# Implied Probability: How to Convert Betting Odds to Win Percentage

> How to convert American, decimal, and fractional odds to implied probability - plus how to remove the vig and find true value in player props.

**Date:** 2026-03-05  
**Author:** HeatCheck HQ  
**Tags:** Implied Probability, Betting Odds, Beginner, Guide, Expected Value  
**Full article:** https://heatcheckhq.io/blog/implied-probability-calculator  
**Live picks & dashboards:** https://heatcheckhq.io

---

A prop priced at -110 implies a 52.4% chance of hitting. A prop at +200 implies 33.3%. If your analysis says the real probability is higher than what the odds imply, you've found value. If it's lower, you haven't—no matter how good the matchup looks on paper.

This is the foundational math behind every profitable bet. The calculation takes five seconds. Skipping it means you're placing bets without knowing whether the price is fair.

## The Formulas

### American Odds

**Negative odds (favorites):** Implied Probability = |Odds| / (|Odds| + 100)

| Odds | Implied Probability |
|:----:|:---:|
| -110 | 52.4% |
| -120 | 54.5% |
| -130 | 56.5% |
| -150 | 60.0% |
| -200 | 66.7% |
| -300 | 75.0% |

**Positive odds (underdogs):** Implied Probability = 100 / (Odds + 100)

| Odds | Implied Probability |
|:----:|:---:|
| +100 | 50.0% |
| +110 | 47.6% |
| +150 | 40.0% |
| +200 | 33.3% |
| +300 | 25.0% |

### Decimal Odds

Implied Probability = 1 / Decimal Odds. So 1.91 decimal equals 52.4%. Clean and simple.

### Fractional Odds

Implied Probability = Denominator / (Numerator + Denominator). So 10/11 equals 11/21 equals 52.4%.

## Understanding the Vig

Add up the implied probabilities of both sides of a bet—the total exceeds 100%. That excess is the sportsbook's vigorish, their built-in margin.

A standard -110/-110 line: 52.4% + 52.4% = 104.8%. The 4.8% overround means the book profits regardless of the outcome. Both sides can't truly add to more than 100%—the extra 4.8% is the tax you pay to bet.

**Vig varies by market:**

| Market | Typical Vig |
|--------|:---:|
| NFL spreads | 4–5% |
| NBA player props | 4–8% |
| MLB player props | 5–10% |
| SGP props | 8–15% |
| Futures | 15–40% |

Player props typically carry 4–8% vig. The thinner the market, the wider the vig. Higher vig means you need a larger edge to justify the bet.

## Removing the Vig: Finding True Probability

Raw implied probabilities include the vig. To estimate the true probability the market assigns to each side, strip it out.

**Example:** Over -115 (53.5% implied) and Under -105 (51.2% implied). Total: 104.7%.

Divide each side by the total to normalize: True over probability = 53.5% / 104.7% = 51.1%. True under probability = 51.2% / 104.7% = 48.9%.

**Why this matters:** most bettors compare their analysis to the raw implied probability, which overstates both sides. At -110, the raw implied probability is 52.4%. The vig-free probability is 50.0%. If your analysis says 54%, your real edge against the true market probability is 4%—not 1.6%. That distinction changes how you size the bet and evaluate your performance.

## Putting It to Work: A Player Prop Example

**The prop:** De'Aaron Fox over 6.5 assists at -120.

**Calculate implied probability.** -120 gives you 120 / 220 = 54.5%. You need Fox to clear 6.5 assists more than 54.5% of the time for this bet to be profitable.

**Check the matchup.** The [Defense vs Position dashboard](/nba/defense-vs-position) shows tonight's opponent ranks 28th against PG assists—they give up the 3rd most in the league. Favorable.

**Check recent form.** The [Streak Tracker](/nba/streaks) shows Fox has cleared 6.5 assists in 8 of his last 10 games, averaging 8.2 over that span.

**Check game environment.** Game total is 228.5—moderately high. The Kings play at the 6th fastest pace. More possessions mean more assist opportunities.

**Estimate your probability.** Favorable matchup + strong recent form + pace-friendly environment. Your estimate: roughly 62%.

**Calculate the edge.** 62% minus 54.5% = 7.5% edge. That's strong. This is a bet worth making.

## Common Mistakes

**Not removing the vig before calculating edge.** If the over is -110 (52.4% implied) and you estimate 54%, you might think your edge is 1.6%. After removing the vig, the true market probability is 50%, making your real edge 4%. Understating your edge means you underbet winners.

**Treating implied probability as a prediction.** Implied probability reflects the market's pricing, not mathematical truth. A prop at -200 (66.7% implied) doesn't mean the over hits 66.7% of the time. It means the book set the juice at -200—which could reflect actual probability, public money distortion, or risk management. It's a starting point for comparison, not an endpoint.

**Ignoring probability on plus-money bets.** A prop at +200 implies 33.3%. Many bettors see +200 and think "big payout" without calculating what it requires. You need the prop to hit 33% of the time just to break even. If your analysis says 25%, the big payout doesn't matter—you're losing money over time.

**Forgetting the break-even rate.** Implied probability equals the break-even win rate at those odds. At -110, you need 52.4% to break even. At -130, 56.5%. At -150, 60.0%. Every bet starts from behind because of the vig. Your analysis needs to show a probability above the break-even rate for the bet to have positive expected value.

## From Implied Probability to Expected Value

Once you know both the implied probability and your estimated probability, you can calculate expected value.

**EV = (Your Probability x Payout) - ((1 - Your Probability) x Stake)**

At -110 with a 57% estimated probability: EV = (0.57 x $100) - (0.43 x $110) = $57.00 - $47.30 = +$9.70 per bet. That's 8.8% ROI. Place 100 bets like this and your expected profit is $970.

You don't need to be right on every bet. You need to find bets where the odds underestimate the true probability. Over hundreds of bets, the math does the work.

## Quick Reference Table

| American Odds | Implied Probability | Vig-Free (vs -110 other side) |
|:---:|:---:|:---:|
| -300 | 75.0% | 73.2% |
| -200 | 66.7% | 64.6% |
| -150 | 60.0% | 57.7% |
| -130 | 56.5% | 54.1% |
| -120 | 54.5% | 52.2% |
| -110 | 52.4% | 50.0% |
| +100 | 50.0% | 47.8% |
| +110 | 47.6% | 45.5% |
| +150 | 40.0% | 38.2% |
| +200 | 33.3% | 31.8% |
| +300 | 25.0% | 23.8% |

For common odds, you can do this in your head. At -110, roughly 52%. At -150, 60%. At +150, 40%. At +200, 33%. After a few weeks, it becomes automatic.

The gap between what the odds imply and what your analysis shows—that's where every profitable bet lives.

[Find edges the market hasn't priced in on the Prop Analyzer.](/check)


---

*Data powered by HeatCheck HQ — sports analytics platform. Free tools at https://heatcheckhq.io*