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How are Bid and Ask calculated? If someone offers to sell a single share of a stock at twice yesterday's price at closing, does that inflated price become the Ask? Or is Ask an average of some kind, like the average selling-price of the top n offers to sell?

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Ask is always the lowest price that someone is willing to sell their shares for, whether it's one lot of 100 shares (the lowest trade size needed to change the ask) share or millions of shares. If someone offers their shares for a lower price, that becomes the new ask. If someone has a bid that's higher than the ask (they're willing to pay more that the other is wanting to sell for), the orders are matched up and a transaction happens.

If someone puts in an offer that's higher than the current lowest offer, their offer goes in a "queue" that's ordered by the price (lowest first) and time (earliest first). Only when all lower offers are filled does that offer get to be the "ask".

And vice-versa for the bid.

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  • I think you're saying that it is the single lowest asking price. Is that right? The queue order is by price by time. So that if there were 100,000 would-be sellers the queue would have 100,000 items in it?
    – Tim
    Jan 28, 2022 at 15:02
  • Should the Ask figure on a quote be changing in real-time as would-be sellers vie to offload shares?
    – Tim
    Jan 28, 2022 at 15:12
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    In the USA, odd lots do not update the bid and ask. See Why didn't my buy limit order become the highest bid?
    – Flux
    Jan 28, 2022 at 16:11
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    "if there were 100,000 would-be sellers the queue would have 100,000 items in it?" Correct - it's called the "order book".
    – D Stanley
    Jan 28, 2022 at 16:32
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    ""Should the Ask figure on a quote be changing in real-time as would-be sellers vie to offload shares?" Yes - as either lower offers are added or the existing low offer is filled by other orders.
    – D Stanley
    Jan 28, 2022 at 17:47

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