How to read a prediction market order book: bids, asks, spreads, depth and why the last price can mislead

Every price you see on Kalshi or Polymarket comes from an order book. It is the list of everyone who wants to buy and everyone who wants to sell, and at what price. Reading it takes a minute to learn and saves money on every trade, because it tells you what you will actually pay, not just what the headline price says.

This guide explains the parts of an order book, how Kalshi and Polymarket show them, and how to use the book to trade better.

The parts of an order book

Bids sit on one side of the book, asks on the other. A trade happens when someone is willing to meet the other side's price, either by placing an order that matches a resting one or by accepting the best price on offer.

A simple example

Here is a simplified order book for a Yes contract, showing the five best prices on each side. The numbers are made up for illustration.

Reading it:

• The best bid is 61¢ and the best ask is 62¢. The spread is 1 cent, which is tight.
• The mid-price is 61.5¢, a reasonable estimate of what the market thinks the probability is: about 61.5%.
• If you buy 1,500 contracts at market, you pay 62¢ each. That clears the first level of asks.
• If you buy 5,000 contracts at market, you pay more. You take the 1,500 at 62¢, then 3,000 at 63¢ and 500 at 64¢, for an average of about 62.8¢. That extra cost is slippage.

Yes and No are the same book

On Kalshi, every market has a Yes side and a No side, but they are two views of one book. A contract pays $1 to Yes holders if the event happens and $1 to No holders if it does not, so the two prices always relate:

• A bid to buy No at 38¢ is the same as an offer to sell Yes at 62¢.
• A bid to buy Yes at 61¢ is the same as an offer to sell No at 39¢.

When you look at the No side, you are looking at the Yes book upside down. Kalshi shows bids for both Yes and No; the best Yes ask is 100 minus the best No bid.

Polymarket works the same way with its Yes and No shares: each pair pays out $1 between them, and the order book for No mirrors the book for Yes.

A real example

On the morning of 5 November 2026, Kalshi's market for that evening's Jacksonville–Baltimore NFL game showed:

Source: Kalshi market data, KXNFLGAME-26NOV05JACBAL, 5 November 2026.

Notice the relationship: the best bid for Jacksonville (81¢) plus the best ask for Baltimore (19¢) is $1. Buying Baltimore at 19¢ is economically the same as selling Jacksonville at 81¢. Our guide to betting on the NFL on Kalshi walks through this market in more detail.

Why the last price can mislead

Most market pages show the last traded price in big numbers. It is often a poor guide to what you will pay:

• It may be old. In a quiet market, the last trade might have been hours ago, before news that has since moved the bids and asks.
• It may be a small trade. One trade of ten contracts at an unusual price can set the "last price" for a market where nobody will actually trade there now.
• It ignores the spread. If the best bid is 40¢ and the best ask is 50¢, the last trade at 45¢ tells you little about where you can buy (50¢) or sell (40¢).

The mid-price and the spread are more useful. If the spread is wide, treat any single price with caution.

How big orders move the price

If you trade more contracts than are available at the best price, you work through the book. This is slippage, and it is the hidden cost of trading large size in a thin market.

Two ways to limit it:

• Use a limit order. It fills only at your price or better. If not enough contracts are available, the rest of your order waits on the book.
• Split the order. Trading smaller pieces over time lets other traders refill the book between your trades.

Our guide to slippage, volume and spreads has more on the cost of size, and limit orders vs market orders explains the two order types.

Spreads: what is normal?

Spreads vary a lot by market. Our study of spreads on both platforms, from late October 2026, found:

• the median spread among markets that had traded in the previous day was about 2 cents on Kalshi and 2.4 cents on Polymarket;
• about a third of active markets had a spread of just 1 cent;
• quieter markets and long-shot outcomes often had spreads of 5 cents or more.

As a rule of thumb, a 1–2 cent spread is cheap to trade; above 5 cents, use limit orders and be patient.

Reading depth: is the price real?

A tight spread with very little depth is fragile. If the best bid is 61¢ for 50 contracts and the next bid is 55¢, a single seller can knock the price six points. Before trusting a price as a probability, check how many contracts sit near it.

This matters most:

• in markets with little volume, such as long-dated political markets or niche props;
• during fast news, when market makers pull orders;
• when two platforms disagree. If one shows a price backed by thin depth, the other may be the better guide.

Who puts the orders in the book?

Most of the depth in busy markets comes from market makers: traders and firms that post bids and asks on both sides and try to earn the spread. Both platforms encourage them. Kalshi runs incentive programmes for liquidity providers, and Polymarket pays liquidity rewards to users whose resting orders sit close to the mid-price (Polymarket rewards).

That explains a few things you will see:

• Spreads tighten as volume grows. More trading attracts more market makers.
• Books thin out before big events. Market makers pull orders when news is about to break, so spreads widen just before a game, a data release or poll closing.
• Round numbers attract orders. Bids and asks cluster at prices like 50¢, 75¢ and 90¢.

Placing a limit order using the book

1. Look at the best bid and best ask. Say they are 61¢ and 62¢.
2. Decide how patient you are. Buying at 62¢ fills now; bidding 61¢ joins the queue and may fill later, or not at all.
3. Check the depth at your price. If 2,000 contracts are already bid at 61¢, your order waits behind them.
4. Set a quantity you are comfortable leaving on the book. A resting order can fill at any time, including just after news that changes the picture.
5. Review open orders regularly. Cancel orders that no longer reflect your view.

Common mistakes

• Treating the last price as the price. Always check the bid and ask, and how many contracts are available at each before you decide on a size.
• Market orders in thin books. One click can buy through several price levels.
• Forgetting resting orders. An old limit order can fill after the news has turned against it. Check your open orders before any big event, such as an injury report, a data release or polls closing.
• Ignoring the other platform. The same outcome may have a tighter spread on Kalshi or on Polymarket.
• Chasing a price in a fast market. When a book moves quickly, a limit order a cent or two away from the last trade may never fill. Decide your price, set it and walk away rather than raising it again and again.

Where to find the order book

• Kalshi shows the order book on each market's page, with bids for Yes and No and the size at each price.
• Polymarket shows the book under each outcome, with bids and asks and the size at each price.
• Through the API, both platforms publish order books: Kalshi at its market order-book endpoint and Polymarket through its central limit order book. Our API guide covers both.
• On PredictDesks, our market pages and comparison tools show prices side by side.

FAQ

What is the spread? The gap between the best price someone will sell at (ask) and the best price someone will buy at (bid).

Which price is the market's probability? The mid-price between the best bid and best ask is a better estimate than the last trade, especially when the spread is wide.

Why did I pay more than the price shown? Because the price shown was probably the last trade or the mid-price, and you bought at the ask, or your order was large enough to move through several price levels.

Are Yes and No separate markets? No. On Kalshi and Polymarket they are two sides of the same book.

How do I avoid paying the spread? Use limit orders and let others trade against you. On Kalshi that can also lower your fee (Kalshi fees).

Not investment advice.