Order books and how they work, reading bids, asks, and the spread.

An order book is a real time list of every resting order for a single contract, sorted into bids that want to buy and asks that want to sell. The best bid is the highest price a buyer will pay right now and the best ask is the lowest price a seller will accept right now, so the difference between them, the spread, is the price of trading at once.

When your order crosses the spread and takes a resting order you are the taker and you usually pay a fee. When your order rests on the book and waits for someone to trade against it you are the maker, and on most venues you pay little or nothing. Size matters as much as price: a large order eats through several levels and fills at a worse average than the top quote, which is what traders call slippage.

What an order book actually is, a queue of intentions

Strip away the screens and an order book is a list. For one contract, say a Yes share that pays one dollar if an event happens and nothing if it does not, the book holds every order that traders have placed but that have not yet traded. Each order says three things: a side, buy or sell, a price, and a size, meaning how many contracts. The exchange sorts these orders so that the most competitive ones sit at the top, then matches a new order against the best available order on the other side. Nothing more mysterious is happening. A price you see on a prediction market is simply the level where the next buyer and the next seller currently agree.

Two rules govern the queue, and they are worth saying plainly because they explain almost everything else. The first is price priority: a better price always trades first, so the highest bid and the lowest ask are next in line. The second is time priority: when two orders share the same price, the one that arrived earlier trades first. This is why placing a resting order early can matter, and why the same price can fill instantly for one trader and sit untouched for another. A central limit order book, the model used by Kalshi, Polymarket and Limitless, is just this queue run continuously and shown to everyone at once.

Because the book is public, it is the single most honest object on a trading screen. The headline price can be stale or thin. The book cannot lie about what is actually there: it shows the exact prices, the exact sizes, and the exact gap you would have to pay to act now. Learning to read it is the difference between trading the price you hoped for and trading the price that exists.

Illustrative book for one Yes contract. Bar length shows the size resting at each price. The best ask is the cheapest place to buy now, the best bid the highest place to sell now, and the 2 cent gap between them is the spread.

Bids, asks, and the best price, the top of the book

Every order book has two sides. Bids are the buy orders, listed from highest to lowest, and the one at the very top is the best bid. Asks, sometimes called offers, are the sell orders, listed from lowest to highest, and the one at the top is the best ask. In Figure 1 the best bid is 59 cents and the best ask is 61 cents. If you want to buy a Yes share immediately, you pay the best ask of 61 cents, because that is the cheapest contract anyone is currently willing to sell. If you want to sell immediately, you receive the best bid of 59 cents, because that is the most anyone is currently willing to pay.

This is the part that surprises new traders. There is no single price. The number a platform shows as the market price is usually the midpoint or the last traded price, but you cannot trade at the midpoint on demand. You buy at the ask and sell at the bid. The two sides are real people and real algorithms with real orders, and you only ever interact with the top of the queue unless your order is big enough to reach deeper.

Prices on these venues run from 1 cent to 99 cents and are read as an implied probability, so 61 cents on a Yes share reads as the market pricing roughly a 61 percent chance. We cover that translation in reading prices as implied probability. For now the point is narrower: the price you actually get is set by the book, not by the headline number.

The spread, the cost of immediacy

The spread is the distance between the best bid and the best ask. In our example it is 2 cents. That gap is not a fee charged by anyone. It is the price the market puts on immediacy, and it is collected by whoever is willing to wait on the other side. If you buy at 61 cents and change your mind a second later, you can only sell at 59 cents, so you are down 2 cents before anything has happened. You pay the spread on the way in and again on the way out. A round trip in this market costs you the full spread.

This makes the spread one of the most underrated costs in event trading, because it is invisible. There is no line item for it. A market quoted at 50 cents with a 1 cent spread is cheap to trade. The same market quoted at 50 cents with a 10 cent spread, meaning a best bid of 45 cents and a best ask of 55 cents, is expensive and the price signal is far less precise. A wide spread tells you two things at once: trading will cost more, and the crowd is less sure where the true price sits. We go deeper in understanding the spread.

