What it means
Shares in large US companies trade at the same time on many exchanges and trading venues. Each one shows its own best bid (the highest price a buyer will pay) and best offer (the lowest price a seller will accept), and these can differ by a cent or two.
The NBBO is built by combining all of those quotes and picking the highest bid and the lowest offer wherever they happen to sit. The gap between the two is the spread, which is the basic cost of trading a stock.
US market rules require brokers and exchanges to protect the best displayed quotes. In practice this means an order generally cannot be filled at a worse price than the NBBO when a better one is visible elsewhere, and brokers have a duty of best execution (getting the most favourable terms reasonably available to their clients).
For a business reader, the NBBO matters because it is how you judge the quality of trading. A corporate treasurer buying shares for an employee plan, or a company running a buyback programme, can compare their actual fill prices against the NBBO at the time to see whether the broker added value or cost.
Nuances apply. The NBBO reflects displayed quotes, and the quantity available at that price may be small, so a large order can walk through several price levels.
Some trades also take place away from public exchanges, and prices can move in the instant between the quote and the fill. Traders also pay attention to the size shown at the best quotes, often called depth.
A stock with a tight spread but only a few hundred shares available can still be costly to trade in volume, which is why institutions split large orders into smaller pieces over time.
In practice
Real-world examples.
Example
A broker executes a client's order to buy 500 shares at $50.04, matching the NBBO offer. The broker's trade report later shows the fill against the NBBO to demonstrate that the client received a fair price.
Example
A company buys back its own shares through a broker and the treasurer finds the average price paid was $0.02 above the NBBO midpoint on each trade. She asks the broker to explain the difference and compares alternative algorithms.
Example
A fintech start-up designing a trading app needs to show users a reliable current price. Its product manager decides to display the NBBO because it reflects the best available prices across the market, not just one exchange.
Formula
Calculation
NBBO bid = Highest bid across all venues
NBBO offer = Lowest offer across all venues
NBBO spread = NBBO offer - NBBO bid
Worked example: three venues show quotes for the same stock.
Venue A: bid $50.00, offer $50.05
Venue B: bid $50.01, offer $50.04
Venue C: bid $49.99, offer $50.06
NBBO bid = highest of $50.00, $50.01 and $49.99 = $50.01
NBBO offer = lowest of $50.05, $50.04 and $50.06 = $50.04
NBBO spread = $50.04 - $50.01 = $0.03
The midpoint is ($50.01 + $50.04) / 2 = $50.025, so the spread as a share of price is $0.03 / $50.025 = 0.06%.Case study
Seen in the real world.
Pinecrest Securities is an illustrative, fictional brokerage that handled orders for small business owners. A client complained that his order to sell 2,000 shares had been filled at $24.96 when the stock seemed to be quoted at $25.00.
The compliance team pulled the market data for the time of the trade. At the moment of the order the NBBO bid was $24.97, and only 300 shares were displayed at that price, so the larger order moved down through several price levels.
In this illustrative story, the firm explained that its fill was within a cent of the NBBO and that the displayed quantity had been the limiting factor. It then added a note in its order screen that large orders may fill beyond the NBBO because only the top-of-book quantity is guaranteed.
Watch out
Common mistakes.
- Assuming a large order will always fill at the NBBO, when only the displayed quantity at that price is available.
- Confusing the NBBO with the last traded price, when the NBBO is a live quote and the last price is a past transaction.
- Believing the NBBO exists for every market in the world, when it is a US concept tied to US market rules.
Questions
People also ask.
Who calculates the NBBO?
Market data processors collect quotes from all the exchanges and publish the combined best bid and offer in real time.
Why does the spread matter?
The spread is the immediate cost of trading, so a narrow spread means lower costs for buyers and sellers.
Does the NBBO cover pre-market and after-hours trading?
Coverage during extended hours can be thinner and rules may differ, so quotes outside normal hours should be treated with extra caution.
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