UK bookmakers traditionally use fractional odds (3/1, 5/2, evens), continental bookmakers use decimal odds (4.00, 3.50, 2.00), and US sportsbooks use American odds (+300, -150). All three formats express the same thing — the payout relative to your stake — but converting between them by hand is error-prone. This calculator handles the conversion instantly and shows the implied probability for each format.

Enter odds in any format and the calculator shows all three formats plus the implied probability (what the bookmaker thinks the true probability is, including their margin). The "overround" or "vig" is the percentage above 100% that all outcomes sum to — it represents the bookmaker's profit margin. Typical overrounds: Premier League match result 102–106%, NBA spread 104–107%, horse racing each-way 115–130%.

For matched betting specifically: this calculator is essential for finding qualifying bets and free bet conversions. Convert the bookmaker's odds to implied probability, calculate the lay stake at Betfair/Smarkets to lock in profit, then verify the expected value before placing. See our parlay calculator for multi-selection bets.