How it works
Bookmakers quote the same price in three styles. Fractional odds (5/2) show profit relative to stake. Decimal odds (3.50) show the total return per $1 staked, including the stake. American odds show the profit on a $100 stake when positive (+250) or the stake needed to win $100 when negative (−150).
The implied probability is the break-even win rate: 1 ÷ decimal odds. Odds of 5/2 (3.50) imply 28.57%, so the bet is only profitable in the long run if the true chance of winning is higher than that.
Fractional odds are reduced to the simplest fraction with a denominator up to 1,000, so some conversions from decimal or American odds show unusual fractions.
Formula
Decimal = fractional + 1 · American = (decimal − 1) × 100 if decimal ≥ 2, else −100 ÷ (decimal − 1) · Implied probability = 1 ÷ decimal
Questions people ask
What does −150 mean in American odds?
You must stake $150 to win $100 profit. That equals decimal odds of 1.667 and an implied probability of 60%.
What are even odds?
Even money is 1/1 fractional, 2.00 decimal and +100 American, with an implied probability of 50%.
Why do the implied probabilities of all outcomes add up to more than 100%?
The excess is the bookmaker’s margin (the overround or vig). To estimate fair probabilities, divide each implied probability by the total.