Let's put the charts away and discuss some simple math that can help you size your positions so that your account can continue to grow.
Position sizing is just one element of risk management, but it is sometimes misunderstood or explained incorrectly because of the differences between the two main types of brokerage accounts: margin accounts and cash accounts. Today, some retail traders also use Roth IRAs, which generally don't allow borrowing on margin and therefore operate more like cash accounts for trading purposes.
While there are different ways to approach position sizing when using margin, I'll be focusing on the basics of position sizing in a cash account. This approach will be based on the capital you have available and a set of rules that can help us arrive at a math-based outcome.
Four Components
There are four components we need to determine when sizing our positions correctly:
Capital Allocation
Position Size
Stop Loss
Take Profit
I'm giving you the outcomes up front so you know what to look for as we work through the process. These four components should also be detailed in your current trading plan. If you don't have a trading plan yet, don't worry—you can start by drafting one. Keep it simple and aim for no more than one page.
Before placing any trade, you need to know your capital allocation, which is the amount of money you are committing to the trade. You also need to determine your stop loss. In this article, I'll show you how to determine your stop loss as a percentage. Finally, you need a plan for taking profits based on your understanding of risk-to-reward.
Before we determine any of these, however, we need to establish the maximum amount we are willing to lose on the trade. One common starting point is the 1% rule, where you limit your potential loss on a trade to 1% of your account capital. The important part is that this amount is determined before entering the trade.
This is what I mean by taking a risk-first approach: always determine your risk first. Always, always, always know how much you are willing to lose before you place the trade.
The Standard 1% Rule
The 1% rule is a common risk-management guideline that limits the amount of capital you are willing to lose on any trade to 1% of your account. In this approach, 1% represents your maximum acceptable loss—not the amount of money you necessarily expect to lose.
I also want to take this a step further: when you have multiple positions open, you should consider your total amount of risk across all positions. If the market moves against several of your positions at the same time, your combined potential loss should still remain within the maximum amount of risk you have established for your account. This is one of the ways position sizing can help protect your account from taking a loss that is too large.
Can you increase your risk above 1%? Yes, but I would keep your maximum risk at 2% or less. The important point is that you establish this maximum before entering the trade and use it consistently.
Now that we have established our maximum acceptable loss, we can use that number to determine our four components: Capital Allocation, Position Size, Stop Loss, and Take Profit.
Bill O'Neil's 8% Rule
In Bill O'Neil's book How to Make Money in Stocks, he introduced what has become known among traders as the 8% rule. The basic idea is that if a stock declines 8% from your purchase price, you exit the position without hesitation. O'Neil's approach treats an 8% decline as a signal that the original analysis or setup may have been wrong.
You don't necessarily have to use 8% as your stop-loss percentage. Depending on your strategy and trading plan, you could use 5%, 6%, or 7%, for example. The important point is to establish your maximum acceptable loss before entering the trade. Having a predetermined exit can help limit a small loss from turning into a much larger one.
Risk isn't talked about nearly enough. The exciting part of trading is often the discussion about how much money you can make, while the amount you can lose is overlooked. But before you put on any trade, you should know exactly how much capital you are willing to risk.
This is where the 8% rule becomes useful for our position-sizing calculation. We now have two important numbers: the amount of our account we are willing to risk and the percentage decline at which we will exit the position. With those two numbers, we can begin calculating our position size and putting together the four components of our trade.
Capital Allocation
Determining your capital allocation is very simple and only requires some basic math. Your capital allocation is the total amount of capital that will be committed to the position, but remember: not all of that capital is at risk. The amount at risk is determined by the maximum loss we established using the 1% rule.
Let's use a $15,000 account as an example. If we are willing to risk 1% of our account, our maximum acceptable loss is $150.
Now that we have established an 8% stop-loss rule, we can use that $150 risk amount to determine how much capital we can allocate to the trade. We divide our maximum acceptable loss by the stop-loss percentage:
$150 ÷ 0.08 = $1,875
This means we can allocate a maximum of $1,875 to this position. If the position declines 8%, the resulting loss would be approximately $150, or 1% of our $15,000 account.
This is an important distinction: $1,875 is the capital allocated to the trade, while $150 is the capital at risk.
