Fill
The execution of your order. A full fill means your entire order traded; a partial fill means only some did, with the rest still resting or canceled.
In thin Kalshi markets you may only get partially filled at your price, with the remainder filling worse or not at all. Watching fills is part of judging whether a market is liquid enough to trade your size.
A fill is the execution of an order, the moment your trade actually happens against another order in the book. An order can be filled completely, partially, or not at all, and the price of the fill is what determines your real cost basis, not the price you saw when you clicked.
Partial fills are common in thin markets: if you want 500 contracts but only 200 are available at your price, you may be filled on 200 and left with the rest resting or canceled, depending on your order type. A large order can also produce multiple fills at different prices as it works through the book, giving you a blended average.
Reviewing your fills is how you measure execution quality. The gap between the price you intended and the average price you actually got is slippage, and tracking it across many trades reveals whether you are losing money to poor execution, wide spreads, or thin markets, costs that are easy to ignore but add up.
You place a market buy for 300 contracts. You receive three fills: 100 at 60, 100 at 61, and 100 at 63 cents, for a blended average of about 61.3 cents. Those individual fills, not your intended 60, set your true cost basis.
Frequently asked questions
What is a fill in trading?
It is the actual execution of your order against another order in the book. Fills can be complete, partial, or split across multiple prices, and the fill price sets your real cost basis.
Why did my order only partially fill?
Because there was not enough liquidity at your price to complete it. The available size filled, and the remainder either rests in the book or is canceled, depending on your order type.
How do fills relate to slippage?
Slippage is the gap between the price you intended and your average fill price. Reviewing fills across many trades shows how much you are losing to execution, spreads, and thin markets.