Before you place a trade, the order book tells you everything you need to know about a market. It shows all active buy and sell orders, the prices they are sitting at, where liquidity exists, and how a larger order might execute.
Why was my realized profit lower than the profit I saw before I closed?
If your realized profit came out lower than the profit showing before you closed, the most common cause is that your sell order was larger than the number of contracts buyers were offering at the top price. Your order then filled at the next price down, and the next, until the full size sold. Your realized profit is based on the average price across the whole order, not the top price you saw before selling.
Example: You sell 1,086 contracts. The highest bid is 97 cents, but only 44 contracts are available at that price. Once those fill, the rest of the order fills at progressively lower prices, down to 41 cents for the last contracts. Averaged across all 1,086 contracts, your effective sell price comes out to about 77 cents. The profit that posts to your account is based on 77 cents, not 97 cents.
This is slippage, and it is expected behavior on any market with limited depth. It is not a platform error, and it does not mean your trade was recorded incorrectly.
How do I avoid losing profit to slippage when I close a large position?
To reduce slippage when you close a large position on Catalyst:
Check the order book before you close. It sits below the order ticket on the market page and shows how many contracts buyers are offering at each price.
Use a limit order for larger sells. A limit order sets the lowest price you will accept, instead of taking whatever price is needed to fill your full size.
Size your position with the order book in mind. A position larger than the market can absorb is harder to exit at the price you expect.
If the order book looks thin and you are not in a hurry, holding to resolution is often better than selling into a shallow market.
Key terms
Term | Definition |
Bid | The highest price a buyer is willing to pay |
Ask | The lowest price a seller is willing to accept |
Spread | The difference between the best bid and the best ask. A wider spread means it costs more to enter and exit a position |
The order book is visible below the order ticket on each market page. Click Order Book to expand it. It shows Price, Shares, and Total for each available price level on both sides of the market.
How liquidity affects your trade
Liquidity refers to how many contracts are available at different price levels. It directly affects how your order executes.
Higher liquidity | Lower liquidity |
Faster fills | Slower fills |
Tighter spreads | Wider spreads |
Less price movement on entry/exit | Larger orders may fill across multiple price levels |
Slippage
When a larger order fills across multiple price levels, your average execution price may differ from the price you saw when you placed the order. This is called slippage and is a normal part of trading in any order book market.
Example: YES is showing at 68¢. You place a $400 market order. There are only $150 worth of contracts at 68¢, then another $250 at 70¢. Your order fills $150 at 68¢ and $250 at 70¢. Your average fill price is approximately 69.3¢, slightly worse than the displayed price.
The same applies when selling, your sell fills against the highest available buy offers, which sit below the displayed price, so a larger sell order will average lower than the quoted price.
You'll see this reminder on every order ticket: "Large orders may fill across multiple price levels in the order book, so your average fill price may differ from the quoted price." This is expected behavior, not an error.
Where Catalyst's liquidity comes from
Catalyst's order book mirrors Polymarket's live order book directly. You are trading against the same pool of liquidity as traders on Polymarket. Catalyst does not add its own liquidity and does not match orders between Catalyst users.
The volume you see is real Polymarket volume, not just activity from other Catalyst traders.
Why prediction markets have more slippage than futures or stocks
Traders coming from futures or forex often notice slippage feels higher here. That is expected.
Futures markets have billions of dollars sitting in the book at any time. Prediction markets are event-specific with a defined end date. Total volume in a prediction market might be $50,000 or $500,000, not billions. On a thinner book, mid-size orders move through several price levels fast.
This is normal for prediction markets and is not specific to Catalyst. Position sizing relative to available liquidity matters more here than in traditional markets.
Why buys show more slippage than sells
Buy orders walk up the ask side of the book. Sell orders walk down the bid side.
On prediction markets, the bid side tends to have more liquidity clustered near the current price because buyers compete with each other for contracts. The ask side is often thinner, so buy orders move through more price levels before completing, and sells tend to fill closer to the expected price.
Buys will almost always show more slippage than sells in the same market. This is normal.
How to reduce slippage
Check the order book before placing larger orders to understand available depth
Use a limit order to cap the price you are willing to pay
Size your orders relative to available liquidity, a $200 order on a market with $500 in depth fills very differently than the same order on a market with $50,000 in depth
Be aware that newer or less popular markets tend to have less liquidity