What it means
An order book collects every resting buy and sell instruction for a security. Buy orders are ranked from highest price to lowest, and the highest of them is the best bid, also called the inside bid or the top of book.
Like the best ask, the best bid carries a size as well as a price. A best bid of $18.40 for 200 shares tells you almost nothing about what a 50,000 share sale would fetch, so professional traders always read price and depth together.
The best bid matters to any business that might need to convert holdings into cash. For a corporate treasurer valuing a portfolio, an executive planning a share sale under a trading plan, or a fund reporting daily prices, the best bid is the realistic answer to the question of what you would actually get today.
In use, the best bid is the reference point for measuring execution quality on a sale. Selling above it means you found a hidden buyer or waited patiently; selling below it means your order was too large for the displayed depth and had to reach down to lower price levels.
One nuance often missed is that displayed bids can be withdrawn instantly. In stressed markets the best bid can fall or vanish within seconds, which is why valuations based on last quoted bids can overstate what an investor could realise in a genuine sale.
In practice
Real-world examples.
Example
A founder selling shares under a scheduled trading plan instructs the broker never to sell below $18.30. On days when the best bid drops beneath that level the plan simply pauses, protecting the founder from dumping stock into a thin market.
Example
A fund administrator marks an illiquid holding at the closing best bid rather than the mid price, following a policy that positions should be valued at the price a seller could realistically achieve. The auditors accept the approach because it is applied consistently.
Example
A corporate bond desk shows a best bid of 98.20 for a $5 million clip. When a client asks for a price on $25 million, the desk quotes 97.60 instead, because absorbing five times the size requires a wider cushion.
Formula
Calculation
Quoted spread = Best Ask - Best Bid; Mid price = (Best Bid + Best Ask) / 2; Realised proceeds on a large sale = Sum of (shares filled at each level x that price).
Suppose a stock shows a best bid of $18.40 and a best ask of $18.44. The quoted spread is $18.44 - $18.40 = $0.04 and the mid price is ($18.40 + $18.44) / 2 = $18.42.
Now assume the buy side of the book holds 2,000 shares bid at $18.40, 4,000 at $18.37 and 6,000 at $18.30, and a shareholder sells 10,000 shares at market. The fills are 2,000 x $18.40 = $36,800, then 4,000 x $18.37 = $73,480, then 4,000 x $18.30 = $73,200, giving proceeds of $183,480 and an average of $183,480 / 10,000 = $18.348 per share. Valuing the same 10,000 shares at the best bid alone would have suggested 10,000 x $18.40 = $184,000, so the shortfall from walking the book was $184,000 - $183,480 = $520.Case study
Seen in the real world.
The following is a fictional illustration. Ainsworth Pension Trust, an invented scheme, held a large stake in a lightly traded regional bank and carried it in the accounts at the last quoted best bid, which had been stable at around $18.40 for months. The stake represented roughly forty days of average trading volume.
When the trustees decided to exit ahead of a strategy change, the investment team discovered the best bid was only ever good for a couple of thousand shares at a time. Selling in size pushed the realised average to $18.35 across the first week and lower thereafter, because each day's selling consumed the visible depth before it refreshed.
The trust changed its valuation policy to apply a liquidity discount for any holding representing more than ten days of volume. The illustrative message is simple: the best bid tells you the price of the next small trade, not the price of your whole position.
Watch out
Common mistakes.
- Valuing an entire holding at the best bid regardless of size. The displayed bid usually supports only a fraction of a large position.
- Assuming the best bid is a firm commitment. Most resting orders can be cancelled at any moment, and in fast markets many are.
- Judging a broker's performance against the best bid at the end of the trade rather than the one that existed when the order was sent.
Questions
People also ask.
What is the difference between the best bid and the mid price?
The best bid is what a buyer will actually pay now, while the mid is the average of the best bid and best ask and is a theoretical reference rather than a tradeable price.
Why do best bid and best ask sometimes cross venues?
Because the same security trades on multiple platforms, and the consolidated best bid is simply the highest of the individual venue bids at that moment.
Does a high best bid mean the security is a good investment?
No, it only tells you what buyers will pay right now; it says nothing about whether the price is justified by the underlying business.
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