Implied Probability Calculator: 3 Formulas to Convert Any Odds Format

What is Implied Probability?

Implied probability is the win percentage baked into betting odds. It tells you what probability the sportsbook or market is assigning to an outcome.

For every set of odds, there's a corresponding implied probability. A +150 underdog has an implied probability of 40%. A -200 favorite has an implied probability of 66.67%. The odds converter runs the calculation instantly. This guide shows you the math.

Implied probability is not the same as true probability. Sportsbooks add vig to both sides, so the implied probabilities sum to more than 100%. If you want the true win probability with the vig removed, use the true probability calculator instead.

How to Convert American Odds to Implied Probability

American odds come in two flavors: positive (underdogs) and negative (favorites). The formula changes depending on which you're working with.

For positive odds (underdogs):

Implied Probability = 100 / (American Odds + 100)

Example: +150 odds

• 100 / (150 + 100) = 100 / 250 = 0.40 = 40%

For negative odds (favorites):

Implied Probability = -American Odds / (-American Odds + 100)

Example: -200 odds

• -(-200) / (-(-200) + 100) = 200 / 300 = 0.6667 = 66.67%

Run any American odds through the implied probability calculator to see the conversion instantly. The tool handles both positive and negative odds without switching formulas.

How to Convert Decimal Odds to Implied Probability

Decimal odds are common in Europe, Australia, and on prediction markets like Polymarket and Kalshi. The formula is simpler than American odds.

Formula:

Implied Probability = 1 / Decimal Odds

Example: 2.50 decimal odds

• 1 / 2.50 = 0.40 = 40%

Example: 1.50 decimal odds

• 1 / 1.50 = 0.6667 = 66.67%

Decimal odds of 2.00 are exactly 50%. Higher than 2.00 = implied probability under 50%. Lower than 2.00 = implied probability over 50%.

How to Convert Fractional Odds to Implied Probability

Fractional odds are traditional UK betting notation. They show profit relative to stake.

Formula:

Implied Probability = Denominator / (Numerator + Denominator)

Example: 3/2 odds

• 2 / (3 + 2) = 2 / 5 = 0.40 = 40%

Example: 1/2 odds

• 2 / (1 + 2) = 2 / 3 = 0.6667 = 66.67%

For more on fractional odds and how they relate to American and decimal formats, see the odds formats guide.

Why Implied Probabilities Sum to More Than 100%

Take a standard NFL point spread:

• Patriots -3 at -110
• Bills +3 at -110

Implied probability of Patriots covering: -(-110) / (-(-110) + 100) = 110 / 210 = 52.38%

Implied probability of Bills covering: -(-110) / (-(-110) + 100) = 110 / 210 = 52.38%

Total: 52.38% + 52.38% = 104.76%

The extra 4.76% is the vig. The sportsbook charges roughly 4.76% on this market. That overround is how the book makes money. If you bet both sides, you lose 4.76% guaranteed.

The implied probabilities include the vig. If you want the fair probabilities without the vig, you need to de-vig the line using the de-vig calculator. That gives you the true probability the market is pricing.

When to Use Implied Probability

Implied probability is the first step in any EV calculation. To find expected value, you compare the implied probability from the odds to your true win probability estimate.

Basic EV formula:

EV = (True Probability × Profit) - ((1 - True Probability) × Stake)

If a bet has 45% true probability and the sportsbook is offering +150 (40% implied probability), you have edge. The sportsbook is underpricing the outcome. That's a +EV bet.

Run the full calculation through the EV calculator to see if the edge is large enough to justify a bet given the vig and your bankroll.

Implied Probability in Prediction Markets

Prediction markets like Kalshi, Polymarket, and Robinhood price contracts in cents on the dollar. A contract trading at $0.65 has an implied probability of 65%.

The conversion is direct: Price = Implied Probability.

Prediction markets don't hide the vig in the odds. The vig shows up as the bid-ask spread or explicit fees. For details on how fees affect expected value, see the prediction market fees guide.

When comparing sportsbook odds to prediction market prices, convert both to implied probability first. The market converter handles the conversion automatically.

Implied Probability and Parlays

Parlay odds are built by multiplying the implied probabilities of each leg. A 3-leg parlay with legs priced at 50%, 50%, and 50% has a combined implied probability of 12.5% (0.50 × 0.50 × 0.50).

The sportsbook compounds the vig on every leg, which is why parlays are high-margin bets. The implied probability of a parlay winning is always higher than the true probability if each leg is individually juiced.

For the full breakdown of parlay math and how the vig stacks, see the parlay math guide.

Common Mistakes When Using Implied Probability

Mistake 1: Treating implied probability as true probability.

Implied probability includes the vig. It's what the sportsbook is charging, not the fair price. Always de-vig before using implied probability as your baseline estimate.

Mistake 2: Adding implied probabilities in parlays.

You multiply probabilities in parlays, not add them. A 2-leg parlay with 50% implied probability on each leg has a 25% implied probability of hitting, not 100%.

Mistake 3: Comparing implied probabilities across platforms without adjusting for vig.

A -110 line on one book and a -105 line on another have different implied probabilities (52.38% vs 51.22%). The difference is vig, not information. Use the hold calculator to see how much vig each sportsbook is charging before comparing lines.

How Implied Probability Connects to Closing Line Value

Sharp bettors don't just look for +EV at bet placement. They track whether their bets beat the closing line. If you bet at +150 (40% implied) and the line closes at +130 (43.48% implied), you got value.

Over a large sample, beating the closing line correlates strongly with long-term profit. For the full explanation, see the closing line value guide.

Implied probability is the common unit for comparing opening lines, your bet, and closing lines. Convert all three to probabilities, then track your edge relative to close.

Frequently asked questions

What is implied probability in betting?

Implied probability is the win percentage embedded in betting odds. It shows what probability the sportsbook assigns to an outcome, including the vig they charge.

How do you convert +200 odds to implied probability?

Use the formula: 100 / (200 + 100) = 33.33%. Positive American odds divide 100 by (odds + 100).

How do you convert -150 odds to implied probability?

Use the formula: 150 / (150 + 100) = 60%. Negative American odds divide the absolute value of the odds by (absolute value + 100).

Why do implied probabilities add up to more than 100%?

Sportsbooks add vig to both sides of a market. The extra percentage over 100% is the vig. It's how the book makes money.

Is implied probability the same as true probability?

No. Implied probability includes the vig the sportsbook charges. True probability is what you get after removing the vig using a de-vig calculator.