Convert odds between decimal, fractional and American formats with implied probability for every value.
Decimal, fractional and American odds are three notations for the same number: the implied probability of an outcome. Decimal 2.50 is fractional 3/2 and American +150, and all three mean the market is pricing a 40% chance.
Conversion is arithmetic, not opinion. Decimal to probability is 1 divided by the decimal. Fractional to decimal is numerator over denominator plus one. American positive to decimal is the number over 100 plus one; negative is 100 over the absolute value plus one.
Switching notation never changes the price. What changes the price is the bookmaker margin, which shows up when the implied probabilities of all outcomes add to more than 100%. On a two-way market at 1.91 each, the implied total is 104.7% — that excess is the cost of the bet.
Convert first, then strip the margin with the no-vig calculator, and you can compare two books that quote in different formats.
Decimal, fractional and American odds are regional conventions for the same quantity. Decimal states the total return per unit, fractional states the profit relative to the stake, and American states how much you win on 100 staked or must stake to win 100.
The conversion trap is the sign flip around evens: American odds jump from +100 to −100 with nothing between, and fractional odds shorter than 1/1 read backwards to anyone used to the other direction. Converting through decimal avoids both problems.
None — they are notations for the same value. Decimal is easiest to compute with, which is why most models use it internally.
A winning $100 stake returns $150 profit, so the total return is $250. In decimal that is 2.50, an implied 40% chance.