Prediction market liquidity: why the quoted edge isn't the real one

Every arbitrage screen quotes the price at the top of the order book. That price is real for the first few contracts and frequently not for the next hundred. On a snapshot of our own board, one in five depth-checked signals had its entire edge disappear once we walked the book for real size.

What is order-book depth on a prediction market?

A prediction market contract has a queue of resting orders at each price. The best ask is simply the cheapest one available — it might be backed by 8 contracts or 8,000. A scanner that reads only the best ask on each venue reports the edge you would get on a single contract, then implicitly invites you to size it at $1,000.

Walking the book means adding up the resting orders in price order until your intended size is filled, then recomputing the edge at the average price you actually paid. The result is usually smaller than the headline, and sometimes negative.

How much of a prediction market edge survives real size?

Measured on our live board on 17 August 2026, across the 71 signals where both legs' books were walked:

• Median quoted edge 2.04% → median fillable edge 1.63% — roughly a fifth of the edge lost to depth alone.
• 40 of 71 (56%) degraded once real size was applied.
• 14 of 71 (20%) collapsed to zero or negative — the edge existed only at the top of the book.
• Among the 50 highest-quoted signals, the median fell from 2.79% to 2.04%. The headline ranking and the tradeable ranking are not the same list.

These are a dated snapshot rather than a constant: the board is rebuilt every 10 minutes and the specific numbers move with it. The pattern does not — the biggest quoted spreads sit disproportionately on the thinnest books, because a thin book is why the price is stale.

Why are the largest quoted spreads usually the least tradeable?

A large gap between two venues means nobody has closed it. On a liquid contract, that is because the gap is not real — it is a stale quote, a mismatched pair, or a market you cannot access. On an illiquid contract, it is because closing it is not worth anyone's time: the edge is genuine but only a few hundred dollars of it exists.

This is why a board sorted by headline ROI is close to useless for deciding what to trade. Sorting by what fills reorders it substantially.

How do you check depth before placing an arbitrage trade?

1. Decide your size first, before looking at the edge. The question is not "what is the spread" but "what is the spread for $1,000".
2. Walk both books to that size and take the weighted average fill price on each leg, not the best quote.
3. Recompute the edge net of fees at those average prices. A Kalshi taker fee of roughly 0.07 x price x (1 - price) per contract eats a meaningful share of a 2% edge.
4. Check the thinner leg. A hedge fills at the pace of its worse side; deep liquidity on one venue does not rescue an empty book on the other.
5. Account for position caps. PredictIt limits roughly $850 per contract per person, which caps the trade regardless of what the book shows.

What does this mean for how much capital you need?

The edge is a percentage of capital locked until resolution, not of the profit collected. A 1.6% net edge is $16 on $1,000 — and that $1,000 is unavailable until the market resolves, which for an election contract can be a year or more. Combine that with per-venue position caps and fixed withdrawal or gas costs, and the practical floor is a few thousand dollars per venue rather than a few hundred.

It also means fill quality matters more than edge size. A reliable 1.5% that fills at size beats a headline 6% that fills twenty contracts, and the second one is what most screens show you.

How PROVISUM reports this

Every signal on the board carries the fillable figure alongside the quoted one, plus how much size actually fits, and a signal whose edge does not survive its own order book is not badged tradeable. The same discipline applies to the published settled record — returns there are net of each venue's fees, not gross.

Keep reading: Prediction market arbitrage: the complete guide · Polymarket vs Kalshi: fees, access & price gaps · Polymarket arbitrage: finding mispriced markets · Oracle risk: when both legs of a hedge lose