What happens if my order exceeds the bid or ask sizes?
Sure. Depending on how you configure your order, it will either be fulfilled partially or wait until it can be fulfilled. You can set a time limit on your order (usually its either 1 day or 60 days, but may vary between brokerages), and allow or disallow partial fulfilment. You should check with your broker. I asked my broker a similar question just 2 weeks ago. With their market orders they will be filled within 3 points from the current market bid/ask. If there is any remaining it will be placed as a limit order at 3 points away from the bid/ask price. For example, if the current ask is 100 @ $1.00 followed by 500 @ $1.01, 300 @ $1.02 and 100 @ $1.03; if you were to place a buy market order for 1000 shares you would get 100 filled at $1.00, 500 filled at $1.01, 300 filled at $1.02 and 100 filled at $1.03.  If, on the other hand, you were to place a buy market order for 2000 shares you would get 100 filled at $1.00, 500 filled at $1.01, 300 filled at $1.02 and 100 filled at $1.03, with the remaining 1000 of your order being placed as a limit order at $1.03. Again, check with your broker, as they may be different in how they treat their market orders. "This is a great question precisely because the answer is so complicated. It means you're starting to think in detail about how orders actually get filled / executed rather than looking at stock prices as a mythical ""the market"". ""The market price"" is a somewhat deceptive term. The price at which bids and asks last crossed & filled is the price that prints. I.e. that is what you see on a market price data feed. ] In reality there is a resting queue of orders at various bids & asks on various exchanges.  (source: Larry Harris. A size of 1 is 1H = 100 shares.) So at first your 1000H order will sweep through the standing queue of fills. Let's say you are trading a low-volume stock. And let's say someone from another brokerage has set a limit order at a ridiculous price. Part of your order may sweep through and part of it get filled at a ridiculously high price. Or maybe either the exchange or your broker / execution mechanism somehow will protect you against the really high fill. (Let's say your broker hired GETCO, who guarantees a certain VWAP.) Also people change their bids & asks in response to what they see others do. Your 1000H size will likely be marked as a human counterparty by certain players. Other players might see that order differently. (Let's say it was a 100 000H size. Maybe people will decide you must know something and decide they want to go the same direction as you rather than take the opportunity to exit. And maybe some super-fast players will weave in and out of the filling process itself.) There is more to it because, what if some of the resting asks are on other venues? What if both you and some of the asks match with someone who uses the same broker as you? Not only do exchange rules come into play, but so do national regulations. tl;dr: You will get filled, with price slippage. If you send in a big buy order, it will sweep through the resting asks but also there are complications."
"What if everyone decided to sell all the shares at a given moment, let's say when the stock is trading at $40? It would fall to the lowest bid price, which could be $0.01 if someone had that bid in place. Here is an example which I happened to find online:  Notice there are orders to buy at half the market price and lower... probably all the way down to pennies.  If there were enough selling activity to fill all of those bids you see, then the market price would be the lowest bid on the screen.  Alternatively, the bid orders could be pulled (cancelled), which would also let the price free-fall to the lowest bid even if there were few actual sellers. Bid-stuffing is what HFT (high frequency trading) algorithms sometimes do, which some say caused the Flash Crash of May 2010.  The computers ""stuff"" bids into the order book, making it look like there is demand in order to trigger a market reaction, then they pull the bids to make the market fall.  This sort of thing happens all the time and Nanex documents it http://www.nanex.net/FlashCrash/OngoingResearch.html Quote stuffing defined: http://www.investopedia.com/terms/q/quote-stuffing.asp I remember the day of the Flash Crash very well.  I found this video on youtube of CNBC at that time.  Watch from the 5:00 min mark on the video as Jim Crammer talks about PG easily not being worth the price of the market at that time.  He said ""Who cares?"", ""Its not a real price"", ""$49.25 bid for 50,000 shares if I were at my hedge fund.""  http://www.youtube.com/watch?v=86g4_w4j3jU You can value a stock how you want, but its only actually worth what someone will give you for it. More examples: Anadarko Petroleum, which as we noted in today's EOD post, lost $45 billion in market cap in 45 milliseconds (a collapse rate of $1 billion per millisecond), flash crashing from $90 all the way to an (allegedly illegal) stub quote of $0.01. http://www.zerohedge.com/news/2013-05-17/how-last-second-flash-crash-pushed-sp-500-1667-1666 How 10,000 Contracts Crashed The Market: A Visual Deconstruction Of Last Night's E-Mini Flash Crash http://www.zerohedge.com/news/2012-12-21/how-10000-contracts-crashed-market-visual-deconstruction-last-nights-e-mini-flash-cr Symantec Flash-Crash Destroys Over $1.5 Billion In Less Than A Second http://www.zerohedge.com/news/2013-04-30/symantec-flash-crash-destroys-over-15-billion-less-second This sort of thing happens so often, I don't pay much attention anymore." The situation you're proposing is an over-simplification that wouldn't occur in practice. Orders occur in a sequence over time. Time is an important part of the order matching process. Orders are not processed in parallel; otherwise, the problem of fairness, already heavily regulated, would become even more complex. First, crossed and locked markets are forbidden by regulators.  Crossed orders are where one exchange has a higher bid than another's ask, or a lower ask than another's bid.  A locked market is where a bid on one exchange is equal to the ask on another.  HFTs would be able to make these markets because of the gap between exchange fees. Since these are forbidden, and handling orders in parallel would ensure that a crossed or locked market would occur, orders are serialized (queued up), processed in order of price-time priority. So, the first to cross the market will be filled with the best oldest opposing order. Regulators believe crossed or locked markets are unfair.  They would however eliminate the bid ask spread for many large securities thus the bid-ask cost to the holder. "You can choose to place successively lower buy limit orders, but whether they get filled or not is not a given; it depends on whether sellers care to accept your bid. In your example of a 49.98 / 50.01 spread, if you place a buy with limit of 49.99, it won't get filled (if the order reaches the market while still at 49.98 / 50.01) immediately, but will be added to the order book.  