How often does a stock price change and where is this defined?
"Stocks prices are determined whenever a buyer and seller agree to trade at a given price. The company (you use AAPL as an example) doesn't set its own stock price. Rather, the investors set the price every time it trades. There's no ""official"" price -- just the last trade. Likewise, you can offer to trade a stock at whatever price you want: that's the definition of a limit order. You might not find a willing buyer or seller at that price, but you can certainly open an order. Stock quotes that you get from your broker or a finance web site reflect the price as last traded. These quotes are updated throughout the trading day and the frequency and delay varies amongst quote providers. Like Knuckle-Dragger suggests in the comments, there are ways to get real-time quotes. It's often more helpful to think in terms of bid/ask instead of ""official price"". See this question for details."
"This is called ""change"" or ""movement"" - the change (in points or percentage) from the last closing value. You can read more about the ticker tape on Investopedia, the format you're referring to comes from there." "Stock price is set to the price with the highest transaction volume at any given time.  The stock price you cited was only valid in the last transaction on a specific stock exchange. As such it is more of an ""historic"" value. Next trade will be done with the next biggest volume. Depending on the incoming bids and asks this could be higher or lower, but you can assume it will not be too far off if there is no crash underway. Simple example stock exchange:" "I've alway thought that it was strange, but the ""price"" that gets quoted on a stock exchange is just the price of the last transaction. The irony of this definition of price is that there may not actually be any more shares available on the market at that price. It's also strange to me that the price isn't adjusted at all for the size of the transaction. A transaction of just 1 share will post a new price even if just seconds earlier 100,000 shares traded for a different price. (Ok, unrealistic example, but you get my point.) I've always believed this is an odd way to describe the price. Anyway, my diatribe here is supposed to illustrate the point that the fluctuations you see in price don't really reflect changing valuations by the stock-owning public. Each post in the exchange maintains a book of orders, with unmatched buy orders on one side and unmatched sell orders on the other side. If you go to your broker and tell him, ""fill my order for 50,000 shares at market price"", then the broker won't fill you 50,000 shares at .20. Instead, he'll buy the 50 @ .22, then 80 @ .23, then 100 @ .30, etc. Because your order is so large compared to the unmatched orders, your market order will get matched a bunch of the unmatched orders on the sell side, and each match will notch the posted price up a bit. If instead you asked the broker, ""open a limit order to buy 50000 shares at .20"", then the exchange will add your order to the book: In this case, your order likely won't get filled at all, since nobody at the moment wants to sell at .20 and historically speaking it's unlikely that such a seller will suddenly appear. Filling large orders is actually a common problem for institutional investors: http://www.businessweek.com/magazine/content/05_16/b3929113_mz020.htm http://www.cis.upenn.edu/~mkearns/papers/vwap.pdf (Written by a professor I had in school!)" Most stocks are not actively trades by lots of people.   When you buy or sell a stock the price is set by the “order book” – that is the other people looking to trade in the given stock at the same time. Without a large number of active traders, it is very likely the pricing system will break down and result in widely changing prices second by second.   Therefore for the market to work well, it need most people to be trading at the same time. Here in Australia a stock price is usually highest just before a dividend and lowest just after a dividend. If you buy just after the dividend then you missed out until next time. There may be many other reasons why a stock may exhibit yearly, quarterly and monthly cycles. So how often do investors really lose money? The short answer is, every day. Let's first examine your assumptions: If the price of the share gets lower, the investor can just wait until   it gets higher. What are the chances that it won't forever, or for   years? There are many stocks whose price goes down and then down further and then to zero. The most apparent example is, of course, Enron. The stock went from about $90 per share to zero in about 18 months. For it to have been sold at $90, obviously, someone had to buy it. Almost no matter where they sold it, they lost money. If they didn't sell it, when the stock was worthless, they lost money. https://en.wikipedia.org/wiki/Enron#/media/File:EnronStockPriceAugust2000toJanuary2001.svg There are more modern examples of companies that are declining in a rapidly changing market. For example, Sears Holdings is getting beat down by Amazon and many other on-line retailers. I suspect that if you buy it today and wait for it to go higher, you will be disappointed. https://www.google.com/finance?q=NASDAQ%3ASHLD&ei=E8_fWIjWGsSGmAGx7b_IAw The more common way to lose money is to either not have a plan or not stick to the plan. Disciplined investors typically plan to buy quality stocks at a fair price and hold them long enough for increasing sales and profits to bring the stock price up. If, later, he hears a bit of bad news about his stock and decides to sell out of panic or fear and become a trader instead of keeping to the plan to remain a disciplined investor, he is likely to lose money. He will lose because no-one can predict accurately that a stock is going down and will never recover; nor can he predict accurately when a stock is going up and will never falter. The chance of bankruptcy (especially for huge companies like Apple) is   really low, as I see it, but I may be wrong. Thousands of people lost billions of dollars thinking that about Enron, too. I too believe Apple is a fine stock with excellent prospects, but technology changes and markets change. Twenty or thirty years from now, it may be a different case. "I always liked the answer that in the short term, the market is a voting machine and in the long term the market is a weighing machine. People can ""vote"" a stock up or down in the short term.   