Prediction Market Calculator: Fee and Break-Even Math
Price to probability, Polymarket and Kalshi fee rules, and the break-even odds a trade needs once fees are paid. The sums most calculator widgets skip.
Jump to a section (10)
- Opening
- What a prediction market calculator should compute
- What are implied probabilities in a prediction market?
- Payout per contract and the Polymarket taker fee
- Break-even probability after fees
- Kalshi fees: why the formula output is not the bill
- Rounding rules a calculator has to copy
- Intel Q&A
- Source file
- More intel
A prediction market calculator should run four sums, not one: contract price to implied probability, payout per contract, the exchange fee from the venue's published schedule, and the break-even probability once that fee is paid. The last number is the one that matters, and most calculator widgets never show it. On Polymarket a taker buying a crypto contract at 50¢ needs the event to land 51.75% of the time to break even, not 50%.
What a prediction market calculator should compute
Price, payout, fee and break-even, in that order. Each step feeds the next, and a tool that stops after the first one answers a different question from the one a trader is asking.
The model below treats a binary contract the way both venues' fee documents do: a share priced on a 0-to-1 dollar scale that is worth 1 dollar if it resolves in the holder's favour and nothing if it does not. Fees are the only input that differs by venue, and they are where widgets diverge from the bill.
- Implied probability: the share price read as a percentage.
- Payout per contract: 1 dollar minus the price, before fees.
- Fee: the venue's formula applied to the trade, with its rounding rules.
- Break-even probability: price plus fee per share, read as a percentage.
What are implied probabilities in a prediction market?
The implied probability is the share price expressed as a percentage: a 30¢ share prices the outcome at 30%. Polymarket's own documentation reads price this way, describing its fee as symmetric around 50% probability and pairing a 30¢ trade with a 70¢ one.
That number is the market's price, not a forecast anyone has to accept. A trader who thinks the true chance is higher than the price sees value on the Yes side; one who thinks it is lower sees value on No. The calculator's job is to tell that trader how much higher the true chance has to be once costs are counted. Converting between odds formats is a separate exercise this brief leaves aside: on these venues the price already is the probability.
Payout per contract and the Polymarket taker fee
Polymarket charges only the taker. Its fee page states the formula as shares times rate times p(1 - p), and makers are never charged under that schedule (as of October 2026).
The p × (1 - p) term is why the fee peaks at a 50¢ price and falls toward the extremes. The fee rate depends on the market category:
| Category | Taker fee rate | Maker rebate |
|---|---|---|
| Crypto | 0.07 | 20% |
| Sports | 0.05 | 15% |
| Geopolitics and world events | fee-free | none |
Run the formula at 100 shares of a crypto market priced at 50¢: 100 × 0.07 × 0.5 × 0.5 = 1.75 USDC. The same 100 shares at 50¢ cost $50 and pay $100 if they resolve Yes, so the gross profit of $50 shrinks to $48.25 after the taker fee. Geopolitical and world events markets carry no fee, so there gross payout and net payout are the same number.
Break-even probability after fees
Break-even probability is the all-in cost per share divided by the 1 dollar payout. For a Polymarket taker holding to settlement that is p + feeRate × p × (1 - p), which is the finding most calculators omit.
| Price | Crypto break-even | Sports break-even | Geopolitics break-even |
|---|---|---|---|
| 30¢ | 31.47% | 31.05% | 30% |
| 50¢ | 51.75% | 51.25% | 50% |
| 70¢ | 71.47% | 71.05% | 70% |
The fee hurts cheap contracts more in relative terms. Polymarket confirms that a 30¢ trade pays the same dollar fee as one at 70¢; in a sports market that is $0.0105 per share either way, about 3.5% of a 30¢ stake against about 1.5% of a 70¢ stake.
Makers sit outside this table on Polymarket because they pay nothing. A calculator that does not ask whether the order rests on the book or takes liquidity cannot give the right break-even.
Kalshi fees: why the formula output is not the bill
On Kalshi the model fee is a starting point, not the charge. Kalshi says it takes a transaction fee on expected earnings (Kalshi Help Center, April 2026), and its API documentation layers rounding on top.
Each fill's trade fee is rounded up to the nearest $0.000001. The exchange then adds a rounding fee to bring the balance back to the member's precision grid and can refund accumulated overpayment as a rebate, with net fee never below $0.00. On a small order those adjustments are a visible share of the total, so a calculator that applies only the fee model understates the cost.
The correction runs across an order, not a single fill. Kalshi's fee accumulator carries rounding overpayment across every fill so the total converges to what one equivalent fill would cost.
Two further inputs belong in any Kalshi calculation. Some markets carry different fees, so a single rate across all contracts is wrong by design. And maker fees, where a market has them, are charged only when a trade executes; cancelling a resting order is free.
Rounding rules a calculator has to copy
Fee precision decides the answer at the extremes. Polymarket rounds fees to 5 decimal places and its smallest charge is 0.00001 USDC (as of October 2026).
Anything below that floor rounds to zero, so very small trades near the extremes can pay no fee at all. A calculator that applies the raw formula to a tiny position at a price close to 0 or 1 reports a fee the exchange does not collect. The error is small in dollars, but it is the kind of drift that shows a tool was never checked against the venue's own rules.
The same discipline matters when the calculation spans two venues. A cross-venue position pays Polymarket's category rate on one leg and Kalshi's rounded, market-specific fee on the other; how that cost shapes prediction market arbitrage between venues is covered in the hub brief.
Intel Q&A
How do you calculate the break-even probability on Polymarket?
Add the taker fee per share, feeRate × p × (1 - p), to the price p. At 50¢ in a crypto market with a 0.07 rate that gives 51.75%; in a fee-free geopolitics market break-even equals the price.
Do makers pay fees on Polymarket or Kalshi?
Polymarket charges only takers and never charges makers. Kalshi applies maker fees in some markets, charged only when a resting order executes; cancelling is free.
Why does the fee Kalshi charges differ from the fee formula?
Kalshi rounds the trade fee up, adds a rounding fee to restore balance precision and may refund accumulated overpayment as a rebate. The net fee is never negative.
Is the Polymarket fee the same at 30¢ and 70¢?
Yes, in dollars. The fee is symmetric around 50% probability, so the dollar fee at 30¢ equals the fee at 70¢, which makes it a larger share of the cheaper stake.
Source file
- Polymarket Documentation, Trading Fees, 2026-10-20
- Kalshi API Documentation, Fee Rounding, 2026-10-20
- Kalshi Help Center, Fees, 2026-04-19
