A prediction-market hedge is worth placing only when the net cash flow remains attractive after the actual fill prices, every fee on every leg, and any exit costs. Start with each contract’s settlement rules, walk the order book for your intended size, calculate fees per fill, and test the result under worse execution. A price gap by itself is not guaranteed arbitrage.
The decision rule
Calculate the hedge using executable prices, not the headline bid, ask, or midpoint. For each leg, determine the cash paid at every price level, add the venue’s fee for each executed fill, and include the cost of closing the position if you will exit before settlement. Then enumerate the net cash flow in every outcome covered by the contracts.
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The compact expression is:
Net hedge result = combined contract payout − purchase cash outlay − fees on all fills − execution cost versus the assumed reference prices − other documented transaction costs.
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You can express execution cost either as a higher actual cash outlay or as a separate slippage line against a reference price, but not both. A hedge is a guaranteed positive trade only when every relevant outcome has positive net cash flow under the written settlement terms. If one outcome is uncovered, the contracts can resolve differently, or the calculation depends on your probability estimate, describe it as an expected-value trade or residual-risk hedge instead.
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1. Define what each contract actually pays
Before looking at prices, write the payoff and resolution conditions for each leg. Similar labels do not establish equivalent contracts.
Map the outcomes
For a binary example, one leg might pay $1 if an event occurs and $0 otherwise, while the other is intended to pay $1 if the event does not occur. Write those payments explicitly for every outcome you can identify. Include what happens if the market is cancelled, voided, extended, or resolved under a special rule.
Check timing and definitions
- Read the complete rulebook for each market, including the source used for resolution and the exact cutoff time.
- Confirm that both legs refer to the same event, geographic scope, time window, and definition of the outcome.
- Check whether one contract can settle earlier, be disputed, or be cancelled while the other remains open.
- Record whether the payout is cash, a stablecoin, or another settlement asset and whether conversion or withdrawal costs apply.
A cross-market price difference is not a hedge if one contract pays on a different date, uses a different threshold, or can resolve to an outcome that the other contract does not cover.
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2. Gather the inputs for a size-specific calculation
Use a worksheet for the exact quantity you plan to trade. Record these inputs before sending an order:
- Quantity per leg. Include partial fills and any minimum order size.
- Reference price. This is the quote used in your initial estimate, such as the best available ask at the moment you model the trade.
- Visible depth. Capture the price and quantity available at each book level on the side you will take.
- Fee terms. Identify the current market-specific schedule, whether the order is maker or taker, the fee formula, rounding rule, minimum charge, and any category exemption.
- Exit assumptions. If you will unwind before settlement, estimate the exit side of the book and its fees separately.
- Settlement payout. List the amount paid by each leg in each possible resolution outcome.
Keep a timestamp and a copy of the applicable market rules. Fees and available depth can change between the estimate and the order submission.
3. Estimate slippage by walking the order book
Slippage is the cost of obtaining your intended size at prices worse than the reference price in your estimate. It is not the same as a platform fee or the bid-ask spread.
Walk every visible level
- Start with the best executable price on the side you will trade.
- Take only the quantity available at that level, or the remainder of your order if it is smaller.
- Multiply that quantity by its price to obtain the cash used at the level.
- Continue to the next level until the full intended quantity is filled or the available depth is exhausted.
- Add all level costs and divide by the filled quantity. The result is your estimated volume-weighted average fill price.
For leg i, the book-walk cost is the sum of quantityj × pricej across levels j. The average fill is that cost divided by the total quantity. If your reference price is pref and your planned quantity is Q, estimated slippage in cash is:
Slippage = actual book-walk cost − (Q × pref).
A positive value means the executable book is more expensive than the reference. If the book cannot supply the planned size, reduce the size, accept a different average price, or abandon the hedge; do not extrapolate the top-of-book quote.
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What a book walk does not guarantee
A visible-book estimate is still conditional. The methodology described by Polymarket Price Data’s slippage calculator warns that the estimate does not include queue priority, cancellations between observation and arrival, latency, hidden liquidity, price improvement, or self-impact beyond the entered book. Treat the result as an execution estimate, not a locked-in fill.
4. Calculate fees for each fill and each leg
Apply the venue’s current market-specific formula to the quantity and price that actually execute. Do not apply one fee to the combined notional unless the schedule explicitly says to do so. Multiple fills can have different prices, and rounding can affect the total.
