How the prediction-market EV calculation works
A contract priced at 35 cents implies a 35% probability before trading costs. If your independent estimate is 55%, the raw probability edge is 20 percentage points. That difference is only a starting point: the price you can actually fill, venue fees, spread, and slippage all change the economics.
The calculator treats your stake as the amount spent on contracts. It divides the stake by the contract price to estimate the number of contracts, then applies a $1 payout to each winning contract. Expected value is your probability-weighted gross payout minus the position cost and your estimated trading costs.
| Metric | Calculation | What it answers |
|---|---|---|
| Gross payout | Stake ÷ contract price | How much winning contracts pay at settlement |
| Expected value | Probability × gross payout − stake − costs | Average value under your probability assumption |
| Break-even probability | (Stake + costs) ÷ gross payout | The forecast threshold required for non-negative EV |
| Quarter Kelly | 25% of full Kelly risk, converted to pre-cost stake | A conservative version of model-based bankroll sizing |
Kalshi payout calculator
For a Kalshi Yes or No contract, enter the price in cents and the amount you plan to spend. A winning contract settles at $1, so the calculator shows the estimated contract count, gross payout, net profit, and return after your cost assumption. Use the price you can actually execute, not an old last-traded price.
Polymarket profit calculator
For Polymarket, use the current executable outcome-token price and include expected fees or slippage. The same $1 winning payout math applies, while the EV result compares your probability estimate with the total cost of the position.
Use an executable price, not the headline price
The displayed probability may be a midpoint or an old last trade. An immediate buy usually executes against the best available ask, and a larger order may fill across several price levels. A 42% midpoint with a 45-cent ask should be tested at 45 cents—or at the estimated average fill for your full size.
Learn how bid, ask, spread, and depth interact in the Kalshi order-book guide. You can also compare active contracts in Alphascope prediction-market odds before opening the venue to confirm the live book.
How to estimate fees and slippage
There is no reliable universal fee percentage. Kalshi notes that fees vary by market and that maker fees can apply in some markets. Polymarket's current fee rules depend on the market category and trade price. Both platforms can change their schedules, and slippage depends on the order book at the moment you trade.
Check the current primary sources before entering a cost estimate: Kalshi's official fee guide and Polymarket's official fee documentation. If your planned size would consume multiple price levels, add estimated slippage rather than assuming the best ask applies to every contract.
Expected value is not a guarantee
Positive EV depends on the quality of your probability estimate. A formula cannot correct stale evidence, an ambiguous resolution rule, correlated positions, or overconfidence. Use a range rather than a single probability when uncertainty is material, and test how the result changes at the low end of that range.
Kelly sizing is especially sensitive to forecast error. The calculator shows quarter Kelly as a reference, but zero is a valid position size when the edge is narrow, the market is thin, or the contract rules are unclear. Alphascope provides research tools and calculations, not trade execution or financial advice.
Worked example
Suppose a contract can be bought at 35 cents, your probability estimate is 55%, you plan to spend $100, and you estimate fees plus slippage at 1%. The calculator estimates 285.71 contracts, a $285.71 gross payout if right, $184.71 net profit after the $1 cost estimate, and a $56.14 expected value. The cost-adjusted break-even probability is 35.35%.
Change the probability from 55% to 34% and the same trade becomes negative EV. That sensitivity is the point of the tool: test the assumptions that drive the decision instead of focusing only on the maximum payout.
Frequently asked questions
How do you calculate prediction-market expected value?
Multiply your estimated probability by the $1 payout per winning contract, then subtract the position cost and estimated fees or slippage. Positive expected value means the probability-weighted payout exceeds those costs under your assumptions.
What is the break-even probability for a prediction-market contract?
Without trading costs, a 35-cent contract breaks even at 35%. Fees and slippage raise that threshold. This calculator adds your cost estimate to show a cost-adjusted break-even probability.
Does the calculator use live Kalshi or Polymarket fees?
No. Fee schedules vary by venue, market, price, and order type, so you enter an estimated all-in percentage. Check the venue's current official fee documentation before relying on the result.
What is quarter Kelly?
The Kelly criterion estimates a bankroll fraction from your probability, price, and payoff assumptions. Quarter Kelly uses one fourth of that fraction to reduce sizing, but it can still be too aggressive when your probability estimate is uncertain.