Polymarket & Kalshi odds calculator
- Shares
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- Fee
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- Total cost
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- Payout if right
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- Break-even probability
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- Decimal odds
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- American odds
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- Kelly stake, full / ¼
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Example values. Change any field. Everything is calculated in your browser and nothing is sent anywhere.
A Polymarket or Kalshi price is the market's implied probability. A YES share at 62¢ implies a 62% chance and pays $1 if the event happens, so $100 buys about 161 shares and returns $161.29 if right. It only has positive expected value if your own probability is above the price plus fees.
How to read a prediction market price
Each YES or NO share pays $1 if its side is right and $0 if not, so the price is the market's probability. 62¢ means the market puts the event at about 62%. YES and NO prices add up to roughly $1.
Implied probability = price ÷ $1 Shares = stake ÷ price Payout if right = shares × $1 Break-even = total cost ÷ payout Expected value = your probability × payout − total cost Kelly stake = (your probability − break-even) ÷ (1 − break-even)
Fees on Polymarket and Kalshi
Kalshi publishes its trading fee as 0.07 × contracts × price × (1 − price), rounded up to the next cent, with a lower rate on some markets. Set the rate for your market in the calculator. The fee is largest at 50¢ and shrinks towards 1¢ and 99¢.
Polymarket charges no trading fee on most markets. Some markets do; if yours shows one, enter it as a percentage of the stake.
Converting to sportsbook odds
Decimal odds are payout ÷ cost, so a 62¢ share with no fee is 1.613. American odds for a favourite are −100 ÷ (decimal − 1), here −163; for an underdog they are +(decimal − 1) × 100.
How AI Trading Fleet uses this
A Fleet strategy makes its own probability estimate and compares it with the price. It only proposes a trade when the edge left after fees clears your floor, and the risk check then caps the size per market in dollars. Strategies are judged on calibration with the Brier score: one that drifts past the gate has its size cut, goes back to paper, and is retired if it keeps failing.
Questions
What does a Polymarket price mean?
It is the market's implied probability. A share at 62¢ pays $1 if its side is right, so the market prices the outcome at about 62%.
How are Kalshi fees calculated?
Kalshi publishes the fee as 0.07 × contracts × price × (1 − price), rounded up to the next cent, with a lower rate on some markets. Check the fee schedule for your market.
Does Polymarket charge trading fees?
Most Polymarket markets have no trading fee. Some markets do, and the market page shows it.
What is expected value on a prediction market?
Your probability times the payout, minus what the position costs you with fees. Positive expected value means that, if your probability is right, the trade makes money on average over many trades.
Should I bet the full Kelly stake?
Full Kelly assumes your probability is exactly right and swings hard when it is not, so many traders use a quarter or half of it. Nothing here is investment advice.
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