How betting odds really work
Decimal, fractional, American — and why implied probabilities always sum to more than 100%.
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An odd is not a prediction. It is a price. Like a stock price, it reflects the market's collective view — plus the operator's margin.
Three formats, same information: Decimal (2.00), fractional (1/1) and American (+100) all express 50% implied probability. Formula: implied probability = 1 / decimal odds.
The hidden margin: A football match priced at 2.10 / 3.40 / 3.60 implies 47.6% + 29.4% + 27.8% = 104.8%. Those 4.8% are the bookmaker's overround.
Why it matters: Top leagues sit at 4–6% margin. Live betting 8–12%. Five-leg accumulators compound to 25%+. Once you see this, you read odds differently.
Practical takeaway: Always compare books. 5–10% odds differences on the same outcome are common — and the long-term difference between losing and breaking even.