Betting odds formats: decimal, fractional and American

The short answer

Odds formats display the same underlying price in different notation. Converting the format does not change the implied probability or market margin.

Read decimal odds

Decimal odds show the total return for each unit staked, including the returned stake. A price of 2.50 therefore represents a total return of 2.50 for a stake of 1 if the selection wins; the profit portion is 1.50.

To find implied probability, divide 1 by the decimal odds and multiply by 100. Decimal 2.50 implies 40% before accounting for the bookmaker margin.

Convert fractional odds

Fractional odds show potential profit relative to the stake. Odds of 3/2 describe 3 units of profit for every 2 units staked. Add 1 to convert the fraction into decimal form: 3 divided by 2, plus 1, equals decimal 2.50.

To convert decimal odds back to fractional form, subtract 1 and express the result as a simplified fraction. Decimal 1.50 becomes 0.50, or 1/2.

Understand positive and negative American odds

Positive American odds show the profit on a stake of 100. +150 means 150 profit for 100 staked and converts to decimal 2.50. Divide the positive number by 100, then add 1.

Negative American odds show how much must be staked to make 100 profit. -200 converts to decimal 1.50: divide 100 by 200, then add 1. The minus sign describes the notation; it does not mean a negative return.

Compare the price, not the notation

Changing the display format does not change the underlying price. Decimal 2.00, fractional 1/1 and American +100 all imply 50% before margin. Use one preferred format when comparing services to reduce transcription mistakes.

Always compare the same market, settlement rules and timestamp. A familiar-looking format does not make a price fair, accurate or profitable.

• Decimal to probability: (1 / odds) × 100.
• Fractional to decimal: numerator / denominator + 1.
• Positive American to decimal: value / 100 + 1.
• Negative American to decimal: 100 / absolute value + 1.

Practical checklist

• Convert every quote into one consistent format.
• Check the implied probability after conversion.
• Compare identical markets and settlement rules.
• Remember that notation does not remove margin or risk.