Last Updated: 2026-10-27
A cheaper quote is useful only if it covers the same result and can be filled at the size shown. A market price is not a verified probability, and a peer-to-peer venue does not make a losing prediction profitable. This comparison uses hypothetical arithmetic, not personal trading results or a ranking of operators.
Start with the rule, not the logo
Write down the event, outcome, settlement source and time zone. For sports, compare regulation time versus overtime, draws, postponements, abandoned games and player-participation conditions. “Team A wins” can mean different things under two sets of rules. Polymarket's resolution documentation makes the rules, rather than the title alone, decisive.
Then preserve the executable quote and quantity, with a timestamp. A last-traded price is not an offer to sell to you. A displayed cash-out amount is not guaranteed to remain available. A difference between two nonidentical contracts is exposure to different results, not a locked arbitrage.
Three numbers that are often confused
Consider a synthetic two-outcome event with no draw, refund or extra fee. Both sportsbook sides quote American odds of -110: risk $110 to win $100, receiving $210 in total if successful.
Scroll sideways to see every column. Keyboard: focus the table region and use the arrow keys.
| Measurement | Calculation | Meaning |
|---|---|---|
| Break-even win probability | 110 / 210 = 52.38% | Required win probability at those odds, before other costs |
| Two-sided overround | 2 × 110 / 210 − 1 = 4.76% | Sum of the quoted implied probabilities above 100% |
| Expected loss if the true chance is 50% | 0.5 × $100 − 0.5 × $110 = −$5 | 4.55% of the $110 stake, not 4.76% |
Overround is a property of the quoted market. Realized bookmaker hold depends on accepted stakes and outcomes. Neither is the fraction of individual customers who lose, and neither proves a particular operator's realized profit.
Put a binary share on the same scale
Suppose a contract pays $1 for a winning outcome and $0 otherwise. Buying one share at $0.52 risks $0.52 for $0.48 profit if it wins. With no costs, its break-even probability is 52%.
For a $110 cost budget, a continuously divisible, fee-free example buys 211.538 shares and pays $211.54 if successful, versus the $210 sportsbook payout above. That $1.54 difference is before fees, rounding and liquidity. It is not proof that today's Polymarket price is better; neither quote here is live.
If your assumed probability is 60%, expected profit is $0.60 − $0.52 = $0.08 per share, or 15.38% of its $0.52 cost. If that assumption is wrong and the chance is 50%, expected profit becomes −$0.02 per share. Expertise or confidence does not establish the input probability.
Polymarket's fee documentation describes market-dependent taker fees. Use the exact product and market's current parameters. Do not apply a blanket zero-fee claim or mix the international and US products. Keep separate rows for entry and exit fees, funding/withdrawal costs, and the price available for the intended quantity. If your calculation already uses executable buy and sell prices, do not subtract the same spread again.
A bonus is a separate, conditional cash flow
A hypothetical $100 bonus bet whose stake is not returned, at +100 odds and a true 50% win chance, pays $100 on a win and $0 on a loss. Its expected cash payout is $50, not $70 or $100. This excludes any paid qualifying bet, expiry, minimum odds, wagering requirement and withdrawal restriction. Those conditions can change the value substantially. Do not increase spending to meet a promotion whose full cash flows you have not calculated.
A comparison record you can actually use
Scroll sideways to see every column. Keyboard: focus the table region and use the arrow keys.
| Field | Evidence to save |
|---|---|
| Eligibility | Exact operator/product, permitted location, account requirements and dated terms |
| Matching outcome | Both rule texts, including cancellation and tie treatment |
| Position | Stake or share quantity; maximum cash loss; payout for each possible result |
| Cost | Fillable price/size, fees and payment costs; bonus conditions listed separately |
| Exit | Available counterparty or cash-out terms; consequences if no exit is available |
| Records | Accepted order/bet, fees, settlement, deposits and withdrawals |
Check geographic restrictions and the relevant operator's terms before funding an account. A working website or wallet is not permission to participate; do not evade restrictions. This guide does not determine local legality or tax treatment. Preserve transaction records rather than relying on a promise that a platform will send a particular form.
If the rules, eligibility or costs cannot be established, the comparison is incomplete. A spreadsheet can compare cash flows; it cannot prove an advantage, guarantee withdrawal speed or make gambling safe. Set limits on time and money independent of any quoted odds, and stop if losses are prompting more spending. If gambling is causing harm, seek a qualified local support service.
Risk and affiliate disclosure: Educational analysis, not financial advice or a recommendation to gamble. The entire stake can be lost. This site may earn commissions from referral links elsewhere; this comparison contains no referral link, paid ranking or claimed personal test.