How do we know the number of shorted shares of a stock?
For a company listed on NASDAQ, the numbers are published on NASDAQ's site. The most recent settlement date was 4/30/2013, and you can see that it lists 27.5 million shares as held short.  NASDAQ gets these numbers from FINRA member firms, which are required to submit them to the exchange twice a month: Each FINRA member firm is required to report its “total” short interest positions in all customer and proprietary accounts in NASDAQ-listed securities twice a month. These reports are used to calculate short interest in NASDAQ stocks. 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 sale of shares on vesting convolutes matters. In a way similar to how reinvested dividends are taxed but the newly purchased fund shares' basis has to be increased, you need to be sure to have the correct per share cost basis. It's easy to confuse the total RSU purchase with the correct numbers, only what remained.  The vesting stock is a taxable event, ordinary income.  You then own the stock at that cost basis. A sale after that is long or short term and the profit is the to extent it exceeds that basis.  The fact that you got these shares in 2013 means you should have paid the tax then. And this is part two of the process. Of course the partial sale means a bit of math to calculate the basis of what remained. "An investment is sold when you sell that particular stock or fund. It doesn't wait until you withdraw cash from the brokerage account. Whether an investment is subject to long term or short term taxes depends on how long you held that particular stock. Sorry, you can't get around the higher short term tax by leaving the money in a brokerage account or re-investing in something else. If you are invested in a mutual fund, whether it's long or short term depends on when you buy and sell the fund. The fact that the fund managers are buying and selling behind your back doesn't affect this. (I don't know what taxes they have to pay, maybe you really are paying for it in the form of management fees or lower returns, but you don't explicitly pay the tax on these ""inner"" transactions.) Your broker should send you a tax statement every year giving the numbers that you need to fill in to the various boxes of your income tax form. You don't have to figure it out. Of course it helps to know the rules. If you've held a stock for 11 1/2 months and are planning to sell, you might want to consider waiting a couple of weeks so it becomes a long term capital gain rather than short term and thus subject to lower tax." (Note: I am omitting the currency units.  While I strongly suspect it's US$ I don't know from the chart.  The system works the same no matter what the currency.) A call or a put is the right to sell (put) or buy (call) shares at a certain price on a certain day. This is why you see a whole range of prices.  Not all possible stock values are represented, the number of possibilities has to be kept reasonable.  In this case the choices are even units, for an expensive stock they may be spaced even farther apart than this. The top of the chart says it's for June.  It's actually the third Friday in the month, June 15th in this case.  Thus these are bets on how the stock will move in the next 10 days. While the numbers are per share you can only trade options in lots of 100. The left side of the chart shows calls.  Suppose you sell a call at 19 (the top of the chart)  The last such trade would have gotten you a premium of 9.70 per share (the flip side of this is when the third friday rolls around it will most likely be exercised and they'll be paying you only 19 a share for a stock now trading at something over 26.)  Note the volume, bid and ask columns though--you're not going to get 9.70 for such a call as there is no buyer.  The most anybody is offering at present is 7.80 a share. Now, lets look farther down in the chart--say, a strike price of 30.  The last trade was only .10--people think it's very unlikely that FB will rise above 30 to make this option worthwhile and thus you get very little for being willing to sell at that price.  If FB stays at 26 the option will expire worthless and go away.  If it's up to 31 when the 15th rolls around they'll exercise the option, take your shares and pay you 30 for them.  Note that you already gave permission for the trade by selling the call, you can't back out later if it becomes a bad deal. Going over to the other side of the chart with the puts: Here the transaction goes the other way, come the 15th they have the option of selling you the shares for the strike price.  Lets look at the same values we did before.  19?  There's no trading, you can't do it. 30?  Here you will collect 3.20 for selling the put.  Come the 15th they have the right to sell you the stock for 30 a share.  If it's still 26 they're certainly going to do so, but if it's up to 31 it's worthless and you pocket the 3.20 Note that you will normally not be allowed to sell a call if you don't own the shares in question.  