Order Book Spread Explained: Bid‑Ask Basics · PolyArb

This guide explains the order book spread in plain terms so you can read and act on it as a new Polymarket trader. You will learn what the spread is, why it exists, how tick size and liquidity affect it, and practical ways traders use the spread when placing limit and market (FAK) orders.

Key takeaways

• The spread is the difference between the best bid and best ask on the CLOB; it is the visible cost to cross the book.
• Tick size, liquidity, and fees determine how tight the spread can be; near-zero spreads require deep resting liquidity.
• Spreads matter for execution: market (FAK) orders pay the spread plus taker fees; limit orders can capture or improve the spread.
• For binary markets the two outcome prices should sum to $1.00 at fair value; spreads on both legs create arbitrage opportunities but carry operational risks.

What the order book spread is

In a Central Limit Order Book (CLOB) the spread is simply best ask minus best bid. For example, if the best bid for YES is $0.45 and the best ask is $0.47, the spread is $0.02. This is the immediate cost to someone who wants to buy with a market-style order (FAK) or to someone who wants to sell immediately.

On Polymarket the spread is quoted in pUSD, and tick sizes are typically $0.01, tightening to $0.001 as prices approach extremes. The spread is a market-implied friction: it compensates liquidity providers for risk and houses structural costs such as latency and adverse selection.

Why spreads exist

Three immutable drivers create and sustain spreads:

• Liquidity depth: Thin books have wider spreads because a single large order would move price significantly. Deep books compress spreads.
• Information and adverse selection: Market makers widen spreads when they fear trading with better-informed participants.
• Market mechanics: Tick size, fees, and discrete order sizes mean prices only move in set increments, which can impose a minimum non-zero spread.

On Polymarket, makers pay zero maker fees while takers face variable taker fees (0%–1.8% depending on category). That asymmetric fee structure encourages posted liquidity but does not eliminate spreads.

Tick size and its practical impact

Tick size defines the smallest price increment a resting order may use. On Polymarket the usual tick is $0.01, tightening to $0.001 when prices are near 0.96 or 0.04. Tick size affects spreads in two ways:

• Minimum spread: When tick = $0.01, the spread cannot be smaller than $0.01 unless both sides share the same price level; a $0.01 tick therefore sets a floor on how tight spreads can be.
• Price improvement granularity: Smaller ticks allow makers to step in with better prices by $0.001, which attracts order competition and narrows spreads.

When you see a spread equal to the tick size, expect that competition can quickly compress it if a new maker posts a better quote.

How to read the spread as a trader

• For impatient buyers: a market (FAK) order immediately crosses the spread and executes against resting asks. You pay best ask plus taker fees and whatever slippage larger fills cause.
• For opportunistic makers: post a limit order inside the spread (if tick granularity allows) to capture the spread as you become the new best on one side.
• For arbitrage-aware traders: in binary markets, watch both legs. If bestAsk(YES) + bestAsk(NO) < $1.00 there is an intra-market edge, but capturing it requires immediate fills and exposes you to fill risk and resolution/settlement risk.

Always check orderbook depth: a narrow spread with no depth behind best quotes is fragile. A visible $0.01 spread with only a few cents of size can disappear the moment someone executes a modest market order.

Spread dynamics through a market lifecycle

Spreads are not static. They evolve with time, news, and liquidity:

• Early life / low liquidity: wide spreads as makers wait for more information.
• Liquid mid-life: spreads tighten as more makers and takers arrive and competitive posting increases.
• News or volatility: spreads widen sharply because makers pull or widen quotes to avoid adverse selection.
• Endgame / close to resolution: spreads can behave oddly—tick changes, order sizes shrink, and you may see both very tight and very wide spreads depending on participation.

On Polymarket, the CLOB and the WebSocket market feed give real-time best_bid_ask and tick_size_change events. Use those feeds to monitor whether a current spread is stable or likely to move.

Practical rules for placing orders

• If you need immediate execution, use a market (FAK) order but budget for the spread and taker fees. Remember FAK will execute immediately or cancel, protecting you from waiting fills.
• If you can wait, post a limit order at or just inside the current best opposite price; you become a maker and pay no maker fee when filled.
• Size intelligently: if depth behind the best quote is shallow, break larger buys into tranches to avoid moving through multiple ticks and widening realized spread.
• Watch tick_size_change events: when tick tightens to $0.001, you can use smaller price improvements to capture more of the spread.

Example scenarios (what to watch for)

• Tight spread, deep book: good environment for small market orders and scalping spreads via limit placement.
• Tight spread, shallow depth: fragile. A single market order can eat multiple price levels and create slippage larger than the quoted spread.
• Wide spread with no news: often a sign of low maker participation; consider limit posting rather than aggressively crossing.

How this affects your trading

Understanding the order book spread helps you choose execution style, manage expected costs, and size orders. You should treat the quoted spread as the starting point for execution cost, not the final number — slippage, taker fees, and partial fills change the outcome. For binary intra-market arbitrage, edge may appear as a spread discrepancy across legs, but capturing it requires considering settlement timing, UMA resolution risk, and smart-contract operations for CTF splits and merges.

The next step is to pair this knowledge with live market feeds. Use the Polymarket WebSocket market feed to watch best_bid_ask and tick_size_change, and refer to the CLOB API for order placement if you build tooling. If you want a deeper operational guide to arbitrage, see /guides/intra-market-binary-arbitrage-explained and /guides/polymarket-clob-explained.

In closing, the book spread explained is the first tool you need as a trader. Read it, monitor depth, and let spread dynamics inform whether to post a limit order or take liquidity.

Frequently asked questions

What is the difference between spread and midpoint?

The spread is best ask minus best bid. The midpoint is the average of best bid and best ask. Midpoint is useful as a reference for fair value; the spread is the cost to immediately cross the book.

If the spread is $0.01 should I always use a market order?

Not always. A $0.01 spread with shallow depth can still cause slippage for larger sizes. Use market (FAK) for speed and accept taker fees; use limit orders if you can wait to capture the spread as a maker.

How does tick size change affect my limit order strategy?

When tick size tightens (e.g. to $0.001 near extremes), you can post limit orders with smaller price improvements to step in front of existing makers. That increases your chance to capture spreads but requires finer price management.

Do maker fees affect the spread on Polymarket?

Maker fees on Polymarket are zero, which encourages posted liquidity. Variable taker fees (0%–1.8%) still matter because they widen the effective cost for someone crossing the spread.

Can I rely on narrow spreads being stable?

No. Narrow spreads can be fragile if depth is thin or volatility increases. Always check book depth, recent trades, and tick_size_change events before assuming a tight spread will persist.