Fills are executed trades—partial or complete matches against an order. When your buy or sell order matches with available liquidity on Polymarket, the transaction settles instantly, and you may receive partial fills.
Fills are executed trades—partial or complete matches against an order. When your buy or sell order matches with available liquidity on Polymarket, the transaction settles instantly, and you may receive partial fills.
A fill is the execution of your order—whether partial or complete. When you place a buy or sell order in a prediction market, it doesn't execute instantly just because you submitted it. Instead, your order sits in the order book waiting for a matching order on the opposite side. A fill occurs the moment your order finds a counterparty match and the transaction is executed at a specific price and quantity. On Polymarket, you might place an order to buy 100 shares of "Yes" at 65¢, and that entire order could fill immediately if there's sufficient liquidity at that price—meaning other traders have already placed sell orders at 65¢ or lower. Alternatively, the exchange might execute only 60 shares at your requested price and leave the remaining 40 shares in the order book—this is called a partial fill. Partial fills are common in less liquid markets where not enough supply or demand exists at a single price level to match your entire order size.
The term "fill" originates from traditional equities and derivatives trading, where brokers and market makers needed precise language to communicate trade execution to clients and back-office systems. In those markets, a fill referred to the quantity and price at which an order was executed, and the term became standard across all financial exchanges. The prediction market industry adopted the same terminology because the underlying mechanics are fundamentally identical—when you place an order, you're seeking a counterparty; when that counterparty is found, the trade fills. On Polymarket, fills matter because they directly determine your entry price, your cost basis, your P&L, and your risk on a position. Understanding fills helps traders distinguish between the order they intended to place and the actual execution they received—a critical distinction when prices move rapidly or when liquidity dries up during market-moving news.
On Polymarket, you encounter fills in several practical contexts. When you place a market order—buying or selling at the best available price right now—you typically receive an immediate fill or near-immediate fill at the offer (for buys) or bid (for sells), though the exact price may differ slightly from what you saw on screen due to latency or market movement. If you place a limit order specifying an exact price and size you're willing to trade, that order waits in the order book until either the market price reaches your limit or another trader chooses to cross your bid. In the "My Activity" panel or "Open Orders" section, you can see all your fills—both completed orders that filled entirely and any partial fills still awaiting execution. Each fill entry shows the timestamp, the fill size, the fill price, and the remaining quantity if the order was only partially executed. This transparency is essential for tracking your position entries, calculating your true cost basis, and managing risk across multiple tranches of a trade.
Partial fills can catch inexperienced traders off guard. Imagine you want to close a position and place a sell order for 500 shares at 35¢. If the market only has 300 shares of buy interest at 35¢ or better, you will receive a partial fill of 300 shares, and 200 shares will remain in the open order book. Some traders expect all-or-nothing execution and are surprised when they receive a partial fill; they may have intended to sell all 500 shares only if they could do so at that exact price, not in pieces. Others strategically use partial fills to test liquidity and gauge market interest, then adjust pricing on the remaining shares if the initial tranche fills quickly. Another common misconception is that all fills at the same price happened simultaneously. In reality, Polymarket's order book may execute your order in rapid succession across multiple matching cycles, but all those executions appear in your activity feed grouped as fills at the same price point, even though they technically matched microseconds apart. This can matter for tax reporting and for understanding the true timing of your exposure.
Related concepts deepen your understanding of how fills work in practice. An open order is one that has not yet been completely filled—some or all of the original quantity still awaits execution in the order book. A market order typically executes as a single aggressive fill against resting liquidity and is usually filled immediately or within milliseconds. A limit order, by contrast, may accumulate multiple fills as the market gradually reaches different price levels and matches with your standing bid or offer. The concept of fill rate—what percentage of your intended order size actually executed—matters for calculating slippage and evaluating strategy execution quality. Understanding the distinction between a taker (your order aggressively crosses the spread and takes existing liquidity, usually filling instantly) and a maker (your order rests in the order book and waits for others to cross your bid or ask) clarifies why some orders fill instantly while others hang in the book for hours or days. Orderbook depth and liquidity also directly impact whether you receive partial fills or full fills and at what prices.
You place an order to buy 1,000 shares of "Will the Fed cut rates by 25bp in June?" at 62¢ per share. The order book has 700 shares available at 62¢ from other traders, so you receive a partial fill of 700 shares immediately. Your remaining 300 shares stay in the open order book at 62¢, waiting for more sellers to appear; if the market price rises above 62¢, your order will never fill.