In the stock market, why is the “open” price value never the same as previous day's “close”?
"What most of these answers here seem to be missing is that a stock ""price"" is not exactly what we typically expect a price to be--for example, when we go in to the supermarket and see that the price of a gallon of milk is $2.00, we know that when we go to the cash register that is exactly how much we will pay.  This is not, however, the case for stocks. For stocks, when most people talk about the price or quote, they are really referring to the last price at which that stock traded--which unlike for a gallon of milk at the supermarket, is no guarantee of what the next stock price will be.  Relatively speaking, most stocks are extremely liquid, so they will react to any information which the ""market"" believes has a bearing on the value of their underlying asset almost (if not) immediately.  As an extreme example, if allegations of accounting fraud for a particular company whose stock is trading at $40 come out mid-session, there will not be a gradual decline in the price ($40 -> $39.99 -> $39.97, etc.)-- instead, the price will jump from $40 to say, $20.  In the time between the the $40 trade and the $20 trade, even though we may say the price of the stock was $40, that quote was actually a terrible estimate of the stock's current (post-fraud announcement) price. Considering that the ""price"" of a stock typically does not remain constant even in the span of a few seconds to a few minutes, it should not be hard to believe that this price will not remain constant over the 17.5 hour period from the previous day's close to the current day's open.  Don't forget that as Americans go to bed, the Asian markets are just opening, and by the time US markets have opened, it is already past 2PM in London.  In addition to the information (and therefore new knowledge) gained from these foreign markets' movements, macro factors can also play an important part in a security's price-- perhaps the ECB makes a morning statement that is interpreted as negative news for the markets or a foreign government before the US markets open.  Stock prices on the NYSE, NASDAQ, etc. won't be able to react until 9:30, but the $40 price of the last trade of a broad market ETF at 4PM yesterday probably isn't looking so hot at 6:30 this morning... don't forget either that most individual stocks are correlated with the movement of the broader market, so even news that is not specific to a given security will in all likelihood still have an impact on that security's price. The above are only a few of many examples of things that can impact a stock's valuation between close and open: all sorts of geopolitical events, announcements from large, multi-national companies, macroeconomic stats such as unemployment rates, etc. announced in foreign countries can all play a role in affecting a security's price overnight. As an aside, one of the answers mentioned after hours trading as a reason--in actuality this typically has very little (if any) impact on the next day's prices and is often referred to as ""amateur hour"", due to the fact that trading during this time typically consists of small-time investors.  Prices in AH are very poor predictors of a stock's price at open." Quoted from money.howstuffworks.com: NASDAQ has come up with an auction approach called the opening cross. Here's how it works. In the morning, a computer program looks at all the orders that have come in overnight in each different stock. Based on those orders, the program picks a price level that would be the best opening price. However, it also looks to see if there's a trade imbalance. For example, if a company announced bad news after the market closed, there might be 10 times more sell orders than buy orders. NASDAQ then broadcasts the price and imbalance information to its network of dealers with the goal of offsetting the imbalance. It then lets dealers place orders. This all happens very quickly, in a time window of two minutes or so, right before the market opens. Dealers can place orders, and those orders are factored into the opening price. Further reading here: Opening Price calculation A stock's price does not move in a completely continuous fashion.  It moves in discrete steps depending on who is buying/selling at given prices. I'm guessing that by opening bell the price for buying/selling a particular stock has changed based on information obtained overnight. A company's stock closes at $40.  Overnight, news breaks that the company's top selling product has a massive defect.  The next morning the market opens.  Are there any buyers of the stock at $40?  Probably not.  The first trade of the stock takes place at $30 and is thus, not the same as the previous day's close. It's done by Opening Auction (http://www.advfn.com/Help/the-opening-auction-68.html): The Opening Auction Between 07.50 and   a random time between 08.00 and   08.00.30, there will be called an auction period during which time,   limit and market orders are entered   and deleted on the order book. No   order execution takes place during   this period so it is possible that the   order book will become crossed. This   means that some buy and sell orders   may be at the same price and some buy   orders may be at higher prices than   some sell orders. At the end of the random start period,   the order book is frozen temporarily   and an order matching algorithm is   run. This calculates the price at   which the maximum volume of shares in   each security can be traded. All   orders that can be executed at this   price will be filled automatically,   subject to price and priorities. No   additional orders can be added or   deleted until the auction matching   process has been completed. The opening price for each stock will   be either a 'UT' price or, in the   event that there are no transactions   resulting form the auction, then the   first 'AT' trade will be used. The simple answer: The opening price is the price of the first trade of the day and the closing price is the price of the last trade of the day.  