Implied Probability & Expected Value (EV) in Sports Betting

The 30-second answer

Implied probability is the win rate hidden inside a price: 1 divided by the decimal odds. Expected value (EV) is what a bet earns on average per unit staked if you could place it thousands of times. Profitable betting is nothing more than repeatedly finding bets where your true probability is higher than the implied probability.

Step 1 — Convert the price

• Decimal 1.91 → 1 / 1.91 = 52.4%
• Decimal 2.50 → 1 / 2.50 = 40.0%
• American -150 → 150 / (150 + 100) = 60.0%
• American +200 → 100 / (200 + 100) = 33.3%

Step 2 — Remove the vig

Both sides of a market add up to more than 100%. A soccer moneyline might be Home 2.10 (47.6%), Draw 3.40 (29.4%), Away 3.60 (27.8%) = 104.8%. Divide each by 1.048 to get the fair probabilities: 45.4%, 28.1%, 26.5%. Those are the numbers your own estimate has to beat.

Step 3 — Calculate EV

EV = (win probability × profit if won) − (loss probability × stake).

Suppose you believe the home team wins 50% of the time and the price is 2.10.

• Win: 0.50 × 1.10 = +0.55
• Loss: 0.50 × 1.00 = −0.50
• EV = +0.05 per unit (+5%)

A positive number means the bet is +EV. Repeat that bet 200 times at 1 unit each and you expect roughly +10 units, even though you will lose about half of them.

Step 4 — Turn EV into a stake

Edge alone doesn't tell you how much to bet. Flat staking (a fixed unit every time) is the simplest; the Kelly criterion scales the stake to the size of the edge. Most professionals use a fraction of Kelly to reduce variance. See flat betting vs Kelly.

Where do "true" probabilities come from?

This is the whole game. Sources include:

• Statistical models — expected goals, efficiency ratings, Elo, player projections.
• Closing line value — if you consistently beat the closing price, the market is telling you your estimates are good.
• Sharp book comparison — low-margin books approximate the fair price better than recreational ones.

Saint Picks publishes a fair price, projected edge and confidence with every selection so you can see the EV calculation rather than just the pick. The bet analyzer runs the same math on any slip you paste in.

Common mistakes

• Judging a bet by whether it won instead of whether it was priced right. Good bets lose; bad bets win.
• Forgetting to remove the vig before comparing.
• Confusing "likely" with "valuable". A 1.10 favourite at 91% implied can be badly overpriced.
• Betting +EV edges with stakes so large that variance ends the bankroll before the edge shows up.