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."
To add to @Victor 's answer; if you are entering a market order, and not a limit order (where you set the price you want to buy or sell at), then the Ask price is what you can expect to pay to purchase shares of stock in a long position and the Bid price is what you can expect to receive when you sell stock you own in a long position. All the time. For high volume stocks, it may be tough to see exactly what's going on, e.g. the bid/ask may be moving faster than your connection to the broker can show you.  What I've observed is with options. The volume on some options is measured in the 10's or 100's of contracts in a day. I'll see a case where it's $1.80/$2.00 bid/ask, and by offering $1.90 will often see a fill at that price. Since I may be the only trade on that option in the 15 minute period and note that the stock wasn't moving more than a penny during that time, I know that it was my order that managed to fill between the bid/ask. "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." &gt; At what price is the slow customer sending the order? Or are they sending a market order?  So if the high frequency fund sees someone wants to meet the ask at 1.01 they'll push that person back in favor of executing their trade first. They'll then raise the price to 1.02 (that's high though) and make the other person pay that. This happens literally in microseconds. In order to see whether you can buy or sell some given quantity of a stock at the current bid price, you need a counterparty (a buyer) who is willing to buy the number of stocks you are wishing to offload. To see whether such a counterparty exists, you can look at the stock's order book, or level two feed. The order book shows all the people who have placed buy or sell orders, the price they are willing to pay, and the quantity they demand at that price. Here is the order book from earlier this morning for the British pharmaceutical company, GlaxoSmithKline PLC.  Let's start by looking at the left-hand blue part of the book, beneath the yellow strip. This is called the Buy side. The book is sorted with the highest price at the top, because this is the best price that a seller can presently obtain. If several buyers bid at the same price, then the oldest entry on the book takes precedence. You can see we have five buyers each willing to pay 1543.0 p (that's 1543 British pence, or £15.43) per share. Therefore the current bid price for this instrument is 1543.0. The first buyer wants 175 shares, the next, 300, and so on. The total volume that is demanded at 1543.0p is 2435 shares. This information is summarized on the yellow strip: 5 buyers, total volume of 2435, at 1543.0. These are all buyers who want to buy right now and the exchange will make the trade happen immediately if you put in a sell order for 1543.0 p or less. If you want to sell 2435 shares or fewer, you are good to go. The important thing to note is that once you sell these bidders a total of 2435 shares, then their orders are fulfilled and they will be removed from the order book. At this point, the next bidder is promoted up the book; but his price is 1542.5, 0.5 p lower than before. Absent any further changes to the order book, the bid price will decrease to 1542.5 p. This makes sense because you are selling a lot of shares so you'd expect the market price to be depressed. This information will be disseminated to the level one feed and the level one graph of the stock price will be updated. Thus if you have more than 2435 shares to sell, you cannot expect to execute your order at the bid price in one go. Of course, the more shares you are trying to get rid of, the further down the buy side you will have to go. In reality for a highly liquid stock as this, the order book receives many amendments per second and it is unlikely that your trade would make much difference. On the right hand side of the display you can see the recent trades: these are the times the trades were done (or notified to the exchange), the price of the trade, the volume and the trade type (AT means automatic trade).  GlaxoSmithKline is a highly liquid stock with many willing buyers and sellers. But some stocks are less liquid. In order to enable traders to find a counterparty at short notice, exchanges often require less liquid stocks to have market makers. A market maker places buy and sell orders simultaneously, with a spread between the two prices so that they can profit from each transaction. For instance Diurnal Group PLC has had no trades today and no quotes. It has a more complicated order book, enabling both ordinary buyers and sellers to list if they wish, but market makers are separated out at the top. Here you can see that three market makers are providing liquidity on this stock, Peel Hunt (PEEL), Numis (NUMS) and Winterflood (WINS). They have a very unpalatable spread of over 5% between their bid and offer prices. Further in each case the sum total that they are willing to trade is 3000 shares. If you have more than three thousand Dirunal Group shares to sell, you would have to wait for the market makers to come back with a new quote after you'd sold the first 3000. You definitely cannot be guaranteed to get the bid or ask if you are selling more than are available/desired at those prices. What prices you do get depends on who is watching that contract and how willing they are to trade with you. This question is not much different from the question of whether you can easily get into or out of a large position in an illiquid small stock easily.  You can get out quickly if you are willing to take pennies on the dollar, or you may get a reasonable price if you take a long time to get out of (or into) your position.  You can't normally do both. In general taking large positions in illiquid assets is not something people want to do without lining up a buyer/seller beforehand.  Instead see if you can achieve your objective with liquid investments. Traders sometimes look at the depth of the book (number of outstanding limit orders) to try and gauge the sentiment of the market or otherwise use this information to formulate their strategy.  If there was a large outstanding buy order at $49.50, there's a decent chance this could increase the price by influencing other traders. However, a limit order at $2 is like an amazon.com price of $200,000 for a book.  It's so far away from realistic that it is ignored.  People would think it is an error. Submitting this type of order is perfectly legal.  If the stock is extremely thinly traded, it might even be encouraged because if someone wanted to sell a bunch and did a really bad job of it, the price could conceivably fall that far and the limit order would be adding liquidity.  I guess.  Your example is pretty extreme. It is not uncommon for there to be limit orders on the book that are not very close to the trading price.  They just sit around.  