PHTAYA Betting Margin Explained With Two Worked Examples

Every quoted price already has a cut built in before the game starts — that cut is the betting margin, and it’s why the odds on both sides of a market never add up to exactly 100%. Once you can read a decimal price using how to read betting odds before comparing margins, this page shows the margin formula and works a two-way market and a three-way market through it so the number stops being abstract.

What Betting Margin Actually Measures

Betting margin is the gap between what a market’s odds imply and true 100% probability. The formula is short:

1. Convert every side of the market to an implied probability: 1 ÷ decimal odds.
2. Add every side’s implied probability together.
3. Subtract 100% from that total — the remainder is the margin.

Bookmaker Margin: Why Implied Probabilities Exceed 100%

A bookmaker margin exists because a book prices every side of a market slightly short of its true chance, so the implied probabilities always overshoot 100% once added together. That overshoot is where the book’s own edge sits, regardless of which side wins.

Decimal price	Implied probability
1.50	66.67%
1.91	52.36%
3.00	33.33%

A price closer to 1.00 implies a probability closer to 100%; a price further from 1.00 implies a smaller one. Either way, the two sides of a market are priced to sum above true 100%.

A Two-Way Betting Margin, Worked Through

Suppose a hypothetical basketball moneyline where both the favourite and the underdog are priced at 1.91 decimal — a standard two-way market with no draw available.

Side	Decimal price	Implied probability
Favourite	1.91	52.36%
Underdog	1.91	52.36%
Total	—	104.71%

104.71% minus true 100% leaves a 4.71% betting margin on this market.

A Three-Way Overround, Worked Through

Overround is the same calculation applied to a market with three outcomes instead of two — the kind used in sports that allow a draw, such as football. Suppose hypothetical prices of 2.10 (home), 3.30 (draw) and 3.60 (away).

Outcome	Decimal price	Implied probability
Home win	2.10	47.62%
Draw	3.30	30.30%
Away win	3.60	27.78%
Total	—	105.70%

The overround here is 5.70% — a full percentage point higher than the two-way example above, because a third priced outcome gives the book a third place to build its margin into.

Why Basketball Markets Skip the Three-Way Calculation

PBA and NBA games always produce a winner through overtime, so a basketball moneyline never needs a draw price and stays a two-way market like the 4.71% example above. The three-way calculation only applies where the sport itself allows a tie.

Both examples on this page use the same odds shown on the moneyline betting and spread betting pages, and the decimal-to-probability conversion behind every price here works exactly as set out earlier in this page.