What is positive expected value (+EV) betting?

A bet has positive expected value (+EV) when the price you're offered pays more than the outcome's true chance of happening justifies. Make enough +EV bets and, on average, you should come out ahead. The difficulty is that nobody knows the true chance; +EV betting is really about making a better estimate than the sportsbook you're betting with. "Value betting" usually means the same thing.

The formula

For a bet at decimal odds d on an outcome with true probability p:

EV per unit staked = p × d − 1

If that's above zero, the bet is +EV. Example: you believe a game is a genuine 50/50, and a sportsbook offers 2.10 (+110) on one side. EV = 0.50 × 2.10 − 1 = +0.05, or +5% of your stake. On a $100 bet you expect to make $5 on average.

Notice what that doesn't mean. This bet either wins $110 or loses $100. The $5 is the long-run average over many similar bets, not the result of this one.

Where "true probability" comes from

Since the true probability is unknown, +EV bettors estimate it. The most common shortcut is to use a sharper market as the reference:

1. Take a market that's considered efficient (high limits, quickly corrected).
2. Remove that sportsbook's margin to get no-vig (fair) probabilities.
3. Look for other sportsbooks offering better prices than that fair line.

That's what most positive-EV scanners automate. Their output is only as good as the reference line: if the "sharp" price is stale or wrong, the "edge" can be imaginary.

How you know it's working

With a few percent edge and outcomes near 50/50, results are dominated by luck for a long time. Two habits help:

• Track closing line value (CLV). If you regularly get better prices than the final market price, your estimates are probably good. See Closing line value (CLV) explained.
• Judge on volume, not weeks. Hundreds of bets can still show a loss with a genuine edge.

Beating the closing line is also what gets accounts limited: Massachusetts regulators found that players who consistently beat the closing line were more likely to have their limits lowered.^[1]

The costs people forget

A +5% edge isn't 5% profit:

• Subscriptions. A scanner that costs $99 a month needs about $1,980 of turnover at a 5% edge just to break even, and roughly $4,950 at 2%. Model your own numbers with the break-even calculator.
• Missed and partial bets. Prices move before you can bet, and stakes get cut.
• Limits. The better you are, the sooner your best sportsbooks shrink your stakes.