A useful habit is to treat the spread as a tax on impatience. If you must trade now, you accept it. If you can wait, you can often place a resting order inside the spread and let someone else pay it to you instead. That choice between paying the spread and earning it is the maker and taker distinction, and we come to it shortly.

Depth and the ladder, how size moves price

Depth is the amount of size resting at each price, and it decides what happens when your order is large. A small order trades against the top of the book and gets the best price. A large order is not so lucky. Once it clears the size at the best level, it has to keep filling against the next level, then the next, paying a little more each step. The result is an average fill that is worse than the top quote. That difference is slippage, and the only thing that controls it is depth.

Work through it with the asks from Figure 1. Suppose you send a market order to buy 500 Yes contracts. There are only 150 available at the best ask of 61 cents, so you take all of them, then 200 at 62 cents, then 80 at 63 cents, and the last 70 come from the 64 cent level. You wanted 61 cents. You ended up paying an average of about 62.1 cents. The table below traces the fill level by level, and slippage explained takes the idea further.

Our worked example using the illustrative book in Figure 1, as of June 2026. The top quote was 61 cents but the 500 lot averaged 62.14 cents because depth thinned above the best ask. In a deeper book the same order would have filled closer to 61 cents.

Bar height is the number of contracts taken at each price. The dashed line marks the average fill of 62.14 cents. The deeper your order reaches, the further your average drifts from the top quote. Illustrative, June 2026.

Market orders and limit orders, taker and maker

There are two basic ways to interact with a book. A market order says fill me now at whatever the book offers. It guarantees you trade but not the price, and as we just saw, a large market order pays slippage. A limit order says fill me only at this price or better. It guarantees the price but not the trade, because if the book never reaches your level, your order simply waits, or expires unfilled.

This maps directly onto the language of fees. When your order crosses the spread and immediately takes a resting order, you are the taker, because you took liquidity that was already there. When your order rests on the book and waits for someone to trade against it, you are the maker, because you made liquidity available for others. Most venues reward makers and charge takers, because resting orders are what give everyone else something to trade against. A maker adds depth to the ladder. A taker removes it.

The practical lesson is that the cheapest way to trade is rarely the fastest. If you place a limit order to buy Yes at 60 cents, one cent inside the 59 to 61 spread, you might get filled at a better price than the 61 cent ask and pay a lower fee for the privilege. The trade may also never happen. Whether that is worth it depends on how badly you want the position and how fast the market is moving. The full trade off is covered in limit orders versus market orders.

Maker and taker fees, who pays and why

Fees on an order book venue usually sit on the taker. Kalshi, which is regulated by the Commodity Futures Trading Commission as a designated contract market, charges a taker fee that rises and falls with the contract price. Per Kalshi's published fee schedule as of February 2026, the taker fee is calculated as 7 cents times the price times one minus the price, per contract, so it is largest near 50 cents and smallest near the extremes. Maker fees are 25 percent of that figure and, on most standard markets, are zero, so a resting limit order often costs nothing to enter.

Polymarket runs a hybrid order book where matching happens off chain and settlement happens on chain on the Polygon network. Per Polymarket's documentation as of 2026, makers pay no fee and can earn rebates, while takers pay a small fee that is zero on many markets and a couple of cents per contract on others. Limitless, an onchain venue on the Base network, also charges only takers: per its published fee documentation as of June 2026, limit orders pay nothing, while taking buys cost between 0.40 and 3.00 percent and taking sells cost between 0.42 and 1.50 percent, scaled by where the price sits. The table compares the three.

Figures taken from each platform's own published terms, read in June 2026. Fee models change often, so verify the current rate on the platform before you trade. For a full cross platform breakdown see our cross platform fees table.

Order books versus automated market makers, two ways to find a price

Not every prediction market uses an order book. The main alternative is an automated market maker, often shortened to AMM, where a smart contract holds a pool of funds and quotes a price from a formula rather than from a queue of human orders. You trade against the pool, and the price moves along a curve as you buy or sell. Some onchain venues and some play money platforms use this model because it provides a price even when few people are trading. Limitless, for instance, offers both an order book and an AMM style mode for certain markets, and its published terms describe a flat fee on AMM trades alongside the dynamic taker fees on the order book.

