Enter the current market price, your estimated true probability, and your bankroll. The calculator tells you your expected value, edge, and the optimal Kelly Criterion bet size.
Current odds on the market
Your estimated true probability
Total capital available
Results
Expected value
+16.7¢
per $1 wagered
Your edge
+10.0%
vs market price
Kelly bet
$250.00
25.0% of bankroll
Half-Kelly (safer)
$125.00
12.5% of bankroll
This is the current price shown on Polymarket or Kalshi — the percentage probability the crowd has priced in. For example, a market at 60¢ = 60%.
What do you think the true probability is, based on your own research? If you think the market is wrong and the real probability is higher, enter your estimate here. If it's lower, you have an edge on the NO side instead.
The calculator shows your expected value per dollar wagered, your edge over the market, and the Kelly Criterion bet size. Use the half-Kelly figure in practice — it gives nearly the same growth rate with much lower variance.
Positive edge — bet YES
No edge — skip
Edge on NO side
A market trading at price p implies the crowd thinks there's a p% chance of YES. Your edge is the difference between your estimate and the market price. No edge means no reason to bet.
EV = (your_prob × net_payout) − (1 − your_prob)
net_payout = (1 / market_price) − 1
Kelly % = (p × b − q) / b
where p = your probability, q = 1 − p, b = net_payout
Prediction market prices are expressed as percentages representing the market's implied probability of an event occurring. A market at 60% means the crowd collectively believes there's a 60% chance of YES. If you buy YES at 60¢ and the event happens, you receive $1 — a 67% return. If you think the true probability is higher than 60%, you have a positive expected value.
The Kelly Criterion is a formula that calculates the optimal fraction of your bankroll to bet: Kelly % = (p × b − q) / b, where p is your probability of winning, q = 1 − p, and b is the net payout (1/market_price − 1). Kelly maximizes long-run bankroll growth. Most traders use half-Kelly (half the recommended amount) to reduce volatility while still growing their bankroll efficiently.
Expected value (EV) is the average profit per dollar wagered given your probability estimate. Positive EV means the bet is profitable on average; negative EV means it loses money on average. EV = (your probability × net payout) − (1 − your probability). A bet is only worth placing if your estimated probability is higher than the market price implies.
No. Full Kelly is mathematically optimal only if your probability estimates are perfectly accurate — and they never are. In practice, half-Kelly (betting half the recommended amount) is the standard approach for experienced traders. It grows your bankroll nearly as fast as full Kelly while dramatically reducing the risk of large drawdowns from estimation errors.
Start with base rates — how often has this type of event happened historically? Then adjust based on specific evidence you have. For election markets, look at polling aggregators and fundamentals models. For economic data releases, look at consensus forecasts and recent trends. Your estimate is only worth more than the market's if you have specific information or a systematic approach the crowd lacks.
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