True Odds Calculator

The True Odds calculator removes the bookmaker's margin (vig) from market odds to calculate the fair probability of each outcome. This helps you identify value bets by comparing true odds to bookmaker odds.

When to use it

Use the True Odds calculator when:

• Assessing whether a bookmaker's odds offer value
• Comparing odds across different bookmakers to find the best value
• Calculating fair probabilities for use in other calculators (like Expected Value)
• Understanding how much margin a bookmaker has built into their odds

How it works

Bookmakers build profit margin into their odds by making the total implied probability of all outcomes add up to more than 100%. This excess is called the "overround". The greater the overround, the greater the vig built into the odds.

The calculator uses mathematical models to remove this margin and reveal the true fair odds. Different models (devigging methods) make different assumptions about how the bookmaker applied the margin.

Understanding the inputs

Devigging method

Choose from four different methods:

• Power (Recommended)
• Additive (MPTO)
• Multiplicative (EM)
• Shin

Unless you know better, we recommend using the Power method.

Selections

Add each possible outcome in the market with its bookmaker odds:

• Selection Odds: The decimal odds offered by the bookmaker for this outcome

You must add at least 2 selections. Add all possible outcomes in the market for best results (e.g., for a football match, add home, draw, and away).

Understanding the outputs

For each selection, the calculator shows:

1. Implied Probability: The probability implied by the bookmaker's odds (includes margin).
2. True Probability: The fair probability after removing the bookmaker's margin.
3. True Odds: The fair odds based on the true probability. These are the odds that would be fair if there was no bookmaker margin.

At the bottom, you'll see:

• Total Implied Probability: Sum of all implied probabilities (should be >100% if there's bookmaker margin)
• Overround: The bookmaker's margin percentage (Total Implied - 100%)
• Total True Probability: Sum of all true probabilities (should equal 100% after devigging)

Practical example

You're looking at an AFL match. TAB offers:

• Geelong: 2.10
• Draw: 21.00
• Collingwood: 1.80

Enter these three odds with the Power method selected.

The calculator shows:

Geelong:

• Implied Probability: 47.62%
• True Probability: 44.17%
• True Odds: 2.264

Draw:

• Implied Probability: 4.76%
• True Probability: 3.5%
• True Odds: 28.602

Collingwood:

• Implied Probability: 55.56%
• True Probability: 52.34%
• True Odds: 1.911

Market Summary:

• Total Implied Probability: 107.94%
• Overround: +7.94%
• Total True Probability: 100.00%

TAB has built a 7.94% margin into this market. The true odds show that Collingwood (true odds 1.911) at 1.80 represents poor value - you're being offered worse odds than fair. Geelong (true odds 2.264) at 2.10 also represents slightly poor value, though not as bad.

Finding value bets

A bet has value when the bookmaker odds are higher than the true odds:

• Value: Bookmaker Odds > True Odds
• Poor Value: Bookmaker Odds < True Odds

In the example above, if another bookmaker offered Geelong at 2.30, that would exceed the calculated true odds (2.30 > 2.264).

The bigger the difference, the more value. Professional bettors look for situations where bookmaker odds are significantly higher than true odds.

Common issues

The true probabilities don't add up to exactly 100%. Why?

They should add very close to 100% (within 0.1%). Small rounding differences can occur. If the total is significantly off, check that you've entered all possible outcomes in the market.

Which devigging method should I use?

Unless you know better, we recommend using the Power method.

Can I use this for markets with many outcomes?

Yes, the calculator works for markets with many outcomes (e.g., horse racing with 15 horses). Enter all outcomes for best accuracy. However, markets with many outcomes often have higher overround percentages. Some devigging methods may not work with a large number of outcomes, or with longshots.

The overround is negative. Is that possible?

Yes, rarely. A negative overround means the total implied probability is less than 100%, creating an arbitrage opportunity. If you can bet on all outcomes at these odds, you're guaranteed profit. This is extremely rare and usually indicates a pricing error or special promotion that will be quickly corrected.

How does this help with matched betting?

True odds help you calculate fair probabilities for the Expected Value calculator. When a promotion asks you to estimate the chance of something happening (e.g., "team scores first"), you can use True Odds to remove margin from related markets and get fairer probability estimates.