Odds Formats (decimal/fractional/american)
The three main ways to express the same price, decimal, fractional and American, each convertible to the others and to an implied probability.
Definition
Odds can be written three ways for identical prices. Decimal (for example 2.50) shows total return per unit staked, so profit is stake x (odds - 1) and implied probability is 1 divided by the odds. Fractional (for example 3/2) shows profit relative to stake. American uses a baseline of 100: a positive number (+150) is the profit on a 100 stake, a negative number (-200) is the stake needed to win 100. Being able to convert between them lets you compare prices across books and, crucially, read the implied probability that reveals the margin.
Worked example
Decimal 2.50 = fractional 3/2 = American +150, all implying 1 / 2.50 = 40%. Decimal 1.50 = 1/2 = American -200, implying 66.7%. Any decimal below 2.00 corresponds to a negative American price.
Why it matters
Fluency across formats stops beginners mispricing bets on unfamiliar sites, and converting any price to implied probability is the first step every professional takes to spot the margin and any value.