What Is the NBBO and Why Can Your Fill Differ From It?

Highest bid, lowest offer, across every US exchange trading the stock. That pair is the national best bid and offer. The term is defined in the SEC’s Regulation NMS.

A single stock can be quoted on many exchanges at the same time, each with its own best bid and best offer, and the NBBO pulls them together by taking the top bid from any venue and the top offer from any venue, which means the two halves of the quote may well sit on two different exchanges. Consolidated quotes and trades are distributed through the securities information processors, often called SIPs, which collect data from the exchanges and publish the combined picture that most of the market treats as the reference.

What your screen is actually showing

Maybe the NBBO. Maybe not.

Some platforms display the consolidated quote. Others, particularly on free or delayed data, show the quote from a single venue, which can be wider or slightly off from the national best at any moment. A chart’s price line is different again. It plots trades. A trade is history the moment it prints. Check the market-data settings page for the feed you are on, and if it does not say, ask.

Why your fill can differ

The quote moved. In an active stock the NBBO can change many times between your look and your click.

The size was small. The NBBO is one price level. It holds only the shares displayed there. A larger order walks through the levels behind it.

Odd lots. Orders smaller than a round lot, traditionally 100 shares for most stocks, are treated differently in how quotes are built and published, so there can be interest at better prices than the headline quote displays.

Price improvement. Sometimes you get a better price than the NBBO. When a market maker or venue fills a retail order inside the spread, even by a fraction of a cent, the fill beats the quote you saw, and this is the practice at the center of the argument over payment for order flow, where brokers are paid to send orders to exactly those firms.

A worked example with sizes

Neither fill is an error. The first order was larger than the size at the top of the book, so it took the next prices available, while the second was small enough to be filled inside the spread by someone willing to do so.

What the NBBO leaves out is depth. Level 2 quotes show more of the book, venue by venue, though even that is only what is displayed, and hidden or reserve orders can sit at prices nobody on the screen can see.

Checking your own fills

Pull up a recent execution. Note the time, the fill price and the size. Compare it with the NBBO at that moment if your platform stores quote history, or at least with the bid and offer you saw when you sent the order.

The NBBO is also the yardstick in formal execution statistics. Market centers publish execution quality reports under SEC Rule 605, and those reports measure fills against the NBBO at the time each order arrived, which is why the same comparison is a sensible one to run on your own trades, just at a much smaller scale.

A fill at or inside the NBBO on a small order is ordinary. A large order filling at an average worse than the offer usually means it was bigger than the displayed size, and a limit order would have capped that at a price you chose. Why a fill price differs from the chart covers the related confusion between trade prints and quotes.