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Updated September 25, 2026 · facts checked against the dated sources listed below
A few mistakes turn up again and again on prediction markets, and each is cheaper to avoid than to learn. Every one below comes with a worked number, so you can run the same check on your own trades. This is educational material, not financial advice.
A market pays on its written rules, not on what its title seems to say. Polymarket's Will the U.S. invade Venezuela by January 31, 2026? defined an invasion as a military offensive intended to establish control over any portion of Venezuela. After the US operation that captured Nicolás Maduro, its Yes price went from 2.5¢ at 04:00 UTC on January 3, 2026 to about 60¢ four hours later, fell to about 5¢ by January 5, and the market resolved No. A buyer at 60¢ lost the whole stake.
See also: How Polymarket markets resolve · What moves Polymarket prices
The chance on the page is a midpoint; you buy at the ask and pay a taker fee on most markets. On September 25, 2026, US recession by end of 2026? showed 10%, a 9.5¢ midpoint, but 100 Yes shares cost 10¢ each plus 100 × 0.05 × 0.10 × 0.90 = $0.45 in fees, $10.45 in all, so the break-even chance was 10.45%. Selling straight back at the 9¢ bid would have returned $9 minus a $0.41 fee, $8.59: a $1.86 loss, about 18% of the stake, with no change in the odds.
See also: Polymarket spread and slippage calculator · Polymarket fee calculator
Even a real edge loses often. If you think a 50¢ contract has a 60% chance, the Kelly formula, (0.60 − 0.50) ÷ (1 − 0.50), gives 20% of your bankroll as the most to stake, and only if your 60% is right; a fraction of it cuts the swings. With a 40% chance of losing each time, three losses in a row happen 6.4% of the time. At 20% a stake, that run leaves 51.2% of the bankroll; at 50% a stake, it leaves 12.5%.
See also: Kelly criterion calculator · Bankroll planner · Risk of ruin calculator
A 3¢ share pays 33 times its price if it wins, which is why long shots tempt. If the true chance is 2%, each dollar staked returns 67 cents on average (2 ÷ 3), a 33% expected loss before fees, and at a 0.05 fee rate the fee adds about 5% to a 3¢ price (0.05 × 0.97). Cheap outcomes can be mispriced in either direction, so check how often they have come true rather than assuming; this site's long-shot study tracks that for resolved Polymarket markets.
See also: Favorite-longshot bias on Polymarket · Expected value calculator
A 90¢ favorite still fails about one time in ten if prices are fair. Buy ten independent 90¢ favorites and the chance that all ten win is 0.9 to the tenth power, about 35%. At 95¢ you risk 95¢ to make 5¢, so a single loss wipes out the profit from 19 wins.
See also: Why favorites lose · How accurate is Polymarket?
A 95¢ share that pays $1 returns 5.3%, and the calendar decides whether that is good. If it resolves in 30 days, that is about 64% a year on a simple basis; if resolution takes 120 days, about 16%. Polymarket settles an undisputed proposal about two hours after it is made, but a disputed one takes four to six days, and many markets first wait for an official result; this site's resolution-time study shows how long settlement has taken after the scheduled end.
See also: How long Polymarket takes to resolve · Polymarket APY calculator
The price on the screen may hold only for the first few hundred shares. Suppose the best bids are 200 shares at 60¢, 300 at 58¢ and 500 at 55¢, and you sell 1,000 shares at market. You receive $120 + $174 + $275 = $569, an average of 56.9¢, or $31 less than the 60¢ bid suggested, before fees. Polymarket notes that there is no guarantee a trade of a given size can happen without moving the price.
See also: How to read a Polymarket order book · How to spot a thin order book
No. It is the midpoint of the best bid and ask. Takers pay shares × fee rate × price × (1 − price) on most markets, so work out the fee before you trade.
Kelly's formula, (your probability − price) ÷ (1 − price), gives an upper limit when your estimate is right. A fraction of it leaves room for being wrong; the bankroll planner shows how different sizes play out.
Only when your chance is clearly higher than the price plus fees. In this site's sample of resolved Polymarket markets, cheap outcomes happened more often than their prices implied, but the wins clustered in a few events.
Settlement waits for the result and the resolution process. An undisputed Polymarket proposal settles about two hours after it is made; a disputed one takes four to six days.
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