Do not allocate more than your calculated amount if you want to maintain the 1% risk limit with an 8% stop. Always keep a risk-first mindset when executing your trades. Determine how much you are willing to lose first, and then let that number determine how much capital you can put into the position.
Position Size
Now that we have determined our maximum capital allocation of $1,875, let's say we find a stock with a good setup trading at $37.50 per share. To determine our position size, we simply divide our capital allocation by the current stock price:
$1,875 ÷ $37.50 = 50 shares
Therefore, 50 shares is the maximum position size we can take based on our $1,875 capital allocation.
Remember, we are working from a risk-first approach. The $1,875 tells us how much capital we can allocate to the trade, while the 50 shares tells us how many shares that allocation allows us to purchase. Going above 50 shares would mean allocating more capital than our calculation allows and could cause us to exceed our planned risk.
Always determine your risk first, then let the math determine the size of your position.
Stop Loss
With the 8% rule, we are calculating our stop loss as a percentage of the stock's entry price. Let's continue with our example and assume the stock is trading at $37.50 per share. If the stock drops 8% from our entry price, our plan is to exit the trade.
There are a couple of ways to calculate the stop price. I'll show you both, but the first method is the simpler one.
Since we are using an 8% stop, subtract 8% from 100% to get 92%. We then multiply our entry price of $37.50 by 92%:
$37.50 × 0.92 = $34.50
Our calculated stop price is therefore $34.50.
You can also calculate the stop price in two steps. First, determine the dollar amount represented by 8% of the stock price:
$37.50 × 0.08 = $3.00
Then subtract that amount from the entry price:
$37.50 − $3.00 = $34.50
Both methods give us the same result. The first method is simpler, but I wanted to show you both so you understand where the number comes from.
The main objective is to determine your stop price before entering the trade. In this example, our entry price is $37.50 and our calculated stop is $34.50. We have already determined that we can own a maximum of 50 shares, so if the stock reaches our stop, the planned loss would be approximately:
50 shares × $3.00 = $150
That brings us back to our original 1% risk limit on a $15,000 account.
What I do personally is place my stop-market order immediately after my position is filled, using my calculated stop price. This helps ensure that I have an exit order in place if the trade moves against me. Keep in mind that a stop-market order does not guarantee an exact execution price, particularly in a fast-moving market.
The important principle is simple: know your risk, calculate your stop, and have your exit plan in place before the trade moves against you.
Take Profit
Now that we know how much we are willing to risk and where our stop loss will be, we can determine our take-profit level.
This is where risk-to-reward comes into the picture. Risk-to-reward compares the amount we are willing to lose on a trade with the amount we are targeting as a potential profit.
In his book How to Make Money in Stocks, Bill O'Neil discussed the importance of seeking a favorable risk-to-reward relationship and advocated aiming for a 3:1 reward-to-risk ratio. In other words, for every dollar we are willing to risk, we should look for the potential to make three dollars.
In our example, we are risking $150 on the trade. Using a 3:1 risk-to-reward ratio, our potential profit target would therefore be:
$150 × 3 = $450
We are risking $150 to target a potential $450 profit.
Remember that our entry price is $37.50 and our stop price is $34.50. The difference between those two prices is $3.00 per share, which represents our planned risk per share.
To calculate a 3:1 profit target, we multiply that $3.00 risk per share by 3:
$3.00 × 3 = $9.00
We then add the $9.00 potential gain to our entry price:
$37.50 + $9.00 = $46.50
Our 3:1 take-profit price is therefore $46.50.
If we own 50 shares and the stock reaches $46.50, the potential gain would be:
50 shares × $9.00 = $450
Our trade would therefore have a planned $150 risk and a $450 potential reward, giving us a 3:1 risk-to-reward ratio.
The important point is that your take-profit target should be established as part of your trading plan rather than decided emotionally after you enter the trade. Just as we determined our maximum risk and stop loss before entering the position, we should also know what price or conditions will cause us to take profits.
A 3:1 risk-to-reward ratio does not mean that a trade will reach your profit target, nor does it guarantee that you will make three times what you risk. It simply provides a predefined framework for comparing your potential reward with your planned risk.