This is a safety measure as the risk in selling a call without the stock is infinite.  If the stock somehow zoomed up to 10,000 when the 15th rolls around you would have to come up with the shares and the only way you could get them is buy them on the open market--you would have to come up with a million dollars.  If there simply aren't enough shares available to cover the calls the result is catastrophic--whoever owns the shares simply gets to dictate terms to you.  (And in the days of old this sometimes happened.) "I'm not a financial expert, but saying that paying a $1 dividend will reduce the value of the stock by $1 sounds like awfully simple-minded reasoning to me. It appears to be based on the assumption that the price of a stock is equal to the value of the assets of a company divided by the total number of shares. But that simply isn't true. You don't even need to do any in-depth analysis to prove it. Just look at share prices over a few days. You should easily be able to find stocks whose price varied wildly. If, say, a company becomes the target of a federal investigation, the share price will plummet the day the announcement is made. Did the company's assets really disappear that day? No. What's happened is that the company's long term prospects are now in doubt. Or a company announces a promising new product. The share price shoots up. They may not have sold a single unit of the new product yet, they haven't made a dollar. But their future prospects now look improved. Many factors go into determining a stock price. Sure, total assets is a factor. But more important is anticipated future earning. I think a very simple case could be made that if a stock never paid any dividends, and if everyone knew it would never pay any dividends, that stock is worthless. The stock will never produce any profit to the owner. So why should you be willing to pay anything for it? One could say, The value could go up and you could sell at a profit. But on what basis would the value go up? Why would investors be willing to pay larger and larger amounts of money for an asset that produces zero income? Update I think I understand the source of the confusion now, so let me add to my answer. Suppose that a company's stock is selling for, say, $10. And to simplify the discussion let's suppose that there is absolutely nothing affecting the value of that stock except an expected dividend. The company plans to pay a dividend on a specific date of $1 per share. This dividend is announced well in advance. Everyone knows that it will be paid, and everyone is extremely confidant that in fact the company really will pay it -- they won't run out of money or any such. Then in a pure market, we would expect that as the date of that dividend approaches, the price of the stock would rise until the day before the dividend is paid, it is $11. Then the day after the dividend is paid the price would fall back to $10. Why? Because the person who owns the stock on the ""dividend day"" will get that $1. So if you bought the stock the day before the dividend, the next day you would immediately receive $1. If without the dividend the stock is worth $10, then the day before the dividend the stock is worth $11 because you know that the next day you will get a $1 ""refund"". If you buy the stock the day after the dividend is paid, you will not get the $1 -- it will go to the person who had the stock yesterday -- so the value of the stock falls back to the ""normal"" $10. So if you look at the value of a stock immediately after a dividend is paid, yes, it will be less than it was the day before by an amount equal to the dividend. (Plus or minus all the other things that affect the value of a stock, which in many cases would totally mask this effect.) But this does not mean that the dividend is worthless. Just the opposite. The reason the stock price fell was precisely because the dividend has value. BUT IT ONLY HAS VALUE TO THE PERSON WHO GETS IT. It does me no good that YOU get a $1 dividend. I want ME to get the money. So if I buy the stock after the dividend was paid, I missed my chance. So sure, in the very short term, a stock loses value after paying a dividend. But this does not mean that dividends in general reduce the value of a stock. Just the opposite. The price fell because it had gone up in anticipation of the dividend and is now returning to the ""normal"" level. Without the dividend, the price would never have gone up in the first place. Imagine you had a company with negligible assets. For example, an accounting firm that rents office space so it doesn't own a building, its only tangible assets are some office supplies and the like. So if the company liquidates, it would be worth pretty much zero. Everybody knows that if liquidated, the company would be worth zero. Further suppose that everyone somehow knows that this company will never, ever again pay a dividend. (Maybe federal regulators are shutting the company down because it's products were declared unacceptably hazardous, or the company was built around one genius who just died, etc.) What