Polymarket terms published July 10, 2026
The official Polymarket Trading Fees help article says taker fees apply only to certain markets, makers are not charged under the stated schedule, and fees are set by protocol and applied when a trade matches. Its formula is:
Fee = C × feeRate × p × (1 − p)
- C is the number of shares traded.
- p is the share price.
- feeRate is the rate for that market category.
The article says fees are rounded to five decimal places and the minimum charge is 0.00001 USDC. It also states that geopolitical and world-events markets are fee-free under the schedule described in that article. The article’s exact wording is, “Makers are never charged fees.” That statement is limited to the markets and schedule it describes; verify the live terms shown for the specific market before trading because categories, rates, and exclusions can change.
Calculate the formula for each executed quantity and price, then apply the stated rounding rule. If you later sell or otherwise unwind, calculate the exit trade’s fee and execution cost as another leg.
Kalshi terms and the current schedule
Use the official Kalshi fee-schedule page for current rates and market-specific changes. A Kalshi PDF effective February 5, 2026, Fee Schedule for Feb 2026 – 2.5.26 Update, describes fees on immediately matched orders, gives the general calculation mechanics, and directs users to the current schedule. Because the live page is the authority for current rates, do not treat figures from the February document as current without checking that page.
Keep fees separate from spread and slippage
The fee is an explicit charge. Spread and book impact are execution costs created by the prices at which you fill. If you calculate your actual cash outlay from the book-walk prices, do not subtract the same price difference again as a separate slippage charge.
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5. Combine payouts and costs by outcome
After calculating both legs, list the net cash flow for every possible resolution. A simple two-leg hedge can be written as:
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Net cash flow in outcome k = payout from leg A in outcome k + payout from leg B in outcome k − all purchase costs − all purchase fees − exit costs and fees.
For a guaranteed hedge, the smallest of those outcome-specific net cash flows must still be positive after costs. If the minimum is negative, the trade is not guaranteed arbitrage even if the average or most likely outcome looks profitable.
Guaranteed payout versus expected value
A guaranteed calculation uses only contractual payouts and costs. An expected-value calculation adds your probability estimate for each outcome. For expected value, multiply each outcome’s net cash flow by your estimated probability and add the results; that number can be positive while one or more outcomes lose money. Label it accordingly and account for the possibility that your probability is wrong.
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The following numbers are illustrative, not a quoted market rate. They show how to avoid counting slippage twice.
Step 1: Set the intended hedge
Suppose you plan to buy 100 shares of leg A and 100 shares of leg B. Under the written settlement rules, exactly one leg pays $1 per share and the other pays $0, so the combined payout is $100 in either covered outcome.
Step 2: Walk both books
- Leg A has 60 shares offered at $0.46 and 40 shares at $0.48. The cost is (60 × $0.46) + (40 × $0.48) = $46.80, or a $0.468 average fill.
- Leg B has 50 shares offered at $0.48 and 50 shares at $0.50. The cost is (50 × $0.48) + (50 × $0.50) = $49.00, or a $0.490 average fill.
The reference estimate used $0.46 for leg A and $0.48 for leg B, or $94.00 total. Actual book-walk cost is $95.80, so estimated slippage against the reference is $1.80.
Step 3: Apply an illustrative fee formula
Assume, solely for this example, that the applicable fee rate for each leg is 0.02 and that the venue uses the Polymarket formula. Leg A’s fee is 100 × 0.02 × 0.468 × (1 − 0.468) = $0.49795 after five-decimal rounding. Leg B’s fee is 100 × 0.02 × 0.490 × (1 − 0.490) = $0.49980. Total fees are $0.99775.
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Using actual fills, net result is $100.00 − $95.80 − $0.99775 = $3.20225 before any other documented cost. Using the reference-price method produces the same result: $100.00 payout − $94.00 reference outlay − $1.80 slippage − $0.99775 fees = $3.20225.
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If the two contracts did not cover exactly opposite outcomes, or if one could be cancelled under a different rule, the $100 combined payout assumption would fail and this would not be a guaranteed hedge.
6. Stress-test the apparent margin
A small positive result can disappear when execution changes. Recalculate at least three adverse cases:
- Higher fill prices: move the next fill on each leg to a worse visible level or add a conservative increment if the book is thin.
