Odds show up in three common formats -- American (-200, +150), decimal (1.50, 2.50), and fractional (1/2, 3/2) -- and they all describe exactly the same underlying probability, just formatted differently for different regions and traditions. Converting between them is pure arithmetic, not analysis of any specific game or matchup.
American odds to decimal converter, with the formula
Negative American odds (like -200) convert to decimal with 1 + 100/|odds|, giving 1.50. Positive American odds (like +150) convert with 1 + odds/100, giving 2.50. The negative sign marks the favorite; the positive sign marks the underdog.
How to convert betting odds to probability
Once you have decimal odds, implied probability is simply 1 / decimal. For -200 (decimal 1.50), that's 66.67%. For +150 (decimal 2.50), that's 40%. The two sides of a matchup don't sum to exactly 100% in practice -- the gap is the bookmaker's built-in margin.
Negative odds implied probability calculator: worked example
Take -110, a common number for point-spread bets: decimal odds are 1 + 100/110 = 1.91, and implied probability is 1/1.91 = 52.38%. That's higher than a true coin flip (50%), which is exactly the margin built into the line.
An odds payout calculator for a specific stake
Odds alone don't tell you what you'd actually collect. Total payout is stake × decimal odds, and profit is stake × (decimal odds − 1). On a $100 stake at -200, that's a $150 payout and $50 profit.
The Betting Odds Converter handles all three formats plus implied probability and stake payout in one screen -- enter odds in whichever format you have, see the rest instantly.
Try the Betting Odds Converter →
This is a math and format-conversion reference, not betting or financial advice.