Definition
Odds conversion means translating a betting line from one format to another and into implied probability. The three common formats are American, decimal, and fractional.
For negative American odds: implied probability = |odds| / (|odds| + 100).
For positive American odds: implied probability = 100 / (odds + 100).
Decimal odds = total return per $1 staked. Implied probability = 1 / decimal odds.
Worked Example
A −110 point spread means a bettor risks $110 to win $100.
Implied probability = 110 / (110 + 100) = 110 / 210 = 52.38%.
Converted to decimal odds: 1 + (100 / 110) = 1.9091.
Converted to fractional odds: 100/110, reduced to 10/11.
So −110, 1.91 decimal, and 10/11 fractional all describe the same payout shape: a $110 stake returns $210 total if it wins, including the original stake.
For +150 odds, implied probability = 100 / (150 + 100) = 40.00%, and decimal odds are 2.50.
Why It Matters
Odds conversion helps a bettor compare prices across sportsbooks, calculators, and markets that use different formats. It also turns a line into implied probability, which is needed before comparing the sportsbook price to a personal projection.