The trade off is worth understanding. An order book gives you transparency and, in a liquid market, very tight spreads, because competing makers narrow the gap. Its weakness is that a thin book has nothing in it, so a quiet market can be costly to trade. An automated market maker always has a price and never an empty book, but the formula can charge you more on large trades and the quoted price can lag a fast moving event. Neither is better in the abstract. What matters is depth and cost in the specific market in front of you, which is exactly what the book or the pool will show you if you look.

This is also why liquidity is the quiet hero of the whole subject. A great book is one with depth on both sides and a narrow spread. We treat that idea on its own in liquidity and why it matters and look at the traders who supply it in market makers and liquidity.

How the major venues run their books, Kalshi, Polymarket, Limitless

The three best known order book venues differ in where the book lives and who oversees it. Kalshi runs a fully centralised book inside a regulated US exchange. Orders are matched on Kalshi's own systems and the venue is overseen by the Commodity Futures Trading Commission, which means there is a named regulator and a published rulebook behind the price you see. For a US trader who wants a readable book under a clear legal framework, that is the headline feature.

Polymarket runs a hybrid model: a central operator matches orders off chain for speed, then each match settles on the Polygon blockchain so the trade is recorded onchain. The book behaves like a normal exchange book to use, but the settlement layer is public and verifiable. Limitless goes further onto Base, an Ethereum layer two network, with separate Yes and No books for each market and USDC as the collateral. Its own terms, updated 19 June 2026, list the United States among the jurisdictions it does not offer trading to, so a US reader should treat it as a reference point for how onchain books work rather than a venue to open. Availability and legality are the deciding factors, and they differ by person and place, so always confirm your own standing first. Our regulated versus offshore guide draws the line in detail.

Reading a book before you trade, a short checklist

Before you place an order, three quick reads on the book will tell you most of what you need. First, check the spread. A narrow spread means cheap immediacy and a confident crowd. A wide one means the opposite, and may mean you should use a limit order rather than pay to cross. Second, check the depth near the top, not just the best price. If you plan to trade 1,000 contracts but only 80 rest at the best ask, you already know your fill will drift, so size your expectation to the ladder, not the headline.

Third, check both sides for balance. A book stacked heavily on bids with thin asks, or the reverse, can move sharply on a modest order, and a lopsided book is easier to push around. None of these reads tells you whether the outcome will happen. They tell you what it will cost and how stable the price is. That is the right job for the book, and a disciplined trader keeps the two questions separate, as we stress in common mistakes new traders make.

What an order book cannot tell you, the limits of the ladder

A book is a precise picture of supply and demand at one instant, and that is all it is. It cannot tell you whether the event will resolve Yes or No. It cannot tell you whether the price is fair, only where buyers and sellers currently meet. It can be thin one moment and deep the next as orders arrive and cancel, so a snapshot is not a promise. And it can be influenced: a large resting order can be placed to create an impression and pulled before it trades, which is why most regulated venues prohibit such practices and why you should never read a single big order as a signal of truth.

Read well, the book is the most useful tool on the screen for managing cost and risk. Read as a crystal ball, it will mislead you. The price is an implied probability set by people who can be wrong, the spread is a cost, the depth is a constraint, and the outcome is still unknown. Hold those four facts together and the order book becomes exactly what it should be: a clear, honest map of what it costs to act, and nothing more.

Understanding an order book lowers your trading cost. It does not change the fact that event contracts can lose money, and trading more often or in larger size raises that risk, not your skill. Trade only with money you can afford to lose, set your own limits, and step away if it stops feeling like a considered decision.

If trading is affecting your wellbeing or finances, free and confidential help is available. In the United States you can call or text the National Problem Gambling Helpline on 1-800-522-4700, available 24 hours a day. 18+ or the legal age in your region.

This is an explainer, not a recommendation, and we never point you to a platform you cannot legally use. To see how each venue structures its book, fees and availability for your region, start with the platform profiles and the cross platform data, then confirm your own eligibility before doing anything.

Questions people ask, answered plainly