How it works

How ComboCheck works

ComboCheck works out the fair price of a Polymarket combo by multiplying the legs' probabilities, adds Polymarket's taker fee to the price you were quoted, and turns the gap between the two into a 0 to 100 deal score against real combo trades read from the Polygon blockchain. Every number comes from public data, and every step is below.

Step 1: where the trades and prices come from

Polymarket combos settle on the Polygon blockchain. ComboCheck reads the combo contracts' event logs, which record which legs make up each combo and every trade in it: the price, the size and the fee. Leg names and outcomes come from Polymarket's public market listings. Wallet addresses are never shown.

Step 2: the fair price of the combo

For each leg, ComboCheck takes the midpoint of its Polymarket order book at the time of the trade, as the probability of that outcome. The fair price of the combo is those probabilities multiplied together. This assumes the legs are independent: close to true for legs from different games, not for legs from the same game.

Step 3: the taker fee and the all-in price

The all-in price is the quote plus Polymarket's taker fee, 0.05 x p x (1 - p) per share, where p is the price. The margin is how far the all-in price sits above fair: all-in divided by fair, minus one. A positive margin means the buyer paid more than fair.

Step 4: the margin over fair and the deal score

The deal score is the share of similar recent trades, from the last 7 days, whose margin was worse than yours. "Similar" means the same sport, number of legs, pre-game or live, and same-game or not, widening step by step when there are too few. Scores of 80 and above read "Great price", 60 to 79 "Good price", 40 to 59 "Typical price", 20 to 39 "Worse than most" and below 20 "Poor price". With too few similar trades the score is estimated from the margin alone and says so.

What is left out, and why

• Combos whose fair price is under 0.5 cents or over 99.5 cents, where a tiny price error makes a huge margin.
• Live (in-play) trades are kept apart: order books move within seconds during a game, so their fair prices are less reliable.
• Same-game combos are shown separately, because independence does not hold for them.

How fresh the numbers are

New trades are read within about 30 seconds of settling on chain. The summary numbers are rebuilt every 10 minutes; the day runs from midnight UTC.

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