Eight tools for the math behind every bet, free to anyone. No-vig fair odds, Kelly stake sizing, parlay math, EV, hedging — all the math, none of the gimmicks.
Why use it: Every sportsbook bakes a profit margin (the "vig" — typically 4-7%) into their odds. The true fair odds are hidden underneath. Devigging strips out the vig to show what a fair, no-margin book would offer.
How it helps: Compare your own win probability to the devigged fair probability. If your model says 56% but fair is 52%, you have a 4 percentage point model-vs-market gap — a positive-EV opportunity by the math.
Example: Both sides at -110 → 52.4% / 52.4% implied, but fair is 50% / 50%. Book is taking ~4.8% hold.
Why use it: Stop guessing whether a bet is "good." EV (expected value) tells you exactly how much you should expect to win or lose per dollar wagered over the long run.
How it helps: If EV is positive, the bet is mathematically profitable in the long run. If negative, the book has the mathematical advantage — it's -EV over time.
Example: If you think a team has a 55% true win probability at -110 odds, EV on a $100 stake is +$4.76. You would average +$4.76 profit per identical bet over thousands of trials.
Why use it: The Kelly Criterion is the mathematically optimal bet size given your model-vs-market gap. Bet too small, you leave growth on the table. Bet too big, one bad streak ruins your bankroll.
How it helps: Type in your bankroll, your win probability, and the odds — get the stake size the Kelly formula produces for maximum long-run growth. We default to quarter-Kelly because full Kelly is too volatile in practice. Quarter- to half-Kelly is common to survive variance.
Example: 55% win prob at -110 odds with a $1000 bankroll → bet $24 at quarter-Kelly. Full Kelly would be $97 (10% of bankroll — too risky for most).
Why use it: Calculate the combined odds and payout when combining 2-12 bets into a single ticket. Helps you decide if a parlay payout justifies the lower hit rate vs straight bets.
How it helps: See exactly what you would win if all legs hit, and your implied probability of cashing the whole ticket. Most parlays are negative-EV because vig compounds across legs — this calc shows you the math clearly.
Example: Three -110 legs → +595 combined odds, $695 payout on a $100 stake. Implied probability of all hitting: ~12.5%.
Why use it: After a long-shot bet cashes (or the first leg of a big parlay hits), you can reduce your remaining outcome risk by betting the opposite side. Hedging reduces risk — it doesn't remove it, since execution and line movement still matter.
How it helps: Shows how much to wager on the hedge to balance the outcomes whether the original bet wins or loses.
Example: $100 at +500 wins → $600 payout if it cashes. Hedge with $240 at -150 on the opposite side → ~$260 profit either way.
Why use it: Find out how much "vig" or hold % the sportsbook is taking on a two-way market. Sharper books charge less, soft books charge more.
How it helps: Shows which books offer the best posted price. Low-hold books = closer to fair odds = better long-term value. High-hold books are taking more from you on every bet.
Example: -110 / -110 = 4.55% hold (mainstream). -105 / -105 = 2.38% hold (sharp). +100 / +100 = 0% hold (zero-vig promo).
Why use it: Sportsbooks display odds in different formats — American (-110), decimal (1.91), fractional (10/11). Convert any format to a probability percentage to see what the book thinks the chances actually are.
How it helps: Compare the book implied probability to your own win probability instantly. If your model says higher than the implied, your number differs from the book's.
Example: -110 → 52.4% implied. +200 → 33.3%. -200 → 66.7%. Type one format and the others auto-fill.
Why use it: Sportsbooks offer "risk-free" promos where if you lose, you get a refund as site credit (NOT cash). The true cash value depends on how much of that credit you can actually convert to cash.
How it helps: Calculate the actual cash value of any risk-free promo before claiming it. Higher odds + higher conversion rate = better value. A common assumption is ~70% credit-to-cash conversion.
Example: $500 risk-free bet at +100 odds with 70% conversion → +$175 expected cash value (35% of stake). Higher odds with the same conversion rate produces better EV.