The majority of trades are done by algorithmic traders and institutional traders and they don't tend to do this, but a retail investor may choose to submit an order like that, just hoping against hope. Also, buy orders are not likely to push prices down, no matter what their price is.  A sell order, yes (even if it isn't executed). Any time a large order it placed for Buy, the sell side starts increasing as the demand of Buy has gone up. [Vice Versa is also true]. Once this orders gets fulfilled, the demand drops and hence the Sell price should also lower. Depending on how much was the demand / supply without your order, the price fluctuation would vary. For examply if before your order, for this particular share the normal volume is around 100's of shares then you order would spike things up quite a bit. However if for other share the normal volume is around 100000's then your order would not have much impact. The principle of demand-supply law will not work if spoofing (or layering, fake order) is implemented. However, spoofing stocks is an illegal criminal practice  monitored by SEC. In stock market, aggressive buyer are willing to pay for a higher ask price pushing the price higher even if ask size is considerably larger than bid size, especially when high growth potential with time is expected. Larger bids may attract more buyers, further perpetuating a price increase (positive pile-on effect). Aggressive sellers are willing to accept a lower bid price pushing the price lower even if ask size is considerably smaller than bid size, when a negative situation is expected. Larger asks may attract more sellers, further perpetuating a price fall (negative pile-on effect).  Moreover, seller and buyers considers not only price but also size of shares in their decision-making process, along with marker order and/or limit order. Unlike limit order, market order is not recorded in bid/ask size. Market order, but not limit order, immediately affects the price direction. Thus, ask/bid sizes alone do not give enough information on price direction. If stocks are being sold continuously at the bid price, this could be the beginning of a downward trend; if stocks are being sold continuously at the ask price, this could be the beginning of a upward trend.  This is because ask price is always higher than bid price. In all the cases, both buyers and sellers hope to make a profit in a long-term and short-term view You have no guarantees. The stock may last have traded at $100 (so, the market price is $100), but is currently in free-fall and nobody else will be willing to buy it for any more than $80. Or heck, maybe nobody will be willing to buy it at all, at any price. Or maybe trading on this stock will be halted. Remember, the market price is just what the stock last traded at. If you put in a 'market order', you are ordering your broker to sell at the best available current price. Assuming someone's willing to buy your stock, that means you'll sell it. But if it last traded at $100, this doesn't guarantee you'll sell at anything close to that. In practice, it would not work. If you put a bid in that was really out of line, even if it got filled, the exchange would reverse it.  Other than that it really depends on what the current bid/ask spread it, and what volume its trading, as well as how the market feels. Say the current bid is 11 and you put an order in with bid 11.5, it would soak up all the orders on the market up to the volume your buying. But once your order is filled the market will be determined by what the next order's bid/ask is. It might stay where you moved it too if others feels thats a fair price. But if every other order on the market is still at 11, then the price isnt gonna move.  tl;dl unless your a market maker, you could not realistically affect the price. But how does the quantity matching happen? For example, if I want to buy 1000 shares at $100, but there is only one seller to sell 10 shares at $100, what happens then? This depends on the type of order you've placed. If you placed a fill-or-kill order, your order to buy or sell a certain number of shares is routed to the trading floor for immediate execution. If the order cannot be immediately filled, it is cancelled (killed) automatically. Note that the order must be filled in its entirety. Partial fills are not allowed. In your example, your buy order wouldn't be filled because it couldn't be matched to a sell order of the same volume. This is similar to an all-or-none order, which is an order that contains  A condition instructing the broker to fill the order completely or not at all. If there is insufficient supply to meet the quantity requested by the order then it is canceled at the close of the market. In this case, if your order wasn't matched to an order of the same volume by the time the market closes, it's cancelled. If you simply placed a market/limit order, and (in the case of the limit order), part of your order was matched to another order with the right price, that part of your order will be filled, while the rest will remained unfilled. What happened here is pretty obvious: You were trying to sell 2000 shares and apparently didn't mark your order to permit partial execution.  While they had a buyer at 94.66 they didn't want 2000 shares.  Thus your order went unfilled. The obvious thing would happen. 10 shares change owner at the price of $100. A partially still open selling order would remain. Market orders without limits means to buy or sell at the best possible or current price. However, this is not very realistic. Usually there is a spread between the bid and the ask price and the reason is that market makers are acting in between. They would immediately exploit this situation, for example, by placing appropriately limited orders. Orders without limits are not advisable for stocks with low trading activity. Would you buy or sell stuff without caring for the price? This sometimes happens to me. It depends on how liquid the option is. Normally what I see happening is that the order book mutates itself around my order. I interpret this to mean that the order book is primarily market makers. They see a retail investor (me) come in and, since they don't have any interest in this illiquid option, they back off. Some other retail investor (or whatever) steps in with a market order, and we get matched up. I get a fill because I become the market maker for a brief while. On highly liquid options, buy limits at the bid tend to get swallowed because the market makers are working the spread. With very small orders (a contract or two) on very liquid options, I've had luck getting quick fills in the middle of the spread, which I attribute to MM's rebalancing their holdings on the cheap, although sometimes I like to think there's some other anal-retentive like me out there that hates to see such a lopsided book. :) I haven't noticed any particular tendency for this to happen more with puts or calls, or with buy vs sell transactions. For a while I had a suspicion that this was happening with strikes where IV didn't match IV of other strikes, but I never cared enough to chase it down as it was a minor part of my overall P/L.