Find the value in aprediction market price
Enter the market's YES price, your own probability estimate, and your bankroll. You'll see which side holds the edge, its expected return, and a stake sized to your chosen Kelly fraction.
Position inputs
YES price, $0.01 – $0.99
Your fair-value % for YES
Used to size the Kelly stake
Positive expected value
Back YES at $0.63
Your edge
+9.0%
ROI
14.3%
expected return per $1 staked
Payback period
7.00 bets
repeated bets to earn back one stake
Expected value
+14.3%
per $1 staked
Kelly bet size
12.2%
of bankroll · 0.50× Kelly
Recommended stake
$121.62
on YES @ $0.63
EV per share
+9.0¢
$0.09 profit per share
How this was calculated
- Market-implied probability
- Read straight from the YES price
- Your estimated probability
- Chance YES resolves in your favour
- Edge
- Your probability minus the entry price
- Full-Kelly fraction
- (edge) ÷ (1 − entry price)
- Sized stake (0.50× Kelly)
- Fraction of bankroll to risk
Kelly maximises long-run growth but assumes your probability is accurate. Most professionals size down to a fraction of full Kelly to blunt estimation error and variance. This tool is for education, not financial advice.
How to read this calculator
This tool shows your edge and the Kelly-optimal stake. Full-Kelly is aggressive — many traders use half-Kelly instead, betting only 50% of the Kelly fraction to balance growth with lower variance.
Want ROI, payback period, and a head-to-head against the industry baseline?
Want to understand the result?
Learn how a share price becomes an implied probability, how expected value differs from certainty, and why many traders use fractional Kelly.