 0.48 | -10.00 | -4.80 |
| Total EV | +0.92 |
Add the two contributions: +5.72 minus 4.80 leaves +0.92. So the expected value of this bet is +0.92 per 10 staked. Over many bets like this, you would expect to average roughly 92p of profit for every 10 you put down. That is a positive expected value bet. The price is paying you more than the risk deserves, according to your estimate.
It will not feel like that on any single bet. You either win 11 or lose 10. The +0.92 only emerges across a long run of similar spots.
A worked -EV example
Now flip it. Same player, but the bookmaker has shortened the price to 1.80 because money has poured in. That kind of shift is worth recognising, and our look at how betting lines move explains why prices drift before kick-off. The implied probability is now 1 / 1.80, about 55.6%. Your estimate of the true chance has not changed: you still think it is 52%. This time the price implies a higher chance than you believe is real.
If they win, your profit is 8 (an 18.00 return minus the 10 stake). If they lose, you lose 10.
| Outcome | Your probability | Profit or loss | Contribution to EV |
|---|---|---|---|
| Player wins | 0.52 | +8.00 | +4.16 |
| Player loses | 0.48 | -10.00 | -4.80 |
| Total EV | -0.64 |
The sum is +4.16 minus 4.80, which gives -0.64. The expected value is now negative: about minus 64p per 10 staked. Same player, same view of their real chance, but a worse price has turned a smart bet into a losing one over time. This is exactly how a small change in odds flips a wager from +EV to -EV without the underlying match changing at all.
Comparing the two bets side by side
The only thing that moved between the examples was the price. Your probability estimate stayed at 52% both times. That alone decided whether the bet had value.
| Bet | Odds | Implied chance | Your estimate | EV per 10 staked |
|---|---|---|---|---|
| Bet A | 2.10 | 47.6% | 52% | +0.92 |
| Bet B | 1.80 | 55.6% | 52% | -0.64 |
The rule that falls out of this is simple. When your estimated chance is higher than the implied chance from the odds, the bet is +EV. When it is lower, the bet is -EV. Everything else is detail. This is the heart of value betting, and it is why disciplined bettors care so much about getting the best available price rather than just backing who they think will win. One way to check whether you really are beating the market is tracking your closing line value, which compares your price to the final odds at kickoff.
Why EV is a long-run average, not a promise
This is the part worth repeating, because it is where people get burned. Expected value is a statistical average, not a forecast of your next slip. A +0.92 EV bet still loses 48 times out of every 100 in the example above. You can place ten +EV bets in a row and lose the lot. Variance, the natural swing of short-run results, is real and it can run against you for longer than feels fair.
What EV gives you is a direction over a large sample. If you consistently take +EV prices and your probability estimates are sound, the average pulls you the right way in the long run. If your estimates are wrong, no clever maths saves you, because the formula is only as good as the probability you put into it. There is no sure thing here, and anyone selling one is wrong.
This is educational maths, not a profit guarantee. EV is a way to judge whether a price is fair, nothing more. It does not remove risk, it does not predict the next result, and a long run of +EV bets can still end down. Whether your edge survives once the sample is large also depends on your strike rate over time, which is the territory covered by betting ROI.
Estimating your own probability
EV is only as honest as the probability you feed it, so this step deserves care. The bookmaker’s implied probability is a starting point, but it includes their margin, so it is always a touch inflated across the full market. To find a fairer baseline, you can strip that margin out, which is one of the first things our guide to how betting odds work walks through.
From there, your own read comes from the usual sources: recent form, head-to-head records, injuries, home advantage, motivation and, in many sports, underlying numbers like expected goals or shot data. This is also where value often hides in how the public reads a matchup, a theme covered in our piece on backing underdogs against favorites. None of it is exact. The goal is not perfect probabilities, which do not exist, but estimates good enough that the gaps you spot are real value rather than noise. Browsing the value betting markets at Campeonbet lets you compare the listed price against your own estimate before you commit, which is the practical moment EV thinking earns its keep.
Frequently asked questions
What does +EV mean in betting? +EV is shorthand for positive expected value. It means your estimated chance of the bet winning is higher than the chance implied by the odds, so over a large number of similar bets the average result would be a profit. It does not guarantee the next bet wins.
How do I calculate expected value? Multiply your chance of winning by the profit you would make, then subtract your chance of losing multiplied by the stake you would lose. A positive answer is +EV, a negative answer is -EV. The tricky input is your probability estimate, not the arithmetic.
Can a +EV bet still lose? Yes, often. Expected value is a long-run average across many bets, not a prediction of one result. A bet with positive EV can lose any single time, and you can hit a losing streak of them. The edge only shows up over a large sample.
Is value betting the same as expected value? They are closely linked. Value betting is the practice of placing bets where the price is better than the true odds. Expected value is the number that measures how much better or worse a price is. Value betting is the action, EV is the maths behind it.
Does the bookmaker’s margin affect EV? Yes. The margin inflates the implied probability baked into every price, which lowers the EV of the average bet. Stripping the margin out gives you a fairer probability to compare your own estimate against, which is why margin awareness sits right next to EV.
Conclusion
Expected value is just a way to ask whether a price pays you fairly for the risk: weigh each outcome by how likely it is, and see whether the average lands positive or negative. The two worked examples show how the same match flips from +EV to -EV on price alone, and the long-run framing is the part to hold onto. It is a tool for judging value, not a promise of profit, and a losing run of +EV bets is normal. To carry this forward, see how prices are built and how the margin shapes them in our guide to how betting odds work.
BetWise specialises in sports betting guides, betting strategies, odds, and betting market analysis. Through educational content and practical insights, BetWise helps readers build their understanding of sports betting and major sporting events.
