Learning objectives
After this lesson, you should be able to convert American odds into break-even probability, distinguish implied probability from a forecast, and identify the margin built into a two-sided market.
1. Odds are prices, not predictions
Suppose a sportsbook posts -110 on each side of a point spread. At -110, you stake $110 to earn $100 in profit if successful. Your returned amount on a win is $210, including your original stake. A loss costs the $110 staked. Prices can move with information, action and the bookmaker's risk management; the quoted odds are not proof of the true chance of winning.
2. Convert American odds to implied probability
For negative odds -A, implied probability = A / (A + 100). For positive odds +B, implied probability = 100 / (B + 100). Multiply by 100 to express a percentage.
Example: -110 implies 110 / 210 = 52.38%; +150 implies 100 / 250 = 40.00%. These are break-even thresholds before other costs, not forecasts generated from team statistics.
3. Why both sides can add to more than 100%
In a simplified two-outcome spread market with -110 on both sides, the implied probabilities add to 52.38% + 52.38% = 104.76%. The excess 4.76 percentage points is called overround, often loosely called the vig or margin. It does not mean the operator earns exactly 4.76% of every dollar staked: actual hold depends on wager distribution, results, and prices accepted.
4. Estimate a no-vig probability
A simple proportional method divides each outcome's implied probability by the sum. For equally priced -110 sides, each becomes 52.38 / 104.76 = 50%. This is a market-based estimate, not independent proof that the outcomes are equally likely. Other no-vig methods can give different results in uneven markets, and a bookmaker price may be stale or shaded.
5. A repeatable research workflow
- Identify the exact event, market, period and settlement terms.
- Record the best displayed prices and their timestamps from independent sources if available.
- Convert the prices to implied probabilities.
- Calculate market overround and optionally no-vig estimates.
- Compare the no-vig benchmark with your own independent probability estimate, noting uncertainty.
- Record why you trusted the inputs; if prices or rules are ambiguous, do not treat the calculation as actionable.
6. Common mistakes
- Assuming a -200 price means a team is certain to win. It implies a roughly 66.67% break-even threshold.
- Calling a two-sided overround a certain sportsbook profit.
- Comparing one book's full-game total with another's regulation-only total.
- Treating no-vig probabilities as objectively correct rather than one estimate.
- Ignoring commission, liquidity, limits, rule differences and timestamp mismatches.
Practice
Exercise A: A market shows -120 and +105. What is the overround? Answer: 120/220 = 54.545%; 100/205 = 48.780%; sum 103.326%; overround about 3.33 percentage points.
Exercise B: Which is the stronger statement? (1) “-110 means the team wins 52.38% of the time,” or (2) “-110 is approximately a 52.38% break-even threshold for that wager.” Answer: 2. Price-derived probability is not an observed win rate.
Quick quiz
- What is implied probability at +200? 33.33%.
- Is the overround the same thing as realized sportsbook revenue? No.
- Why compare settlement terms before prices? Two markets with different rules may not represent the same outcome.
Recap
- American odds are convertible into break-even probabilities.
- Overround reflects a margin in a set of prices, not certain operator profit.
- A no-vig price is a useful benchmark, but its validity depends on the underlying market and assumptions.
Editorial note: All odds are hypothetical. No outcome or profit is certain. Reviewed date: pending.
Try it in Sharp App: Odds screen
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