- Less available depth: remove some displayed quantity to reflect cancellations before your order arrives.
- Fee or rounding change: use the next higher applicable category rate or the largest fee permitted by the current schedule you are relying on.
- Legging delay: change the second leg’s price to a plausible quote after the first leg executes.
- Exit before settlement: include a second book walk, fee, and spread for closing both positions.
If a small movement in price, depth, or fees turns the minimum outcome negative, call the hedge fragile rather than guaranteed. A wider margin that survives these stresses is more meaningful than a narrow headline difference.
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When several contract pairs or order approaches are available, compare them on the same quantity and settlement basis:
- Total fees: calculate each leg’s fee at its actual price and quantity, including an exit if relevant.
- Maker or taker treatment: a maker order may have different fees and a lower probability of filling; a taker order may fill quickly but pay the applicable taker charge.
- Spread and depth: compare the volume-weighted fill for the intended size, not just the best quote.
- Execution and legging risk: assess the chance that one leg fills while the other moves or remains unfilled.
- Payout equivalence: verify the rulebooks rather than inferring equivalence from market names.
- Capital duration: include how long funds remain tied up until both contracts settle and can be withdrawn.
The best-looking price pair can be inferior after these adjustments. A slightly more expensive leg with deeper liquidity or matching settlement rules may produce a higher reliable net result.
8. A pre-trade worksheet you can repeat
- Copy the resolution language and settlement date for both contracts.
- Write the payout for each leg in every outcome, including cancellation or invalidation cases.
- Choose the exact quantity and identify whether each order is intended as maker or taker.
- Record the timestamped order-book levels available on both sides.
- Walk the book to compute each leg’s average fill and cash cost.
- Calculate slippage against the reference prices, or use actual cash costs directly.
- Open the venue’s current fee schedule and the individual market’s fee terms.
- Apply the formula and rounding rule to every executed fill on each leg.
- Add any funding, conversion, withdrawal, or other transaction cost documented by the venue.
- Calculate net cash flow separately for every resolution outcome.
- Run the adverse-price, reduced-depth, fee, and legging scenarios.
- Place the hedge only if the result meets your required margin after the stress tests; otherwise reduce size, change execution, or skip it.
9. Common calculation failures and fixes
Using the midpoint as the fill
Failure: multiplying the desired size by a midpoint or top-of-book quote that covers only a few shares.
Fix: walk each visible level and use the volume-weighted average price for the intended quantity.
Applying one fee to the whole hedge
Failure: estimating a single percentage on combined notional, ignoring different prices, categories, order types, or rounding.
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Fix: calculate each fill on each leg under the current market-specific schedule.
Counting slippage twice
Failure: using actual book-walk cash costs and then subtracting the same difference from the reference price again.
Fix: choose either actual outlay or reference outlay plus a separate slippage line.
Ignoring the second leg’s execution
Failure: treating the first fill as proof that the hedge is complete.
Fix: model the price, depth, fee, and timing of the second leg and define what you will do if it does not fill.
Assuming labels prove equivalent settlement
Failure: pairing contracts that sound opposite but use different thresholds, dates, sources, or cancellation rules.
Fix: map the written payout for every outcome before comparing prices.
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Failure: carrying forward a rate from an old screenshot, PDF, or third-party calculator.
Fix: check the venue’s live schedule and market-level terms immediately before trading. A secondary methodology such as How We Track Fees: Kalshi & Polymarket Methodology can explain common fee structures, but official schedules control the calculation.
10. Keep the calculation auditable
Save the contract rule links, fee-schedule URL, timestamp, order-book snapshot, intended quantity, fills, fees, and settlement result. Record whether the trade was guaranteed under the rules or depended on an estimated probability. This history lets you distinguish a bad model from an execution problem and prevents a later fee change from being mistaken for slippage.
Final decision framework
Proceed only when the contracts’ written payouts cover the outcomes you intend to hedge, the order book can supply your size at a known average price, every leg’s current fee is included, and the minimum outcome remains acceptable after adverse execution assumptions. If the margin survives those checks, the hedge may be economically worthwhile. If it exists only at the displayed quote, before fees, or under a favorable probability estimate, it is not a fee-and-slippage-adjusted hedge.
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