Fractional Odds Calculator
Convert a probability into fractional (UK-style) betting odds, and calculate the potential profit and payout on a given stake.
About the Fractional Odds Calculator
Fractional odds, the traditional format used in UK and Irish betting, express how much profit a winning bet pays relative to the stake. Odds of 3/2 mean a winning $2 stake profits $3, on top of getting the original stake back.
Formula: Fractional Odds = (1 − Probability) ÷ Probability, simplified to a whole-number ratio · Profit = Stake × (Numerator ÷ Denominator) · Payout = Stake + Profit
A probability of 40% converts to fractional odds of 3/2 — equivalent to decimal odds of 2.5, since decimal odds always equal the fractional numerator divided by the denominator, plus 1. Lower fractional odds (like 1/4) imply a higher probability of winning, while higher odds (like 5/1) imply a lower probability. This calculator also reports the implied probability the fraction represents, so you can sanity-check whether posted odds match your own probability estimate. Bookmakers typically shade these implied probabilities to sum above 100% across a full set of outcomes to build in their margin, so posted odds are usually slightly less generous than a bet's true probability would suggest.