What is Expected Value (EV)?
Expected Value (EV) is a measure of the average outcome of a bet if it were placed many times. Positive EV (+EV) means the bet is expected to return profit over the long run. Negative EV (-EV) means the bet is expected to produce a loss over the long run.
EV does not predict the outcome of a single bet — a +EV bet can lose, and a -EV bet can win on any given night. But over hundreds of bets, positive EV compounds into profit, and negative EV compounds into loss. This is why professional bettors focus on finding +EV opportunities rather than just picking winners.
The EV Formula
EV = (Win Probability × Net Profit per unit) − (Lose Probability × Stake per unit)
Where: - Win Probability = your estimated probability that the bet wins (expressed as a decimal, e.g., 0.55 for 55%) - Net Profit per unit = the amount you win per unit risked (decimal odds − 1) - Lose Probability = 1 − Win Probability - Stake per unit = 1 (normalized)
Simplified: EV = (p × b) − (1 − p) Where p = win probability and b = decimal odds − 1
Example: Over 25.5 Points at −110 American odds (1.909 decimal) - Estimated win probability: 58% (0.58) - b = 1.909 − 1 = 0.909 - EV = (0.58 × 0.909) − (0.42 × 1) = 0.527 − 0.420 = +0.107 - EV% = +10.7% per unit wagered — a strongly positive bet
Converting American Odds to Decimal
EV calculations require decimal odds. Here is how to convert from American format:
For positive American odds (e.g., +150): Decimal = (American / 100) + 1 → (+150 / 100) + 1 = 2.50
For negative American odds (e.g., −110): Decimal = (100 / |American|) + 1 → (100 / 110) + 1 = 1.909
You can also find the implied probability directly: - Positive odds: Implied probability = 100 / (American + 100) → 100 / 250 = 40% - Negative odds: Implied probability = |American| / (|American| + 100) → 110 / 210 = 52.4%
When your estimated true probability exceeds the implied probability, you have a potential edge (+EV). PropLab's EV Calculator handles these conversions automatically — just enter odds and your win probability estimate.
Positive vs. Negative EV: Worked Scenarios
| Scenario | American Odds | Implied Probability | Your Win Estimate | EV per $100 | Decision |
|---|---|---|---|---|---|
| Points Over | -110 | 52.4% | 58% | +$10.70 | +EV — consider betting |
| Assists Under | -115 | 53.5% | 50% | -$6.50 | -EV — skip |
| 3-Pointers Over | +120 | 45.5% | 50% | +$7.00 | +EV — consider betting |
| Rebounds Over | -130 | 56.5% | 52% | -$9.50 | -EV — skip |
| Points Under | -105 | 51.2% | 55% | +$7.40 | +EV — consider betting |
Common EV Mistakes in NBA Prop Betting
1. Ignoring the vig: Standard -110 odds require a 52.4% win rate just to break even. Many bettors evaluate only line quality without accounting for the vig embedded in the odds.
2. Overestimating edge: A 55% win probability estimate that is actually 51% will produce calculated +EV that is actually -EV in practice. Be conservative with your probability estimates.
3. Confusing short-term results with EV: Winning 7 of 10 bets tells you little about whether those bets were +EV. EV is a long-run property, not a short-term outcome measure.
4. Betting -EV to "stay in action": Every -EV bet you place is an expected loss. Volume does not improve negative-expectation bets — it compounds the loss.
5. Not accounting for line shopping: A -EV bet at -115 might be +EV at -108. Getting the best available line is part of EV calculation, not separate from it.
Using PropLab's EV Calculator
PropLab's EV Calculator at /tools/ev-calculator lets you enter odds in American, decimal, or fractional format alongside your win probability estimate. It instantly shows:
- EV% per unit wagered - Expected profit or loss per $100 bet - Edge (your estimated probability minus implied probability) - Implied probability from the sportsbook's odds
Use the calculator in your pre-bet research workflow: enter the line you found, your PropLab model probability estimate, and see whether the bet clears a meaningful +EV threshold before committing capital.
Frequently Asked Questions
- What does +EV mean in betting?
- +EV (positive expected value) means a bet is mathematically expected to return profit over a large sample. A +EV bet can lose on any given night, but consistently betting +EV opportunities should produce profit over hundreds of bets.
- How do I calculate EV on an NBA prop?
- EV = (Your Win Probability × Net Profit per unit) − (Your Lose Probability × Stake per unit). Convert American odds to decimal first (e.g., -110 → 1.909). Then: EV = (p × (decimal odds − 1)) − (1 − p). PropLab's EV Calculator does this automatically.
- What win rate do I need to break even at -110?
- At -110 odds you need to win 52.38% of bets to break even over the long run. At -115, the break-even rate is 53.49%. At -105, it drops to 51.22%. Getting better odds (closer to -100 or positive) directly lowers your break-even threshold.
- Can I have positive EV but still lose money?
- Yes. EV is a long-run expectation. In the short term (under 200 bets), variance means even a strong +EV strategy will produce losing stretches. You need a large enough sample for the mathematical edge to express itself.
- How does line shopping improve EV?
- Getting a better line directly improves EV. If a prop is at -115 on one book and -108 on another, the -108 line has meaningfully better EV even though the underlying bet is the same. Shopping lines is one of the highest-ROI habits in prop betting.
- Does the EV Calculator account for the sportsbook vig?
- Yes. The implied probability automatically reflects the vig embedded in the odds. When your estimated win probability exceeds the implied probability, you have found an edge — but only if your probability estimate is accurate.