Glossary / Prices & odds / Fractional odds
A ParlayAPI glossary entry

Fractional odds

Fractional odds are the traditional UK format, written as profit over stake: 13/10 returns $13 of profit per $10 staked. Still standard in British horse racing; rare in APIs, which favor decimal and American formats.

In practice

# Fractional -> decimal -> American
        13/10  ->  1 + 13/10 = 2.30   ->  +130
        4/6    ->  1 + 4/6   = 1.667  ->  -150
        evens (1/1) -> 2.00           ->  +100

Convert fractional to decimal by adding 1 to the fraction; everything else (implied probability, parlay math) follows from the decimal form. “Odds-on” means a fraction below 1/1 - a favorite.

Where it shows up in ParlayAPI

ParlayAPI serves American and decimal formats (the two API-standard forms); fractional display is a one-line conversion from decimal at the presentation layer, worth knowing when your product serves UK or racing audiences.

Related terms

American oddsThe US-standard plus/minus odds format. Decimal oddsTotal payout per unit staked. Implied probabilityThe win probability an odds price implies. OutrightsWinner markets in soccer, golf, and racing.