And since the stock price change from trade to trade they are usually different. The two answers so far are right, but there's a third factor - for many stocks, there's after hours trading. So the official 4PM close is not what the stock's last trade was when they open again. Regardless, even that after hour price is not the starting point as Muro points out. Besides overnight news events and auction mechanisms there is a more fundamental reason the price of a stock is always moving.  Theoretically the stock price will move slightly even in the unlikely scenario that absolutely nothing of interest happens during the entire night.  Let me go into that in some more detail: Stock valuation using Discounted Cash FLow One of the fundamental reasons that stock value is constantly changing is because underneath every stock there is a company that expects to make some kind of profit or loss in the future. We have to go into the fundamentals of stock value to understand why this is important: One popular way to determine the value of a stock is by looking into the future and summing up all the earnings (or cashflows) it has yet to produce. You have to reduce each amount by a certain factor that gets larger for payments that are farther into the future. Think of it this way: a dollar in hand now is better than a dollar that you get tomorrow. This method of valuation is called Discounted Cash Flow (abbr. DCF; see the wikipedia article on DCF) Time's effect on stock value Now take the Close price C, and the open price O. Let's assume that since there has been no news, the expectations for future earnings are the same for C and O. Remember that the discounting factor for these earnings is dependent on the time until the cashflow occurs? For O, this time is slightly shorter than for C, and therefore the value will be slightly higher (or lower, when the company is expected to incur losses). So now you can see that even without all the external forces that continuously push and pull on the stock price, a stock still changes in value over time. Hope this helps. "It's almost like why don't you wake up in the morning feeling exactly like you slept the earlier night? yeah, once in a while that'll happen, but it's not designed to be that way. Stuff happens. The close of the stock is what happened at 4 PM (for US stocks). The ""open"" is simply the first price ever, or an open price auction like NimChimpsky said. Most things that trade have an open/close cycle, even what seemingly trades all the time (some markets trade 23 hours). Forex trades in different exchanges which have overlapping timing but each market will have an open, high, low and close for each day - for what is the same underlying currency. Also, it's not exactly true that close<>open. Take the GS chart, Oct 1 2010 and Oct 4 2010 (there was a weekend in between). The Oct 1 close was the same as the Oct 4 open. Note that Oct 4 was a down day so it's in red - the open is the upper end of the body (not including the wick), and Oct 1 was an up day so its close was the upper end too. (Candles are drawn so that the open ends of the wicks are the High and Low of the period respectively, and the lower end of the body is the open if it was an up-day, meaning the stock closed higher than it opened, and the body in coloured green below. If the stock went down that day from the open, the body's in red and the lower end is the close. Vice-versa for the other end)  The way to get to this: Go to yahoo finance, choose a stock, go to historical prices, click download data (you should have about 10 years of data), paste into excel, insert a formula to check if prev day's close = current day's open, and I'm sure you'll see at least one instance per stock." It does sometimes open one day the same as it closed the previous day.  Take a look at ESCA, it closed October 29th at 4.50, at opened November 1st at 4.50.  It's more likely to change prices overnight than it is between two successive ticks during the day, because a lot more time passes, in which news can come out, and in which people can reevaluate the stock. "Prices reflect all available information. (Efficient markets hypothesis) A lot can happen between the time a stock closes on one day and opens on another. Particularly in a heavily traded stock such as IBM. Basically, you have a different ""information set"" the following day, which implies a different price. The instances where you are most likely to have a stock where the price opens at the same price is at the previous close is a thinly traded stock on which you have little information, meaning that the ""information set"" changes less from day to day." "Nobody has mentioned the futures market yet. Although the stock market closes at 4pm, the futures market continues trading 24 hours a day and 5.5 days a week. Amongst the products that trade in the future market are stock index futures.  That includes the Dow Jones, the S&P 500. These are weighted averages of stocks and their sectors. You would think that the price of the underlying stock dictates the price of the average, but in this day and age, the derivative actually changes the value of the underlying stock due to a very complex combination of hedging practices. (this isn't meant to be vague and mysterious, it is ""delta hedging"") So normal market fluctuations coupled with macroeconomic events affect the futures market, which can ripple down to individual stocks. Very popular stocks with large market caps will most certainly be affected by futures market trading. But it is also worth mentioning that futures can function completely independently of a ""spot"" price. This is where things start to get complicated and long winded. The futures market factor is worth mentioning because it extends even outside of the aftermarket and pre-market hours of stock trading." A stock is only worth what someone is willing to pay for it. If it trades different values on different days, that means someone was willing to pay a higher price OR someone was willing to sell at a lower price.  There is no rule to prevent a stock from trading at $10 and then $100 the very next trade... or $1 the very next trade. (Though exchanges or regulators may halt trading, cancel trades, or impose limits on large price movements as they deem necessary, but this is beside the point I'm trying to illustrate). Asking what happens from the close of one day to the open of the next is like asking what happens from one trade to the next trade... someone simply decided to sell or pay a different price.  Nothing needs to have happened in between.
