Convert American, decimal, and fractional betting odds to implied probability with EV analysis, Kelly criterion, edge visualization, and vig breakdown.
The implied probability calculator converts betting odds from any format — American (+/−), decimal, or fractional — into the true probability the bookmaker has priced in. Understanding implied probability is essential for identifying value bets where your estimated probability exceeds the bookmaker's.
Beyond simple conversion, this tool computes expected value (EV), return on investment (ROI), Kelly criterion optimal bet sizing, and edge analysis. A visual comparison shows your estimated probability against the implied probability, making it easy to spot value.
The reference table covers common odds ranges, and the vigorish breakdown reveals how much margin the bookmaker is charging — helping you evaluate different sportsbooks and lines. Check the example with realistic values before reporting. Use the steps shown to verify rounding and units. Cross-check this output using a known reference case. Use the example pattern when troubleshooting unexpected results. Validate that outputs match your chosen standards. Run at least one manual sanity check before publishing.
Converting between odds formats by hand is error-prone, and assessing value requires combining implied probability with your own estimate. This calculator handles all conversions instantly and provides the full decision toolkit — EV, edge, Kelly sizing — in one place.
Whether you're a casual bettor checking line value or a quantitative analyst building models, understanding implied probability is the foundation of profitable sports betting.
American (+): Implied = 100/(odds+100). American (−): Implied = |odds|/(|odds|+100). Decimal: Implied = 1/odds. Fractional (a/b): Implied = b/(a+b). EV = (trueProb × profit) − ((1−trueProb) × bet).
Result: Implied Probability = 28.57%, EV = $8.75, Edge = +6.43%
Odds +250 imply a 28.57% chance. If you believe the true probability is 35%, you have a 6.43% edge and positive expected value of $8.75 per $100 bet.
The vig, or juice, is how bookmakers profit. In a fair coin-flip market, both sides would be priced at +100 (50%). Instead, bookmakers might offer −110 on both sides, implying 52.4% each for a total of 104.8%. The 4.8% overround is the vig.
Professional bettors track whether their bets had better odds than the closing line. Consistently beating the close implies genuine predictive edge, even during losing streaks. The closing line is considered the most efficient market price.
Successful bettors are well-calibrated: events they assign 60% probability to actually happen about 60% of the time. Combine calibrated estimates with Kelly or fractional Kelly sizing, and track results over 1,000+ bets to distinguish skill from variance.
It's the probability embedded in the odds. If odds pay 2-to-1 (decimal 3.0), the implied probability is 33.3%. It represents the break-even win rate needed to profit long-term.
A value bet occurs when your estimated true probability of an outcome exceeds the implied probability from the odds. If you believe a team wins 45% of the time but odds imply only 35%, that's a value bet.
A formula for optimal bet sizing: f* = (bp − q) / b, where b = decimal odds − 1, p = true probability, q = 1 − p. It maximizes long-term bankroll growth but can be aggressive — most bettors use half-Kelly.
The excess over 100% is the vigorish (vig) — the bookmaker's margin. On a typical American football spread, implied probabilities might total 104-105%, meaning ~4-5% goes to the house.
Decimal odds are simplest for calculation (payout = bet × odds). American odds are standard in the US. Fractional odds are traditional in the UK. All encode the same information.
Even small edges (2-5%) are profitable long-term with proper bankroll management. But you need hundreds of bets for the edge to manifest — variance is high in the short run.