Polymarket Order Books: Liquidity, Slippage and Real Execution Cost

How the Polymarket order book works

Polymarket uses a central limit order book. Bids are resting offers to buy; asks are resting offers to sell. The spread is the gap between the highest bid and lowest ask. Orders that execute immediately consume liquidity from the opposite side of the book.

The displayed market price may be a midpoint or last trade under the platform’s display rules. It is a useful market summary, not a promise that your order will execute there.

Why the best ask is not the final cost

The best ask applies only to the size available at that level. If you want more shares, the order must consume higher asks until the requested amount is filled or visible liquidity runs out.

Walk the book to estimate VWAP

For each ask level, take the smaller of remaining desired shares and available shares, multiply by price and continue until filled.

If the visible levels cannot fill the request, mark the quote incomplete. Inventing liquidity beyond the book creates a more attractive but false estimate.

Keep four execution measures separate

• VWAP: average price of the modeled fill.
• Price impact: difference between the initial reference price and modeled average.
• Fee: current market-specific charge applied under the protocol rules.
• Completeness: percentage of the intended order supported by visible liquidity.

Combining these into one score hides the reason an entry is expensive or uncertain.

Fees vary by market and price

Polymarket’s current documentation describes market-specific fee settings and a price-sensitive taker-fee formula for fee-enabled markets. A tool should query current parameters rather than hard-code a historical rate.

Fee treatment can also affect the number of shares received. That is why expected payout should use actual modeled shares after fee, not simply divide intended notional by the first visible price.

Why five-minute execution is especially fragile

Between reading the book and submitting an order, other participants can cancel or consume liquidity. Queue position, network delay, order matching, market movement and partial fills can all change the result. An execution quote is therefore a time-stamped estimate, not a reservation.

The correct execution question

Do not ask only whether UP or DOWN is more likely. Ask whether the directional case still offers acceptable value after the intended amount consumes spread, depth and fees. If the answer changes sharply between 10U and 100U, size is part of the strategy—not a detail after the decision.