In the long term, typically, the intrinsic value of a company will be reflected in the price. It's a rule of thumb, not perfect, but it is generally true. I think it's from an old investing book that talks about ""Mr. Market"".   Maybe it's from one of Warren Buffet's annual letters.    Anyone know? :)" At any given time there are buy orders and there are sell orders.  Typically there is a little bit of space between the lowest sell order and the highest buy order, this is known as the bid/ask spread.  As an example say person A will sell for $10.10 but person B will only buy at $10.00.  If you have a billion shares outstanding just the space between the bid and ask prices represents $100,000,000 of market cap. Now imagine that the CEO is in the news related to some embezzlement investigation.  A number of buyers cancel their orders.  Now the highest buy order is $7.  There isn't money involved, that's just the highest offer to buy at the time; but that's a drop from $10 to $7.  That's a change in market cap of $3,000,000,000.  Some seller thinks the stock will continue to fall, and some buyer thinks the stock has reached a fair enterprise value at $7 billion ($7 per share).  Whether or not the seller lost money depends on where the seller bought the stock.  Maybe they bought when it was an IPO for $1.  Even at $7 they made $6 per share. Value is changing, not money.  Though it would be fun, there's no money bonfire at the NYSE. There are lots of good answers on here already.  There are actually lots of answers for this question.  Lots.  I have years of experience on the exchange feed side and there are hundreds and thousands of variables.   All of these variables are funneled into systems owned by large financial institutions (I used to manage these - and only a few companies in the world do this so not hard to guess who I work for).  Their computers then make trades based on all of these variables and equations.  There are variables as whacky as how many times was a company mentioned in an aggregate news feed down to your basic company financials.  But if there is a way to measure a company (or to just guess) there is an equation for it plugged into a super computer at a big bank. Now there are two important factors on why you see this mad dash in the morning: Now most of the rest of the day is also automated trades but by the time you are an hour into market open the computers for the most part have fulfilled their calendar buys.  Everyone else's answer is right too.  There is futures contracts that change, global exchange info changes, options expiring, basic news, whatever but all of these are amplified by the calendar day changing. "When structures recur at different scales, they're called ""fractals"", and there is something called the ""fractal markets hypothesis"" which attempts to analyse stock market movements as fractals and in terms of (related) chaos theory. Whether you can profit from it I have no idea.  If it was easy, everyone would be doing it.  Many of the non-academic pages linked in the search results (previous link) remind me of technical analysis/chartist stuff (which - to me - always seems to be a lot better at explaining things after the event than actually predicting things)." The short float ratio and percent change are all calculated based on the short interest (the total number of shares shorted).  The short interest data for Nasdaq and NYSE stocks is published every two weeks. NasdaqTrader.com shows the exact dates for when short interest is published for Nasdaq stocks, and also says the following: FINRA member firms are required to report their short positions as of settlement on (1) the 15th of each month, or the preceding business day if the 15th is not a business day, and (2) as of settlement on the last business day of the month.* The reports must be filed by the second business day after the reporting settlement date. FINRA compiles the short interest data and provides it for publication on the 8th business day after the reporting settlement date. The NYSE also shows the exact dates for when short interest is published for NYSE stocks, and those dates are exactly the same as for Nasdaq stocks. Since the short interest is only updated once every 2 weeks, there is no way to see real-time updating of the short float and percent change.  That information only gets updated once every 2 weeks - after each publication of the short interest. 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. "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." Theoretically, it could be daily, but depending upon the number of companies in the index, it could be anywhere between daily or once a month or so. Apart from that, there is a periodic index review that happens once every quarter. The methodology for each index is also different, and you need to be aware of it (we had positions on literally hundreds of indices, and I knew the methodology of almost each of them).  If you have say, 2 billion dollars tracking a certain index, even a miniscule change in the composition would be substantial for you. But for certain others, you may just need to buy and sell $10k worth of stocks, and we would not even bother. "The ""price"" is the price of the last transaction that actually took place. According to Motley Fool wiki: A stock price is determined by what was last paid for it. During market hours (usually weekdays from 9:30AM-4:00PM eastern), a heavily traded issue will see its price change several times per second. A stock's price is, for many purposes, considered unchanged outside of market hours. Roughly speaking, a transaction is executed when an offer to buy matches an offer to sell. These offers are listed in the Order Book for a stock (Example: GOOG at Yahoo Finance).  This is actively updated during trading hours. This lists all the currently active buy (""bid"") and sell (""ask"") orders for a stock, and looks like this:  You'll notice that the stock price (again, the last sale price) will (usually*) be between the highest bid and the lowest ask price. * Exception: When all the buy or sell prices have moved down or up, but no trades have executed yet."