Which books we read
The tools read live prices from the US sportsbooks, betting exchanges and prediction markets we track: DraftKings, FanDuel, BetMGM, Caesars, theScore Bet, Fanatics, BetRivers, Hard Rock Bet, Bally Bet, Fliff, Novig, ProphetX and Kalshi. Where a book or market is only available in some states, the tools say so. Offshore books are not included.
Fair odds: how we decide what a price is worth
For each market we take every book that posts the whole market (both sides of a spread or total, every outcome of a moneyline), remove each book's margin (the vig), and take the median fair probability across those books. The fair probabilities for a market are then scaled to add up to exactly 100%, so both sides of a game can never look like value at once. A book that posts only one side still shows its price and can still be the best price, but it never counts toward the fair line. Exchanges and prediction markets show their prices but never set the fair line.
Expected value (EV)
EV compares the best price you can actually bet with the fair probability. A price is +EV when it pays more than the fair odds say it should. Our checks before anything is flagged:
- At least two books must post the complete market for a game line, and at least three for a player prop, or there is no fair line and no EV.
- An EV above 25% is treated as a broken line, not an opportunity, and is never shown as an edge.
- Long shots must clear a higher bar, because small errors in a 20% probability swing EV the most.
- Games posted months ahead are browsable, but their markets are too thin to badge an edge.
- Each signal carries a trust score built from how many books agree, whether any quote looks stale, how large the edge is, and whether market splits point the same way.
Fees are counted, not ignored
Exchanges and prediction markets charge fees that sportsbooks do not. Every price from them competes, and is priced for EV, at what it actually pays after the fee: for example, 2% of net winnings on ProphetX, and the published taker fee on prediction markets. The posted price still displays as posted, so you can check it at the source.
Freshness rules
- A price not updated in 25 minutes before a game, or 3 minutes during a live game, stays visible but can never be the best price, feed the fair line, or create an arbitrage.
- A price with an unknown or impossible timestamp is treated the same way.
- Extreme prices (800 or longer either way, such as -800 or +800) and thin exchange quotes on long shots never win best price.
- The boards show when their prices were last updated.
Arbitrage and middles
An arbitrage appears only when the best prices on both sides of the same line, at two different books, still leave a profit on either outcome after fees are applied. Anything showing more than 12% is treated as a stale line and dropped. Cards with an exchange or prediction-market leg are marked and hidden by default, because a resting exchange order is not a firm sportsbook price. A whole-number spread or total is flagged when both bets could push. Stake splits are shown so both legs return the same amount.
Arbitrage math only holds if both bets are accepted at those prices. Books can move a line or limit an account, so treat every arb as a quote to confirm, not money in hand.
How we check the tools against results
Every line signal the +EV board shows is logged internally with its price, book and time, the first time it appears. After the game it is graded against the official final, and where we can, the closing price is captured to measure closing-line value. This ledger is how we find out whether a rule is working, and rules that do not hold up are changed. Tool signals are graded separately from our picks and are never added to any program's record.
The rules behind every graded result are on how we grade, and the full record lives on the records page. How we are paid is spelled out in the affiliate disclosure, and help is always on our responsible gambling page.
The calculators
The free calculators (odds converter, parlay, no-vig, hedge, arbitrage, middles, Kelly, round robin and bet calculator) run standard, published betting math in your browser. They are checked against worked examples in our test suite before each release, and anyone can embed them.
Frequently asked questions
How does the +EV board decide a bet is +EV?
It removes the vig from every book that posts the full market, takes the median fair probability, and compares that with the best price you can actually bet after any fee. At least two complete books are required for a game line and three for a player prop, and anything above 25% EV is treated as a broken line.
Do the tools use stale odds?
A price older than 25 minutes before a game, or 3 minutes live, still displays but can never be the best price, set the fair line or create an arbitrage.
Can an arbitrage bet still lose money?
Yes. The math holds only if both bets are accepted at the posted prices. Lines move and books limit accounts, so confirm both prices before you bet.