Odds and probability
Understand fractional and decimal odds, implied probabilities, bookmaker margins and expected value. See how prices translate into break-even chances and why estimates remain uncertain.
Explore the related guides below, or return to the complete horse racing guides library. For worked probability examples, see how Probatrix works. All information is educational and outcomes are uncertain.
Guides in this topic
Horse racing odds reflect estimated winning chances, money in the market and a bookmaker's margin; they are not a pure statement…
Bookmakers begin with assessments of runner strength and likely demand, then adjust odds as new information and bets arrive.
Expected value combines the probability of winning with the profit if successful and the loss if unsuccessful.
Implied probability equals 1 divided by decimal odds, multiplied by 100.
For an accumulator, multiply all decimal odds together and then multiply by the stake.
Compare the odds available when a selection was recorded with the official starting price returned at the off.
To convert fractional odds a/b to decimal odds, divide a by b and add one.
A fair price is the odds corresponding to an estimated probability before any bookmaker margin.
Overround is the amount by which all runners' implied probabilities add up to more than 100%.
Probability estimates how likely a specified outcome is; confidence describes how reliable or stable that estimate may be.
A value bet exists when your assessed probability is higher than the break-even probability implied by the available odds.
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