is the stock worth? Zero. It is an investment that you KNOW has a zero return. Why would anyone be willing to pay anything for it? It's no answer to say that you might buy the stock in the hope that the price of the stock will go up and you can sell at a profit even with no dividends. Why would anyone else pay anything for this stock? Well, unless their stock certificates are pretty and people like to collect them or something like that. Otherwise you're supposing that people would knowingly buy into a pyramid scheme. (Of course in real life there are usually uncertainties. If a company is dying, some people may believe, rightly or wrongly, that there is still hope of reviving it. Etc.) Don't confuse the value of the assets of a company with the value of its stock. They are related, of course -- all else being equal, a company with a billion dollars in assets will have a higher market capitalization than a company with ten dollars in assets. But you can't calculate the price of a company's stock by adding up the value of all its assets, subtracting liabilities, and dividing by the number of shares. That's just not how it works. Long term, the value of any stock is not the value of the assets but the net present value of the total future expected dividends. Subject to all sorts of complexities in real life." "(Value of shares+Dividends received)/(Initial investment) would be the typical formula though this is more of a percentage where 1 would indicate that you broke even, assuming no inflation to be factored. No, you don't have to estimate the share price based on revenues as I would question how well did anyone estimate what kind of revenues Facebook, Apple, or Google have had and will have. To estimate the value of shares, I'd likely consider what does my investment strategy use as metrics: Is it discounted cash flow, is it based on earnings, is it something else? There are many ways to determine what a stock ""should be worth"" that depending on what you want to believe there are more than a few ways one could go." "I agree with the answer by @Michael that this number doesn't exist.  It's hard to see what use it would have and it would be difficult to track. I'm writing a separate answer because I also disagree with the premise of your question:  Individual shares of stock have never to my knowledge had such a number.  Your comment about numbers on stock certificates identifies the certificate document, which will generally represent multiple shares of stock.  That number no more identifies a single share of stock than the serial number on a $10 bill identifies any one of the ten dollars it represents.  Even at the ""collective"" unit of $10, when the bill is eventually replaced with a new one, the new bill has a new number.  No continuity." Well, if the short seller has to pay the dividend out of their pocket, what happens to the dividend the company paid out ??  Sounds like there are 2 divdends floating around, the short's, and the company's, but only 1 share of stock. When you want to short a stock, you are trying to sell shares (that you are borrowing from your broker), therefore you need buyers for the shares you are selling.  The ask prices represent people who are trying to sell shares, and the bid prices represent people who are trying to buy shares. Using your example, you could put in a limit order to short (sell) 1000 shares at $3.01, meaning that your order would become the ask price at $3.01.  There is an ask price ahead of you for 500 shares at $3.00.  So people would have to buy those 500 shares at $3.00 before anyone could buy your 1000 shares at $3.01. But it's possible that your order to sell 1000 shares at $3.01 never gets filled, if the buyers don't buy all the shares ahead of you.  The price could drop to $1.00 without hitting $3.01 and you will have missed out on the trade. If you really wanted to short 1000 shares, you could use a market order.  Let's say there's a bid for 750 shares at $2.50, and another bid for 250 shares at $2.49.  If you entered a market order to sell 1000 shares, your order would get filled at the best bid prices, so first you would sell 750 shares at $2.50 and then you would sell 250 shares at $2.49. I was just using your example to explain things.  In reality there won't be such a wide spread between the bid and ask prices.  A stock might have a bid price of $10.50 and an ask price of $10.51, so there would only be a 1 cent difference between putting in a limit order to sell 1000 shares at $10.51 and just using a market order to sell 1000 shares and getting them filled at $10.50. Also, your example probably wouldn't work in real life, because brokers typically don't allow people to short stocks that are trading under $5 per share. As for your question about how often you are unable to make a short sale, it can sometimes happen with stocks that are heavily shorted and your broker may not be able to find any more shares to borrow. Also remember that you can only short stocks with a margin account, you cannot short stocks with a cash account.