What it means
Every traded security has an order book listing prices people are prepared to buy and sell at, together with the quantity available at each price. The sell orders are sorted from cheapest to most expensive, and the cheapest one is the best ask, sometimes called the best offer or the inside ask.
The best ask is a live number that changes constantly as orders arrive, are filled or are cancelled. It is only meaningful alongside its size, because a best ask of $25.02 for 300 shares is a very different proposition from the same price for 30,000 shares.
This matters commercially because the best ask is one half of your true cost of trading. Anyone buying at market crosses the spread, meaning they pay slightly more than the theoretical mid-price, and for a treasury team buying back shares or a fund building a position that difference compounds across every order.
In practice, traders watch the best ask together with the depth behind it. If a large buy order exhausts the shares available at the best ask, the remainder fills at the next price level up, producing an average price worse than the quoted one, an effect called slippage or market impact.
An important nuance is that in fragmented markets the same security trades on several venues, so there is a best ask on each one and a consolidated best ask across all of them. Regulations in most developed markets require brokers to route customer orders to the venue showing the best available price, which is why the consolidated figure is the one that matters.
In practice
Real-world examples.
Example
A company running a share buyback instructs its broker to buy only when the best ask is at or below $25.05. On days when the best ask sits higher, no shares are bought, which keeps the average repurchase price within the board's approved range.
Example
A retail investor places a market order for a thinly traded small-cap and is surprised to fill several cents above the quoted best ask. The best ask was real but only 200 shares deep, so the rest of the order walked up the book.
Example
A market maker in corporate bonds posts a best ask 12 basis points above its best bid on a liquid issue. When a large seller appears and inventory rises, the maker widens the quote and lifts the ask to compensate for the extra risk it is now carrying.
Formula
Calculation
Bid-ask spread = Best Ask - Best Bid; Mid price = (Best Bid + Best Ask) / 2; Effective price on a large order = Total cash paid / Total shares bought.
Suppose a stock shows a best bid of $24.98 and a best ask of $25.02. The spread is $25.02 - $24.98 = $0.04, the mid price is ($24.98 + $25.02) / 2 = $25.00, and the spread as a proportion of the mid is $0.04 / $25.00 = 0.16%, or 16 basis points.
Now assume the sell side of the book holds 3,000 shares at $25.02, 5,000 at $25.05 and 2,000 at $25.10, and an investor sends a market order for 10,000 shares. The fills are 3,000 x $25.02 = $75,060, then 5,000 x $25.05 = $125,250, then 2,000 x $25.10 = $50,200, giving total cash of $250,510 and an effective price of $250,510 / 10,000 = $25.051. Had the entire order filled at the quoted best ask it would have cost 10,000 x $25.02 = $250,200, so slippage was $250,510 - $250,200 = $310.Case study
Seen in the real world.
What follows is an illustrative, fictional case. Kestrel Wealth Advisers is an invented firm that manages model portfolios for several thousand clients and rebalances all of them on the same morning each quarter. For years it sent every trade as a market order the moment the market opened, taking whatever the best ask happened to be.
A review of a single quarter found that the firm's own buying was pushing the best ask higher on the smaller holdings, and that its average fill was roughly 20 basis points worse than the mid price at the time each order was sent. On $180 million of quarterly turnover the cost was material even though no individual trade looked unreasonable.
The firm switched to spreading rebalancing trades across the full session and using limit orders pegged near the mid for the least liquid names. Fills took longer but the average execution price improved. The illustrative point is that the best ask is only a fair price when the size you want is actually available there.
Watch out
Common mistakes.
- Reading the best ask as the price you will get for any size. It is the price for the quantity displayed at that level and nothing more.
- Confusing the best ask with the last traded price. The last trade is history, while the best ask is what is available right now.
- Ignoring the spread when comparing brokers on commission alone. A tighter spread on a better venue can easily outweigh a small difference in commission.
Questions
People also ask.
Is the best ask the same as the offer price?
Yes, ask and offer mean the same thing in market quoting, and the best offer is simply the lowest of them.
Why does the best ask widen away from the bid in volatile markets?
Because market makers face more risk holding inventory, so they demand a larger cushion between the price they buy at and the price they sell at.
Does the best ask exist in every market?
It exists wherever there is a continuous order book, but in dealer markets such as much of corporate credit you may receive individual quotes rather than a single public best ask.
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