By being added to the order book, the markets bid and ask become 49.99 / 50.01.  Your order won't get filled until some seller places a market order or a sell limit order of 49.99 or less.  No guarantee that that will happen, and even if it does, there's nothing to say that your follow-up buy at 49.98 will ever be filled.  In fact, your 49.98 buy order queues up at the ""end of the line"" behind all previously pending 49.98 bids, since your order arrived after those other bids.  Since the initial conditions you supposed had a 49.98 bid, such an order exists (or at least did exist; it might have been cancelled in the intervening moment. Basically, your first buy at 49.99, if it happens, has essentially no influence on whether your second buy at 49.98 will happen.  You can't expect to move the market lower by making a bid that is higher (49.99) than the existing best bid (49.98).  Whatever influence your 49.99 order has is to raise the market's price, not lower it." "The current stock price you're referring to is actually the price of the last trade.  It is a historical price – but during market hours, that's usually mere seconds ago for very liquid stocks. Whereas, the bid and ask are the best potential prices that buyers and sellers are willing to transact at: the bid for the buying side, and the ask for the selling side.  But, think of the bid and ask prices you see as ""tip of the iceberg"" prices.  That is: The ""Bid: 13.20 x200"" is an indication that there are potential buyers bidding $13.20 for up to 200 shares.  Their bids are the highest currently bid; and there are others in line behind with lower bid prices.  So the ""bid"" you're seeing is actually the best bid price at that moment.  If you entered a ""market"" order to sell more than 200 shares, part of your order would likely be filled at a lower price. The ""Ask: 13.27 x1,000"" is an indication that there are potential sellers asking $13.27 for up to 1000 shares.  Their ask prices are the lowest currently asked; and there are others in line behind with higher ask prices.  So the ""ask"" you're seeing is the best asking price at that moment.  If you entered a ""market"" order to buy more than 1000 shares, part of your order would likely be filled at a higher price. A transaction takes place when either a potential buyer is willing to pay the asking price, or a potential seller is willing to accept the bid price, or else they meet in the middle if both buyers and sellers change their orders. Note: There are primarily two kinds of stock exchanges.  The one I just described is a typical order-driven matched bargain market, and perhaps the kind you're referring to. The other kind is a quote-driven over-the-counter market where there is a market-maker, as JohnFx already mentioned.  In those cases, the spread between the bid & ask goes to the market maker as compensation for making a market in a stock.  For a liquid stock that is easy for the market maker to turn around and buy/sell to somebody else, the spread is small (narrow).  For illiquid stocks that are harder to deal in, the spread is larger (wide) to compensate the market-maker having to potentially carry the stock in inventory for some period of time, during which there's a risk to him if it moves in the wrong direction.    Finally ... if you wanted to buy 1000 shares, you could enter a market order, in which case as described above you'll pay $13.27.  If you wanted to buy your shares at no more than $13.22 instead, i.e. the so-called ""current"" price, then you would enter a limit order for 1000 shares at $13.22.  And more to the point, your order would become the new highest-bid price (until somebody else accepts your bid for their shares.)  Of course, there's no guarantee that with a limit order that you will get filled; your order could expire at the end of the day if nobody accepts your bid." You can always trade at bid or ask price (depending if you are selling or buying). Market price is the price the last transaction was executed at so you may not be able to get that. If your order is large then you may not even be able to get bid/ask but should look at the depth of the order book (ie what prices are other market participants asking for and what is the size of their order). Usually only fast traders will trade at bid/ask, those who believe the price move is imminent. If you are a long term trader you can often get better than bid or ask by placing a limit order and waiting until a market participant takes your offer. The order book looks fine, if it were a liquid market. However, a bid that matches with an ask will always be met on a first come first serve basis. There's no other way to do it.   Most traders don't like doing that because they want to try to get a lower price. HFT don't have to worry about meeting the ask because they're just going to pass that cost on to the guy on the hook.   By the time the HFT makes the buy they already know the guy wants to buy at 70.00  They did not know that at the time they placed the buy order. If the buy order from the HFT hasn't been cancelled it means they already found someone.  How? By testing the market with sell orders at the same time they were sending buy orders.  They keep a little bit of stock in reserve to perform these tests. I frequently do this on NADEX, selling out-of-the-money binary  calls. NADEX is highly illiquid, and the bid/ask is almost always from  the market maker. Out-of-the-money binary calls lose value quickly  (NADEX daily options exist for only ~21 hours). If I place an  above-ask order, it either gets filled quickly (within a few minutes)  due to a spike in the underlying, or not at all. I compensate by  changing my price hourly.  As Joe notes, one of Black-Scholes inputs is volatility, but price  determines (implied) volatility, so this is circular. In other words, you can  treat the bid/ask prices as bid/ask volatilities. This isn't as  far-fetched as it seems:  http://www.cmegroup.com/trading/fx/volatility-quoting-fx-options.html When you're selling something through a provider, like Craig's List or newspapers, the only thing that may limit your choices is the provider. They may refuse your post if it's against their rules or the law. But luckily they usually don't limit or enforce certain payment choices.  These private business providers have the right to do so if they want. You don't need to be their customer. They may state their terms for using the service and even refuse service (before any payment is made).  The fun part is that you may do so as well. Just remember to state your terms in your post so the prospective buyers are aware of them. I've found it best to put payment and delivery terms in separate lines so that they are easily noticeable, for example: Nice victorian handbasket with gold embroidery, only used once. Signed by the original author. Comes with a certificate of authenticity. No delivery, only cash payments.