One of the most important questions you need to answer when opening a new trade is how many shares you should buy. Getting your position size correct is one way to reduce risk and increase your odds of having a winning trade.
There are several different ways to size positions but I think the best way is to let the stock itself determine it for you. That way you're not imposing some artificial limit on your trade that the market will not respect. In his book The Art of the Trade R.E. McMaster calls this "backing into the trade."
Say you have a $50,000 futures trading account and decide, in accordance with money management Rule 3 above, that you’re going to risk up to 5 percent of your capital, $2500, on any one trade. This means you have to be wrong 20 times in a row to be wiped out by the market. So far, what’s going on in the market has nothing at all to do with how you’re trading. You determine the number of contracts you trade by computing the dollar risk on each trade from entry point (the price at which you are willing to buy) to protective stop loss (the price at which you decide you will sell), and then divide that dollar amount into $2500. The result gives you the number of contracts you will trade. This way you equalize every trade.
In his example he refers to futures contracts but you can just as easily use the same method for trading stocks. By sizing your trade based off the dollar risk ($2500 in McMaster's example) then you're trading with the charts and not some inflexible rule that ignores the price action. Figuring the actual position size is simple algebra:
Shares = Risk / (Price - Stop)
This is for a long position. A short position is much the same:
Shares = Risk / (Sell - Cover)
Pretty easy and you should be able to tap that out on a pocket calculator pretty easily. However, if you're lazy then I have a simple position sizing web page that you can use.
Here's an example using a Microsoft chart:
In the MSFT chart you can see that the Purchase price will be $28.67 and that the stop point is down below the late March consolidation at $27.45. This is a spread of $1.22. If you divide that into the amount of risk you're willing to take on ($2500), then you get 2,050 shares:
2,050 = 2,500 / ($28.67 - $27.45)
Wait a minute, though. If you multiply 2,050 by the purchase price of $28.67 you get $58,773. That's more than we have in our trading account. You could always buy the extra shares on margin but that would mean that you've just poured your entire account into one trade and we know what they say about putting all your eggs in one basket...
If you have this problem there are two things you can do. The first is to move your stop price down to increase the spread between the purchase and stop. In the MSFT example this would mean lowering the stop to around $24.40. In my opinion this is a bad idea because if the price action were to actually go that low then it would mean your original trade premise was wrong and you should've been out of the stock long before your stop was hit.
The second method to solve the too large position problem is to modify the position sizing formula to
Shares = Equity / Price
to limit the number of shares purchased to whatever money you have on hand. This is better than lowering your stops because you can still keep your stop where it should be but you're risking less money. Of course, if the position size goes low enough then it may not be worth it to actually enter the trade. There is such a thing as putting on too small of a trade.
Do we always need to use support or resistance levels for determining our stop points in order for this to work? Of course not. You can use whatever works for you or makes sense. In addition to chart-based support and resistance I often use pivot points, fibonacci retracements, and trend lines. The important thing is to place your stop where if the price action trades through it you will be convinced that your original trade premise was wrong and then sizing your position accordingly.