Direct Answer
Top-of-book depth is the size available at the best bid and best ask only, the first level of the order book. It shows how much can trade at the current quote before price moves. It is the most immediate liquidity measure: an order larger than the size at the best price will take out the first level and trade at worse prices.
Top-of-Book Depth: Formula, Meaning, Signals, Examples and How to Use It
How Is Top-of-Book Depth Calculated?
Top-of-book depth is the size available at the best bid and best ask only, the first level of the order book. It shows how much can trade at the current quote before price moves.
Formula: Top-of-Book Depth = Bid Size at best bid + Ask Size at best ask (or each side reported separately)
Inputs:
- Best bid size
- Best ask size
- Venue or consolidated quote source
Worked example. 1,200 shares at the best bid and 800 at the best ask give a top-of-book depth of 2,000 shares. The inputs are illustrative numbers, not market data.
Reading it. It is the most immediate liquidity measure: an order larger than the size at the best price will take out the first level and trade at worse prices. It is the quantity behind quote-size tests and the microprice.
Limits. It captures a single level, so it ignores liquidity just behind the best price, and it can flicker with every quote update. Whether it is a single-venue or consolidated figure matters a lot in fragmented markets.
What is Top-of-Book Depth?
Top-of-book depth is the size available at the best bid and best ask only, the first level of the order book. It shows how much can trade at the current quote before price moves.
How to use Top-of-Book Depth
It is the most immediate liquidity measure: an order larger than the size at the best price will take out the first level and trade at worse prices. It is the quantity behind quote-size tests and the microprice. A reading alone is never a buy or sell signal; it needs context from price structure, volume, volatility and the broader market regime.
Strengths
- It reads directly from level-1 quote data, so it is available in every consolidated feed with no deeper book subscription.
- Size at the best quote is what an immediate market order meets first, so it is the capacity most relevant to a small marketable order.
- Reproducible when the formula and data convention are fixed.
Weaknesses and limitations
- Large size at the best quote can reflect many small orders queued for priority, so the number is partly a queue length.
- Many traders cancel and re-post at the touch around news, so the figure changes sharply around announcements.
- Correlated indicators can create false confidence by repeating the same underlying information.
Editorial and risk note
This page is educational content, not individualized investment advice. It should not imply guaranteed prediction, accuracy or outperformance.
Frequently Asked Questions
Is Top-of-Book Depth a buy or sell signal?
No. It is the most immediate liquidity measure: an order larger than the size at the best price will take out the first level and trade at worse prices. It captures a single level, so it ignores liquidity just behind the best price, and it can flicker with every quote update. A trade built on it still needs a trigger, an invalidation rule, a position size and realistic execution assumptions.
What is the best setting for Top-of-Book Depth?
Top-of-Book Depth is computed from best bid size; best ask size; venue or consolidated quote source, so the choices that matter are the data convention and the window it is read over. Test those across instruments, regimes and out-of-sample periods instead of fitting one historical winner.
Can Top-of-Book Depth be used by itself?
It captures a single level, so it ignores liquidity just behind the best price, and it can flicker with every quote update. Whether it is a single-venue or consolidated figure matters a lot in fragmented markets. For that reason it is read next to measures of direction, participation, valuation, liquidity and risk, not alone.
Does Top-of-Book Depth work on every timeframe?
The calculation (Top-of-Book Depth = Bid Size at best bid + Ask Size at best ask (or each side reported separately)) can be run on any bar size, but behavior is not portable: a reading on a five-minute chart describes a very different market window from the same reading on a daily chart. Validate the exact timeframe before relying on it.
Why does Top-of-Book Depth give false signals?
Large size at the best quote can reflect many small orders queued for priority, so the number is partly a queue length. Many traders cancel and re-post at the touch around news, so the figure changes sharply around announcements. Lag, noise and regime changes add to this, because market participants react to new information after the reading is calculated.
How should Top-of-Book Depth be backtested?
Rebuild Top-of-Book Depth from best bid size using point-in-time data, write the rules before testing, include delisted securities where relevant, model realistic fills and costs, reserve a validation sample and compare with a simpler baseline.