Order-Book Depth
Definition
Order-book depth is the quantity of contracts offered at each price level beyond the best bid and ask. Cumulative depth adds them up, showing how many contracts can be bought or sold before the price moves by a given amount. The CFTC describes a liquid market as one 'in which selling and buying can be accomplished with minimal effect on price'; depth is how that is measured.
Why it matters
A good quoted price is worth little if only a few contracts are available at it. For orders of any size, the average execution price depends on depth, and the cost rises with each level taken. Investors should look at the full book, size orders to the depth available, or use limit orders and accept slower fills.
Common misconceptions
- •A tight spread can sit on very little size.
- •Volume measures past trading, not what is available now.
- •Depth can disappear quickly when news arrives, as market makers cancel quotes.
- •The label is only a starting point. Investor outcomes usually depend on the governing documents, cash-flow priority, valuation policy, disclosure quality, and manager discretion surrounding the term, not just the shorthand definition used in marketing materials.
Technical details
Worked example (illustrative): walking the book
Asks: 500 at $0.40, 1,000 at $0.42, 1,500 at $0.45 (cumulative 500, 1,500, 3,000).
Buy 2,000: 500 x $0.40 + 1,000 x $0.42 + 500 x $0.45 = $845.00.
Average price $0.4225: slippage of 2.25 cents against the best ask.
Kalshi standard fees on the three fills add about $34, roughly 1.7 cents per contract.
How investors apply the term
Investors usually apply this term by tying the definition back to the controlling documents and then asking which cash flows, rights, restrictions, or valuation assumptions it changes. A useful review looks at the term alongside fees, leverage, liquidity, reporting frequency, and manager discretion rather than treating it as a standalone concept.
The practical diligence test is simple: identify who benefits if the assumption is wrong, who has control when conditions change, and what evidence the investor receives after closing. Terms that cannot be reconciled to documents, cash movements, and monitoring data should be treated as marketing shorthand until proven otherwise.
Frequently asked questions
- How much can I trade without moving a prediction market?
- As much as is resting at the best price, and no more. Polymarket notes its order book has 'no trading size limits' but that 'large orders may move the price significantly,' and advises: 'Always check orderbook depth before trading in size.'
- How do I calculate the average cost of a larger order?
- Walk it through the levels. If asks show 500 contracts at 40 cents, 1,000 at 42 cents and 1,500 at 45 cents, buying 2,000 takes all of the first two levels and 500 of the third: $200 + $420 + $225 = $845, an average of 42.25 cents, 2.25 cents above the best ask, before fees.
- Does high volume mean a market is deep?
- No. AltStreet's Kalshi study found that the single largest series accounted for 58.6% of recorded volume and the top three for 81.3%, so 'headline exchange volume is a poor guide to capacity in any specific contract family.' The study captured top-of-book quotes only and could not measure depth directly.