How do I convert American odds to implied probability?
Short answer: For negative odds, divide the odds by the odds plus 100: -150 is 150/250 = 60%. For positive odds, divide 100 by the odds plus 100: +130 is 100/230 = 43.5%. Both sides of a bet add up to more than 100%; the excess is the book's margin.
Negative odds (the favourite)
Probability = odds ÷ (odds + 100), using the number without its minus sign. -150: 150 ÷ 250 = 60%. -110: 110 ÷ 210 = 52.4%.
Positive odds (the underdog)
Probability = 100 ÷ (odds + 100). +130: 100 ÷ 230 = 43.5%. +200: 100 ÷ 300 = 33.3%.
Why both sides add up to more than 100%
Over -115 (53.5%) and under -105 (51.2%) add to 104.7%. The extra 4.7 points is the book's margin. Divide each side by the total to get the fair, no-vig probability: 53.5 ÷ 104.7 = 51.1% (full steps: how to calculate a no-vig price).
From probability back to odds
Going the other way is useful when you have your own estimate. If you think a player clears his line 58% of the time, the fair American price is 58 ÷ 42 × 100 = -138. If you think it is 40%, the fair price is 60 ÷ 40 × 100 = +150. A book offering a better number than your fair price is offering more than you think the bet is worth; a worse number means you are paying for it.
Do it without the arithmetic
The free tools page converts odds and strips the vig for you, and the props page shows both books' prices for every line tonight.
Decimal and fractional odds
Decimal odds (common in Europe and at Pinnacle) are simpler: probability = 1 ÷ decimal odds. Decimal 2.50 is 40%. To convert American to decimal: for positive odds, divide by 100 and add 1 (+150 is 2.50); for negative odds, divide 100 by the odds and add 1 (-200 is 1.50). Fractional odds such as 5/2 convert to decimal by dividing and adding 1, so 5/2 is 3.50.
A quick reference
-200 is 66.7%. -150 is 60%. -120 is 54.5%. -110 is 52.4%. Even money (+100) is 50%. +120 is 45.5%. +150 is 40%. +200 is 33.3%. +300 is 25%.
Why the probability matters more than the payout
A price is only a good bet if your estimate of the true probability is higher than the implied one after removing the margin. Long-shot prices look attractive because of the payout, but each one still has to clear its implied probability. Thinking in percentages makes it easier to compare a -150 favourite with a +130 underdog, or the same prop at two books.
Related questions
Market data and education only. OpenSlate does not publish free picks and does not tell you what to bet.
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