Ideally, stock price reflects the value of the company, the dividends it is expected to pay, and what people expect the future value of the company to be. Only one of those (maybe one and a half) is related to current  sales, and not always directly. Short-term motion of a stock is even less directly linked, since it also reflects previous expectations. A company can announce disappointing sales and see its stock go up, if the previous price was based on expecting  worse news. I'd add, this is actually the way any stock opens every day, i.e. the closing price of the prior day is what it is, but the opening price will reflect whatever news there was prior to the day's open. If you watch the business news, you'll often see that some stock has an order imbalance and has not opened yet, at the normal time. So, as Geo stated, those who were sold shares at the IPO price paid $38, but then the stock could open at whatever price was the point where bid and ask balanced.   I snapped a screen capture of this chart on the first day of trading, the daily charts aren't archived where I can find them. This is from Yahoo Finance. You can see the $42 open from those who simply wanted in but couldn't wait, the willingness of sellers to grab their profit right back to what they paid, and then another wave of buying, but then a sell-off. It closed virtually unchanged from the IPO price. The first moment of trading usually occurs even later than that. It may take a few hours to balance the current buy/sell orders and open the stock. Watch CNBC when a hot IPO is about to open and you'll see the process in real time. If you miss it, look at a one day Yahoo chart to see when the open occurred. "The fallacy in your question is in this statement: ""The formulas must exist, because prices can be followed real time."" What you see are snapshots of the current status of the stock, what was the last price a stock was traded at, what is the volume, is the price going up or down.  People who buy and hold their stock look at the status every few days or even every few months. Day traders look at the status every second of the trading day.  The math/formula comes in when people try to predict where the stock is going based on the squiggles in the line. These squiggles move based on how other people react to the squiggles.  The big movements occur when big pieces of news make large movements in the price. Company X announces the release of the key product will be delayed by a year; the founder is stepping down; the government just doubled the order for a new weapon system; the insiders are selling all the shares they can. There are no formulas to determine the correct price, only formulas that try to predict where the price may go." A stock market is just that, a market place where buyers and sellers come together to buy and sell shares in companies listed on that stock market. There is no global stock price, the price relates to the last price a stock was traded at on a particular stock market. However, a company can be listed on more than one stock exchange. For example, some Australian companies are listed both on the Australian Stock Exchange (ASX) and the NYSE, and they usually trade at different prices on the different exchanges. Also, there is no formula to determine a stock price. In your example where C wants to buy at 110 and B wants to sell at 120, there will be no sale until one or both of them decides to change their bid or offer to match the opposite, or until new buyers and/or sellers come into the market closing the gap between the buy and sell prices and creating more liquidity. It is all to do with supply and demand and peoples' emotions. "Trading at the start of a session is by far higher than at any other time of the day. This is mostly due to markets incorporating news into the prices of stocks. In other words, there are a lot of factors that can affect a stock, 24 hours a day, but the market trades for only 6.5 hours a day. So, a lot of news accumulates during the time when people cannot trade on that news. Then when markets finally open, people are able to finally trade on that news, and there is a lot of ""price discovery"" going on between market participants. In the last minutes of trading, volumes increase as well. This can often be attributed to certain kinds of traders closing out their position before the end of the day. For example, if you don't want to take the risk a large price movement at the start of the next day affecting you, you would need to completely close your position." "Take the case where a stock has just two owners, A and B, both at $10. One of them sells his shares to C, at $11.  Now B has made $1 in profit but is no longer an owner of the stock. A hasn't sold anything but his shares are worth 10% more due to the last traded price printed. C has bought shares at $11 and the price is $11, so technically he hasn't lost any money. In a larger market, there are winners and losers every day on a single stock, but they may not remain owners of a stock. There could be days in which those that remain owners are all winners - say when a stock goes up to an all time high and all those that are currently owners have an average buy price lower than the last traded price. And the reverse applies too.  It is of course more complicated. Say you own a stock and let someone else ""borrow"" it for a short-selling opportunity (he sells it in the market). For each uptick in price, you win, the short seller loses, and the guy he sold it to also wins. A person that has a covered call on a stock is not a winner beyond a point. And so on." "Some technical indicators (e.g. Williams %R) indicate whether the market is overbought or oversold.   ... Every time a stock or commodity is bought, it is also sold. And vice versa. So how can anything ever be over-bought or over-sold? But I'm sure I'm missing something. What is it? You're thinking of this as a normal purchase, but that's not really how equity markets operate. First, just because there are shares of stock purchased, it doesn't mean that there was real investor buyer and seller demand for that instrument (at that point in time).  