CostvsPayout
Prediction Market Payout Calculator
Enter a price, a side, and a size (by contracts or by dollars) to see exactly what the position costs, what it pays if you're right, and the probability it needs to clear to break even -- with an optional, fully editable fee.
Position
Profit of $60.00 (150.00% ROI) if YES is right
Breakeven probability: 40.00% -- YES needs to happen at least this often for this trade to pay off on average.
Cost per contract
$0.40
Total cost
$40.00
Total fees
$0.00
Payout per contract if right
$1.00
Total payout if right
$100.00
Profit if right
$60.00
| Cost | Payout | |
|---|---|---|
| Contracts | 100.0 | 100.0 |
| Per contract | $0.40 | $1.00 |
| Total | $40.00 | $100.00 |
| ROI if right | — | 150.00% |
How this is calculated
Cost per contract = price + fee. On the default 100 contracts at 40¢ with no fee, total cost is $40.00, total payout if right is $100.00, and profit is $60.00 -- a 150.00% return.
The fee model has two editable pieces, both defaulting to zero: a percentage of price×(1−price) (the shape both Kalshi and Polymarket actually use, which peaks at 50¢ and shrinks toward the extremes) and a flat percentage of price (a simpler shape some venues might use instead). The two presets above fill in the first field with a rate this page fetched and cited -- use them as a starting point, not a live quote.
This is entry-cost math only. It doesn't model selling before resolution, bid/ask spread, or any fee charged at settlement rather than at trade time -- real platforms vary on all three. Prediction markets carry real risk of loss and may be restricted in some jurisdictions.
Frequently Asked Questions
Why is the payout always $1.00 per contract, no matter the price?
That's the whole design of a prediction-market contract: it pays exactly $1 if the outcome happens, $0 if it doesn't, regardless of what you paid. On 100 contracts bought at 40¢ with no fee, the payout if right is a flat $100.00 -- cost is the only thing that changes with price, not payout.
How does the fee actually change the numbers?
Using Kalshi-style (7%)'s rate as an example -- 7% of price×(1−price) -- a 40¢ contract picks up a fee of $0.02 on top of its 40¢ price. Across 100 contracts, that turns a no-fee cost of $40.00 into $41.68, and drops the profit if right from $60.00 to $58.32 (ROI from 150.00% to 139.92%). Fees hit hardest exactly at 50¢, where price×(1−price) is largest.
Why does breakeven probability equal the cost per contract?
Because cost is what you're risking and $1 is what you get back if right -- the probability at which those two exactly offset, on average, is cost/1, i.e. cost expressed as a percent. At 40¢ with no fee, that's 40%: if YES happens more often than 40% of the time (by your own honest assessment), this trade is profitable on average; less often, it isn't.
Where do these two fee presets actually come from?
Neither is a guess. The Kalshi-style (7%) preset comes from https://www.botforkalshi.com/blog/kalshi-fees-explained (accessed 2026-09-17) -- Kalshi's own official fee-schedule PDF returned an access-blocked response on every direct fetch attempt while building this page, so the rate is corroborated from a source that reproduces Kalshi's published formula, not typed in from memory. The Polymarket preset was fetched directly from Polymarket's own docs. Both are noted as presets, not the last word on either platform's live fee schedule -- always check the platform directly before trading.
Does 'number of contracts' have to be a whole number?
In real trading, yes -- but sizing by dollars (this calculator's other input mode) naturally lands on a fractional contract count, since $100 rarely divides evenly by a given price plus fee. This calculator shows the fractional number so the per-contract math stays exact; round down to the nearest whole contract before placing a real order.
SidebySideCalc's prediction-market calculators apply standard odds, probability, and bankroll-sizing math to the numbers you enter -- they are not trading advice. Prediction markets carry real risk of loss and may be restricted or unavailable in some jurisdictions; nothing here names or endorses a specific trading venue.