How betting odds convert to implied probability
By George Boyle · Updated 2026-09-19 · The Sport Stack
Implied probability is the win rate a betting price assumes. For American odds, a favourite at -150 implies 150 ÷ (150 + 100) = 60%, and an underdog at +200 implies 100 ÷ (200 + 100) = 33.3%. For decimal odds it is simply 1 divided by the price. The two sides of any market add to more than 100%, and the excess is the sportsbook’s margin.
What is the formula for American odds?
For a negative price, divide the size of the number by itself plus 100: at -110 that is 110 ÷ 210 = 52.4%, at -200 it is 200 ÷ 300 = 66.7%. For a positive price, divide 100 by the price plus 100: at +110 that is 100 ÷ 210 = 47.6%, at +350 it is 100 ÷ 450 = 22.2%. The sign tells you which formula to use; the size of the number tells you how far from a coin flip the book thinks the outcome is.
A useful anchor: -110 on both sides of a market is the standard price, and it implies 52.4% for each side. Any time you see a bet at -110, that is the win rate you need to break even on it in the long run.
What about decimal and fractional odds?
Decimal odds are the total return per unit staked, so the implied probability is one divided by the price: 2.50 implies 40%, 1.50 implies 66.7%, and 4.00 implies 25%. Fractional odds of a/b pay a for every b staked, and imply b ÷ (a + b): 5/2 implies 2 ÷ 7 = 28.6%, and evens (1/1) implies 50%.
All three formats describe the same price and convert to the same probability, which is why comparing bets across books and formats is easier in percentages than in odds.
Why do the two sides add to more than 100%?
Add the implied probabilities of both sides of a -110 / -110 market and you get 52.4% + 52.4% = 104.8%. Real outcomes can only sum to 100%, so the extra 4.8 points is the book’s margin, often called the vig or the overround. The bigger the sum, the more the book is charging.
Because of that margin, an implied probability is always a little higher than the book’s actual estimate of the outcome. Removing the margin to recover the book’s real opinion is called devigging, and it is the step between reading a price and judging one.
What does implied probability tell you?
It is the win rate you need to break even at that price, which makes it the yardstick for every bet. If your own estimate of an outcome is higher than the price implies — after the margin has been removed — the bet has positive expected value; if it is lower, no amount of confidence in the pick makes the price right.
It is also the input every sizing rule needs. The Kelly criterion, for instance, sizes a bet from the gap between your probability and the market’s, so the conversion above is the first line of that calculation too.
Common questions
What is the implied probability of -110?
About 52.4%. Divide 110 by 110 plus 100 and you get 0.524. It is the standard price on point spreads and totals, so 52.4% is the win rate a bettor needs to break even on the standard bet.
Does implied probability equal the true probability?
No. It is the probability the price assumes, which includes the sportsbook’s margin and whatever bias the book’s customers have introduced. The two sides of a market always add to more than 100%, so each side is overstated; the market’s real opinion only appears once that margin is removed.
Why is the break-even win rate 52.4% at -110?
Because at -110 you risk 110 to win 100. Win 52.4% of the time and the 100-unit wins exactly cover the 110-unit losses, leaving nothing. Anything below that rate loses money over time even if you pick more winners than losers.
Written by George Boyle, who builds The Sport Stack — the models, the public ledger and these explainers. Corrections and questions: hello@thesportstack.io. Who runs this.