Spread and Slippage

Not all trading costs show up as a line item called a fee. Two of the most important hidden costs in prediction markets are the bid ask spread and slippage. These costs can materially affect your net results, especially in fast moving or low liquidity markets.

This section explains what spread and slippage mean in practical terms, why they occur, and how to think about them before placing an order.

Spread as an Implicit Cost

The bid ask spread is the gap between the best available buy price and the best available sell price. If you buy at the ask, you are paying the higher side of the market. If you sell at the bid, you are accepting the lower side.

That difference is an implicit cost because it is built into the market’s quotes. You may not see it as a platform fee, but it affects your entry and exit prices.

In general, tighter spreads reduce friction. Wider spreads increase friction and make it harder to enter and exit without needing the market to move in your favor.

Slippage as a Difference Between Expected and Executed Price

Slippage is the difference between the price you expected to trade at and the price you actually receive when your order executes.

Slippage can happen even if you understand the quoted bid and ask. It often occurs because:

• the available size at the best price is smaller than your order
• the market moves while your order is executing
• you use a market order in a book with limited depth
• multiple participants are trading at the same time and quotes update quickly

Slippage is usually discussed as an execution effect rather than a fee. It is a function of liquidity and the order book, not a charge set by the platform.

Why Slippage Happens in Prediction Markets

Prediction markets frequently have uneven liquidity. Many markets are active only around key news events, data releases, or late in the market’s life. In these moments, prices can update rapidly.

Slippage becomes more likely when:

• the market is thin and depth is limited
• you trade a larger size relative to the available top of book
• you trade during fast news cycles, when quotes change quickly
• you rely on market orders instead of controlling price with limits

Even in a market that appears calm, slippage can occur if the visible quote is not supported by meaningful depth.

A Simple Example Showing Spread Plus Slippage

Suppose a Yes contract shows:

• bid at $0.49
• ask at $0.51

You place a market buy order for a size that is larger than what is available at $0.51. The order fills partly at $0.51 and partly at $0.53 because the next ask level is higher.

Your average fill might be $0.52. In this case:

• you paid the spread by buying at the ask rather than the bid
• you experienced slippage because you did not get the full fill at the top ask

If you later sell quickly, you might sell near the bid, which could still be around $0.49 or might have moved. Either way, spread and slippage can reduce your net result even if the underlying event likelihood has not meaningfully changed.

Practical Ways to Reduce These Costs

You cannot eliminate spread and slippage entirely, but you can reduce them by trading more deliberately:

• prefer markets with tighter spreads and visible depth
• use limit orders when you want price control
• avoid trading large size in thin markets
• be cautious during fast news moments when quotes update quickly
• check the order book before you submit an order, not after

Spread and slippage are predictable sources of friction. Responsible trading means treating them as real costs and sizing trades with execution realities in mind.