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A Polymarket bot should compare its own probability estimate with the price it can actually trade at—not simply with the probability displayed on the market page. For a YES share bought at price p that pays $1 if YES resolves and $0 otherwise, estimated gross expected profit is q − p per share, where q is the bot’s estimated probability. That is a payoff calculation, not proof the estimate is accurate or the trade profitable: fees, spread, available depth, partial fills and settlement all matter.
“Probability arbitrage” is therefore best treated as a model-driven trading strategy, not risk-free arbitrage. Polymarket’s documentation describes market mechanics; it does not establish that any particular bot or forecasting model has an edge.
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What a Polymarket price tells you
Polymarket prices range from $0.00 to $1.00 USDC and are derived from supply and demand. A winning share pays $1 USDC at resolution, according to Polymarket’s FAQ. That payoff structure makes a share price interpretable as a market-derived probability, but the displayed figure is not necessarily a price available for an immediate trade.
Polymarket Help Center’s “How Are Prices Calculated?” article, dated March 13, 2026, says the displayed probability is ordinarily the midpoint between the best bid and ask. If the spread is wider than $0.10, the displayed value is instead the last traded price. A midpoint summarizes the book; it does not mean a buyer can purchase at that value or a seller can exit there.
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How to calculate fair value and expected value
Start with your own probability
Let q be your model’s estimated probability that YES resolves true, and p the per-share price you can actually obtain. For a binary YES share with a $1 payout on YES and $0 on NO:
Gross expected profit per share = q − p
For example, if a model estimates YES at 0.60 and the executable purchase price is $0.54, the estimated gross edge is $0.06 per share before costs. Those figures are illustrative, not a Polymarket quote or a claim about a working model. The calculation assumes the stated binary payout and that the share is bought at the stated price.
For a YES position of C shares, multiply that per-share expectation by C for a simple gross expected profit estimate. A corresponding way to assess a NO purchase is to estimate the probability of NO resolving true and compare it with the executable NO price. Confirm the contract’s resolution rules: the payoff calculation is only useful if the event definition and settlement outcome are understood.
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Use executable prices and intended size
For a buy, assess the available ask-side liquidity; for a sell, assess the bid side. Inspect enough order-book levels to determine what price the intended quantity could actually receive. A displayed midpoint, last trade or top-of-book quote can misstate the economics if the desired size exceeds available depth or the market moves before the order matches.
A practical net-value calculation starts with the expected payout and subtracts the amount paid, applicable trading fees and execution costs. Spread, market impact and partial fills affect the price actually achieved. Do not label a positive estimate from q − p “profit” until those costs and the uncertainty in q are accounted for.
Distinguish a model edge from arbitrage
A model that assigns a higher probability than the executable market price may identify a potentially favorable trade, but the difference is not riskless arbitrage. The estimate can be wrong, the quote can disappear, and an order may fill only in part. No independently validated calibration or profitability result for a particular Polymarket probability-arbitrage bot is established here. Treat performance claims as unverified unless backed by a reproducible evaluation that accounts for fills, fees and settlement.
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How Polymarket fees affect expected value
Polymarket Help Center’s “Trading Fees” article, dated July 10, 2026, says eligible markets charge takers, makers are not charged fees, and rates vary by category. Its stated formula is:
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Here, C is the number of shares traded, feeRate is the applicable market rate, and p is the share price. Apply the fee to the relevant trade side and use the rate currently specified for that market; do not assume one rate applies across Polymarket. The article says geopolitical and world-events markets are fee-free and that fees are collected at match time. Because market fee schedules can change, check the current market-specific data when evaluating an order rather than hard-coding a general rate into the bot.
For an expected-value calculation, include the fee only where it applies and account for the actual execution price. A maker order may avoid the documented taker fee, but it can remain unfilled; a fee-free or lower-fee trade is not automatically a better trade if price, fill probability or execution impact is worse.
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Choose an order based on price, urgency and fill risk
Polymarket’s order documentation distinguishes market orders, which take available liquidity, from limit orders, which specify a price and can rest on the book. The choice is operational rather than universal:
| Consideration | Market order | Limit order |
|---|---|---|
| Price control | Takes available liquidity; the final average price depends on book depth. | Sets a price constraint; execution occurs only if matched at the specified price or better, subject to order behavior. |
| Urgency | Designed to interact with available liquidity immediately, but the book may be thin or change. | Can wait on the book until filled, expired or canceled; it may never fill. |
| Size and impact | Check whether the available levels can support the intended quantity. | Check the minimum size and price increment; a resting quote does not guarantee execution. |
| Lifecycle | Track the returned order status and subsequent match and settlement events. | Track whether it is live, matched, delayed, expired or canceled, and whether it later settles. |
Polymarket documents GTC and GTD order lifetimes. Orders must respect the market’s minimum price increment (tick size) and minimum order size. The book can provide tick size, minimum size and bid/ask levels, and the market stream documents a tick-size-change event. A bot should read current market parameters rather than assume they are fixed.
Build execution and settlement into the bot
An order response is not the end of the trade lifecycle. Polymarket’s quickstart distinguishes order placement and matching from on-chain settlement, and demonstrates waiting for settlement before checking positions. Its sample order is documentation, not a recommended investment size. Bot logic and accounting should represent these as separate states.
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- Read market state: obtain the relevant book levels, tick size, minimum order size and current applicable fee information.
- Calculate the decision: record the model probability, executable price for the intended size, estimated net value and the assumptions used.
- Submit a valid order: choose market or limit behavior, specify any limit and lifetime, and comply with the market’s size and tick constraints.
- Handle the response: distinguish accepted or live orders from matched or delayed orders. Do not book an unfilled order as a completed position.
- Reconcile the trade: account for fills and partial fills, then wait for on-chain settlement before treating the position as settled or checking the resulting position state.
- Update or cancel when appropriate: re-evaluate a resting order if the probability estimate, price, book or market parameters change, and track its final status.
Keep a timestamped record of the model estimate, best bid and ask, relevant depth, fee inputs, order type and limit, fill quantity and price, order status, and settlement status. These fields make it possible to distinguish a forecasting error from a price move, an unfilled order, a cost estimate error or a settlement delay.
Check geographic eligibility before building around the venue
Polymarket’s transparency page says users in restricted jurisdictions, including the United States, may not trade on Polymarket International or use tools intended to circumvent geographic restrictions. Availability and eligibility can depend on location and current platform rules; consult Polymarket’s current official availability information before using the service. This article does not determine whether a particular person is eligible.
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