How Do Sportsbook Odds Work?

The three formats

American: -110 means stake $110 to win $100. +150 means a $100 stake returns $150 profit.

Decimal: 1.91 means a $100 stake returns $191 total, including your stake. Common outside the U.S.

Fractional: 10/11 is the same price as -110, expressed as profit over stake. Mostly British.

Converting to implied probability

For a negative American price, implied probability = odds / (odds + 100), using the absolute value. -110 gives 110 / 210 = 52.4%.

For a positive price, implied probability = 100 / (odds + 100). +150 gives 100 / 250 = 40%.

For decimal odds, implied probability is simply 1 divided by the decimal price.

Finding the vig

Add the implied probabilities of both sides. Two -110 lines total 104.8%, so the book's margin is 4.8%. Anything under 4% on a two-way market is competitive.

This is why -105 pricing matters: it lowers the total, which is the same as being paid more for the identical bet.