Expected Value in Betting: How to Calculate EV
Expected value is the single metric that separates winning bettors from losing ones. Here's how to calculate it and why nothing else matters long-term.
What is expected value?
Expected value (EV) is the average amount you win or lose per bet if you placed it infinite times. It's a single number that tells you whether a bet is worth taking.
The formula is simple:
EV = (Probability of Winning x Profit) - (Probability of Losing x Stake)
That's it. Every profitable bettor in history is doing some version of this calculation, whether they know it or not.
A concrete example
You find a line at +150 on an NBA underdog. You believe (based on a sharp market or your own model) the true probability is 45%.
- Win probability: 45%
- Lose probability: 55%
- Profit if you win: $150 (on a $100 bet)
- Loss if you lose: $100
EV = (0.45 x $150) - (0.55 x $100) = $67.50 - $55.00 = +$12.50
On every $100 wagered, you expect to make $12.50 on average. That's a 12.5% edge. Take that bet every single time.
The trap: confusing EV with results
You will lose that +EV bet 55% of the time. More than half your bets will lose. This is where most people quit.
A bet can be +EV and still lose. A bet can be -EV and still win. Results over 10 or even 100 bets tell you almost nothing. Expected value only reveals itself over hundreds or thousands of bets.
If your sample size is small, you're not evaluating your strategy. You're evaluating your luck.
Where does the "true probability" come from?
This is the hard part. EV is only useful if your probability estimate is accurate. Common approaches:
- Sharp lines. Pinnacle's closing line is widely considered the most efficient probability estimate in sports betting. De-vig their odds to get the true line.
- Your own model. If you have an edge — injury information, weather data, situational factors the market hasn't priced in — your model's probability is your input.
- Prediction markets. High-liquidity markets on Kalshi or Polymarket can serve as probability benchmarks, especially for non-sports events.
Use the de-vig calculator to strip the vig from any two-way or multi-way market and get the true implied probability.
EV and bet sizing
Knowing a bet is +EV is step one. Step two is sizing it correctly. Bet too much on a +EV proposition and a losing streak can still wipe you out. The Kelly Criterion solves this — it tells you exactly what fraction of your bankroll to wager based on your edge.
The bottom line
Every bet you place is either +EV or -EV. There is no third option. If you're not calculating expected value, you're gambling. If you are, you're investing. And if you want to track whether your edge is real or just luck, closing line value is the metric that tells the truth.
Run any bet through the EV calculator before you place it. The math doesn't lie, and it doesn't care about your gut feeling.