Markets have dedicated middlemen called Market Makers, who are responsible to make sure that there is always someone to buy or sell; this ensures that all instruments have sufficient liquidity.  Market Makers may decide to lower their bid on a stock based on a high number of sellers, or raise their ask for a high number of buyers. During an investor rush to buy or sell an instrument (perhaps in response to a news release), it's possible for Market Makers to accumulate a large number of shares, without end-investors being involved on both sides of the transaction.  This is one example of how instruments can be over-bought or over-sold. Since Williams %R creates over-bought and over-sold signals based on historical averages of open / close prices, perhaps it's better to think of these terms as ""over-valued"" and ""under-valued"".  Of course, there could be good reason for instruments to open or close outside their expected ranges, so Williams %R is just a tool to give you clues... not a real evaluation of the instrument's true value." The next day the market opens trading at 10.50, You haven't specified whether you limit order for $10.10 is to buy or sell. When the trading opens next day, it follows the same process of matching the orders. So if you have put a limit order to buy at $10.10 and there is no sell order at that price, your trade will not go through. If you have placed a limit sell order at $10.10 and there is a buyer at or higher price, it would go through. The Open price is the price of the first trade of the day. Because the stock still has the same value as the money paid for it - you are just exchanging one asset for another (of course the stock value starts to change immediately, but for the accounting the fictional value is the buying price). For the accounting, it is similar to changing a 100$ bill in five 20$ bills - same value, still assets. Theoretically, it's a question of rate of return. If a desired or acceptable rate of return for market makers' capital is X, and X is determined by the product of margin & turnover then higher turnover means lower margin for a constant X.  Margin, in the case of trading, is the bid/ask spread, and turnover, in the case of trading, is volume. Empirically, it has been noted in the last markets still offering such wide-varying evidence, equity options: http://faculty.baruch.cuny.edu/lwu/890/mayhew_jf2002.pdf "No one can claim markets are perfectly efficient. The Grossman-Stiglitz paradox explains one reason why it is impossible for the market to be perfectly effecient, and there are plenty of investors that show it is practically possible to consistently beat the market (e.g. ""The Superinvestors of Graham-and-Doddsville"" or RenTec's Medallion Fund).  However, even if you accept as true that prices behave strangely around round numbers, that isn't a refutation of the efficient market hypothesis. The efficient market hypothesis says that price reflect all available information, so the expected price tomorrow is just the price today (not taking into account the time value of money).  The efficient market hypothesis says nothing about how prices are distributed. Historically, a random walk has been used, but that is neither a consequence of the EMH nor a required assumption.  You could say that prices are more volatile the nearer they are to a round number or new high, but that doesn't necessarily give you an edge in making money off the stock." "It reminds me of the Efficient Market Hypothesis, except that just states in its weakest form that the current market price accounts for all information embedded in previous market prices. In other words, people buying today at 42 know it was selling for 40 yesterday, and the patterns and such. To say that stock is memoryless strikes me as not quite right -- to the extent that stocks are valued based on earnings, much of what we infer about future earnings relies on past and present earnings. One obvious counterexample to this ""memoryless"" claim is bankruptcy. If a stock files bankruptcy, and there isn't enough money to pay senior debt, your shares are worth 0 in perpetuity." Never. Isn't that the whole idea of the limit order. You want a bargain, not the price the seller wants. And when the market opens it is volatile at the most, just an observation mayn't be correct. Let it stabilize a bit. The other thing is you might miss the opportunity. But as an investor you should stick to your guns and say I wouldn't buy any higher than this or sell any lower than this. As you are going long, buying at the right price is essential. You aren't going to run away tomorrow, so be smart.  Probably this is what Warren Buffet said, it is important to buy a good stock at the right price rather than buying a good stock at the wrong price. There is no fixed answer to your question. It can be anything. You can check what analysts, someone with reputation of predicting correctly(not always), say would be the increase/decrease in the price of a stock in the projected future. They do quite a lot of data crunching to reach a price. Don't take their values as sacrosanct but collate from a number of sources and take an average or some sorts of it. You can then take an educated guess of how much you would be willing to pay depending the gain or loss predicted. Else if you don't believe the analysts(almost all don't have a stellar reputation) you can do all the data crunching yourself if you have the time and right tools. Here's an example (US, not Canada) that shows up to a 30% increase for first 6 months after a >30 day lapse, but the best data will come from actual quotes from insurers. If you can do without driving for 2 years it's almost certainly worth dropping coverage and a car for that duration and paying an increased insurance rate for a spell after the lapse. I'm not sure how it works in Canada, but when living with my parents they could not exclude me from their insurance once I was a licensed driver. The insurance company considered me to have access to all vehicles, so my presence increased insurance rates. If you live with your mother, you'll have to check with your insurer to see how that works.