Guide
Expected Value in Betting: What a Bet Is Worth
Quick Summary
- Expected value is the average result of a bet if it could be repeated many times.
- It needs a probability estimate, and your estimate is usually the weakest part of the sum.
- Bookmaker margin pushes most ordinary bets below zero, so do not treat the formula as a way to win.
- 18+ only
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- Checked 5 November 2026
- Set limits first
Expected value, often shortened to EV, is the most useful single idea in betting maths. It turns a vague feeling about a price into a number you can compare. It does not predict the next result, and it cannot remove the risk of losing. This guide walks through the formula with a small example, explains where it goes wrong and shows how it connects to the odds you see on a bet slip.
The formula in plain words
EV equals the chance you win multiplied by what you gain, minus the chance you lose multiplied by what you lose. If the answer is above zero, the bet is worth taking on average. If it is below zero, you give up money on average every time you place it.
Take a simple coin-style example with decimal odds of 2.00 on an event you believe has a 50 percent chance. A stake of 100 returns 200 on a win, so the profit is 100. EV is 0.5 times 100 minus 0.5 times 100, which is zero. Now suppose the odds are 1.90. A win profits 90, so EV is 0.5 times 90 minus 0.5 times 100, which is minus 5 per 100 staked.
What this means for you: a small change in odds can turn a break-even bet into a losing one, and the loss is paid every time.
| Odds (decimal) | Your win chance | Profit if win | EV per 100 staked |
|---|---|---|---|
| 2.00 | 50% | 100 | 0 |
| 1.90 | 50% | 90 | -5 |
| 2.20 | 50% | 120 | +10 |
| 2.20 | 40% | 120 | -12 |
Where the probability comes from
The formula is only as good as your estimate of the chance. Odds themselves imply a probability: dividing one by the decimal odds gives the implied chance, which includes the bookmaker's margin. Our odds guide shows the conversion in more detail.
To find a positive EV bet you must believe the true chance is higher than the implied one. That requires data, such as pitch conditions, team news or form, and it requires honesty about how much you really know. Most bettors overrate their edge, and a few lucky results make this easy to forget.
The table above also shows the last trap: at the same odds, changing your estimate from 50 to 40 percent moves EV from positive to negative.
Why the margin matters
A bookmaker sets prices so that the implied chances of all outcomes add up to more than 100 percent. That excess is the margin. It means that if you backed every outcome at random, you would lose a slice of your stakes over time. It also means that positive EV is rare and tends to be small when it exists.
Casino games work the same way, but with a fixed house edge instead of a margin on odds. You cannot out-estimate a slot, which is why EV for casino games is negative by design and why betting systems fail, as in our piece on the Martingale system.
- Margin is built into every price
- Positive EV bets are rare and small
- A positive EV bet can still lose many times in a row
- Variance hides the true edge for a long time
Using EV without fooling yourself
Use EV as a filter, not a promise. If you cannot explain why the bookmaker's price is wrong, assume it is not. Keep a record of your bets, your estimated chances and the closing odds. If your estimates are consistently worse than the market, the honest conclusion is that you have no edge.
Also remember that EV says nothing about how much to stake. Even a real edge can ruin a bankroll if the stake is too large for the swings. A strict budget, as described in our loss limit guide, matters more than the maths. Cricket fans can apply the same thinking to cricket odds. Players must be 18 or older.
In three steps
Check a bet's value in 3 steps
- 1Convert the odds
Divide one by the decimal odds to see the implied chance, including margin.
- 2Write down your own estimate
State the reasons for your number before looking at the price.
- 3Compute EV
Multiply chance by profit, subtract chance of loss times stake, and skip bets below zero.