Quick answer: Expected value (EV) is the average amount you win or lose per bet if you could repeat that exact bet thousands of times. You work it out by multiplying each possible result by its probability and adding them up. A positive EV means the bet pays more than its risk deserves; a negative EV means it pays less. Every casino game with a house edge has a negative EV, and most sports bets do too once the bookmaker’s margin is included.
The Formula in Plain Terms
For a simple win-or-lose bet:
EV = (chance of winning × profit if you win) − (chance of losing × stake)
With decimal odds there is an even shorter version: EV per peso staked = (probability × decimal odds) − 1. If the answer is −0.03, you lose 3 centavos per peso on average. If it is +0.05, you gain 5 centavos per peso on average.
The key word is average. EV does not tell you what will happen on the next bet. It tells you where your results are heading as the number of bets grows.
Worked Example 1: Red on European Roulette
A European wheel has 37 pockets: 18 red, 18 black and one green zero. A ₱100 bet on red wins ₱100 if the ball lands on red and loses ₱100 otherwise.
- Chance of winning: 18 ÷ 37 = 48.65%
- Chance of losing: 19 ÷ 37 = 51.35%
- EV = (0.4865 × ₱100) − (0.5135 × ₱100) = −₱2.70
That −2.70% is the same number usually quoted as the house edge of European roulette. House edge is simply EV expressed as a percentage, seen from the casino’s side.
Worked Example 2: A Sports Bet at 1.91
Odds of 1.91 on both sides of a match are common for spreads and totals. A ₱100 stake returns ₱191, a profit of ₱91.
| Your true chance of winning | EV calculation | EV per ₱100 |
|---|---|---|
| 50% | 0.50 × 91 − 0.50 × 100 | −₱4.50 |
| 52.4% | 0.524 × 91 − 0.476 × 100 | about ₱0 |
| 55% | 0.55 × 91 − 0.45 × 100 | +₱5.05 |
(Our calculations.) At a genuine coin-flip you lose ₱4.50 per ₱100 on average, because the bookmaker built a margin into the price. Our guide to the vig explains where that margin comes from. To break even at 1.91 you need to win 1 ÷ 1.91 = 52.4% of the time, and to profit you need to win more than that over many bets.
Worked Example 3: A Slot With 96% RTP
For games with many possible outcomes, such as slots, the published return to player (RTP) already is the EV. A 96% RTP means an EV of −4% of every peso wagered.
Play 500 spins at ₱10 and you have wagered ₱5,000. The expected loss is 4% of that, or ₱200. Notice that the expected loss grows with the total amount wagered, not with your starting balance. Re-betting the same ₱1,000 five times counts as ₱5,000 of action.
Why Your Results Do Not Match the EV
Over 500 spins, few players lose exactly ₱200. Some will be up thousands and others down far more. That spread is variance, and it is why EV is easy to ignore in the short term. A losing bet can be a good bet, and a winning bet can be a bad one. EV judges the decision; the result is partly luck.
Over a long run of bets, variance shrinks relative to the total amount wagered, and results drift towards the EV. This is why a casino with a small edge on every bet is reliably profitable across millions of bets, even though individual players win every day.
How Positive EV Is (Rarely) Found
On house-edge casino games, EV is negative by design and no betting pattern changes it. In sports betting, a positive EV only exists when your estimate of the true probability is more accurate than the bookmaker’s price. That is hard, because bookmakers employ specialists and adjust prices quickly.
If you do believe you have a positive-EV bet, the next question is how much to stake. Our guide to the Kelly criterion shows how stake size follows from the size of the edge, and why most people use a fraction of it.
A Quick EV Checklist Before Any Bet
- Convert the odds to a break-even probability (1 ÷ decimal odds).
- Ask honestly whether your chance of winning is higher than that, and why.
- If you cannot explain the reason, assume the EV is negative.
- For casino games, take the house edge or RTP as the EV and budget the expected loss as the cost of entertainment.
Frequently Asked Questions
Is expected value the same as house edge?
They are two views of the same number. House edge is the casino’s expected profit as a percentage of each bet; your EV is the same figure with a minus sign.
Can a betting system turn negative EV into positive EV?
No. Each bet keeps its own EV, so a series of negative-EV bets adds up to a negative total no matter how the stakes are arranged.
Do bonuses change the EV?
They can add value, but wagering requirements usually take much of it back. Our guide to wagering requirements shows how to calculate what a bonus is really worth.
How many bets before results match the EV?
There is no fixed number. Low-variance bets settle faster; high-volatility slots and long-odds bets can take many thousands of plays.
Final Thoughts
Expected value is the single most useful number in gambling because it strips away the excitement and shows the price of each bet. For casino games, that price is always negative, so treat it as the cost of the entertainment rather than something to beat.
Betting is for adults aged 21 and over and always carries the risk of losing money, so only stake what you can afford to lose. If gambling has stopped being entertainment, our responsible gambling page lists the support available in the Philippines.