Quick answer: Implied probability is the percentage chance the sportsbook’s odds suggest an outcome is happening. A line of -200 implies 66.7% probability that the bet wins. A line of +300 implies 25%. Implied probability includes the sportsbook’s vig, so the implied probabilities of both sides of a bet add up to slightly more than 100% (typically 104-108%). The ‘true’ probability after stripping out vig is called the no-vig probability.
The Conversion Formulas
For American odds: negative odds (favorites) convert as |odds| / (|odds| + 100). So -150 = 150/250 = 60%. Positive odds (underdogs) convert as 100 / (odds + 100). So +200 = 100/300 = 33.3%. Decimal odds convert as 1/decimal. So 2.50 decimal = 1/2.50 = 40%. Fractional odds convert as denominator / (numerator + denominator). So 5/2 = 2/(5+2) = 28.6%. Memorize the American odds version because it’s what most US books quote.
Why Vig Inflates Implied Probability
If a coin flip were fairly priced, both sides would be at +100 (50% implied each, totaling 100%). Sportsbooks add vig by pricing both sides at -110 (52.4% implied each, totaling 104.8%). That extra 4.8% is the book’s hold. To find the ‘true’ probability, you have to remove the vig from both sides proportionally. The PropsBot No-Vig Calculator handles this math automatically, but the manual version: divide each side’s implied probability by the total of both sides’ implied probabilities.
Why This Matters for Your Bottom Line
If your model says a bet has 60% true probability and the book is offering it at -125 (55.6% implied probability), you have a 4.4% edge. Over a long sample, that compounds into real profit. PropsBot’s High ROI Signal hits 31.7% verified ROI on 101,881 MLB props by isolating bets with measurable edge over the no-vig probability. The Brier score on those props (0.1903) beats the Vegas closing line (0.1947), confirming the model’s probability estimates are more accurate than the market consensus.
Common Mistakes Bettors Make
First, comparing implied probability to gut-feel probability instead of model-derived probability. Gut feel is biased and inconsistent. Second, ignoring the vig when comparing two sides of the same market. The +100 to -110 swing across books often hides the real edge. Third, using implied probability as a proxy for win rate without considering parlay vig stacking. A 60% implied parlay leg dilutes by another 4-5% when combined with another 60% leg in a parlay.
Frequently Asked Questions
How do you calculate implied probability from American odds?
For negative odds: |odds| / (|odds| + 100). For positive odds: 100 / (odds + 100). So -200 = 66.7% and +300 = 25%.
What’s the difference between implied probability and true probability?
Implied probability includes the sportsbook’s vig. True probability (or no-vig probability) is the implied probability after removing the bookmaker’s margin. True probability is what you compare against your own model’s projection.
What’s a typical bookmaker’s vig in implied-probability terms?
Standard NFL spreads at -110 each side total 104.8%, meaning 4.8% vig. Player props often run 6-10% vig. Live betting and longshot props can run 12-15%.
Can implied probability go above 100%?
Single bets can’t, but the sum of both sides of a market always exceeds 100% because of vig. For multi-outcome markets like first-touchdown-scorer, the sum of all options’ implied probabilities is typically 115-130%.
How does PropsBot use implied probability?
Our model produces a probability estimate for every prop, then compares it to the no-vig probability derived from the book’s line. The gap is the Edge Score. Bets with positive Edge Score are flagged for the High ROI Signal.
Part of the PropsBot.AI Sports Betting Glossary. Updated 2026-05-04.