At this point, we have all four components of our trade:
Capital Allocation: $1,875
Position Size: 50 shares
Stop Loss: $34.50
Take Profit: $46.50 for a 3:1 risk-to-reward target
Most importantly, we arrived at these numbers by starting with our risk. We didn't begin by asking how many shares we could buy or how much money we wanted to make. We first determined how much we were willing to lose, and then used that number to determine the size of the position and the parameters of the trade.
Risk/Reward and Break-Even
Bill O'Neil followed up How to Make Money in Stocks with The Successful Investor in the early 2000s. In that book, he discussed using a standard 3:1 risk-to-reward ratio as a defense mechanism. His point was that you don't have to be right on every trade to make money. In fact, he discussed aiming for a batting average of around 30%.
If you are right on one out of three trades, your batting average is approximately 33%. At a 3:1 risk-to-reward ratio, you would only need to be right one out of four times to break even, which is a 25% win rate.
O'Neil also explained that as you become better at identifying the correct setups, your batting average can improve. My interpretation of this chapter is that the best setups can put you in a position to make substantially more than 3:1 when the trade develops in your favor.
The table below shows the theoretical break-even win rate at different risk-to-reward ratios:
| Risk/Reward | Break-Even Win Rate* |
|---|---|
| 1:1 | 50% |
| 2:1 | 33.3% |
| 3:1 | 25% |
| 4:1 | 20% |
*Break-even rates shown before commissions, fees, slippage, taxes, and other trading costs.
This is an important part of position sizing and risk management that isn't discussed nearly enough, and the subject could easily be an article of its own.
Think about what happens if you find a high-probability setup that wins 60% of the time and produces a 3:1 or better risk-to-reward ratio. The combination of a favorable win rate and favorable risk-to-reward can have a significant effect on the results of a trading strategy.
Let's return to our previous example. We bought 50 shares at $37.50, giving us a total position of $1,875, with a stop at $34.50. We still have $150 at risk, or 1% of our $15,000 account.
Now let's assume the trade continues to work and we take our profit at $49.50.
The difference between our entry price and our profit target is:
$49.50 − $37.50 = $12.00 per share
With 50 shares, our potential gain is:
50 shares × $12.00 = $600
Our original risk was $150, so:
$600 ÷ $150 = 4:1
We have now achieved a 4:1 risk-to-reward ratio rather than the 3:1 target we originally established.
Now let's take this one step further. Suppose we make four trades with the same $150 risk and 4:1 potential reward. We win two trades and lose two trades.
Our two winning trades produce:
2 × $600 = $1,200
Our two losing trades produce:
2 × $150 = $300
Our net result would therefore be:
$1,200 − $300 = $900
We would have a 50% win rate, which is twice the 25% break-even rate for a 3:1 risk-to-reward ratio. More importantly, because our winning trades produced 4:1 rather than 3:1, the two winning trades more than offset the two losing trades.
This is why I believe it is important to think about both win rate and risk-to-reward. Looking at either one by itself doesn't tell us the whole story. A trader can have a relatively low win rate and still have a profitable strategy if the average winning trade is substantially larger than the average losing trade.
The goal isn't to be right on every trade. The goal is to control how much you lose when you are wrong and give your winning trades enough room to make the risk worthwhile.
Conclusion
Position sizing doesn't have to be complicated. The math is actually quite simple once you start with the most important number: how much are you willing to lose? From there, you can determine the four components of your trade: Capital Allocation, Position Size, Stop Loss, and Take Profit.
These four components should be part of every trading plan. Before entering a trade, you should know how much capital you are committing, how many shares you will own, where you will exit if the trade moves against you, and where you plan to take profits if the trade works in your favor.
In our example, a $15,000 account with a 1% maximum risk gave us $150 of acceptable risk. An 8% stop allowed us to allocate $1,875 to the position, which at $37.50 per share gave us a maximum position of 50 shares. From there, we established our stop at $34.50 and identified our potential profit target using risk-to-reward.
The larger lesson is that successful trading isn't about being right on every trade. Losses are part of trading. What matters is controlling the size of those losses and giving your winning trades enough opportunity to make a meaningful contribution to your account. A consistent approach to position sizing allows you to take the same risk framework into your next trade, regardless of whether the previous trade was a winner or a loser.
This is why I believe a risk-first approach is so important. Determine how much you are willing to lose, and then let that number determine the size of your position. Establish your stop before entering the trade, understand your risk-to-reward, and know where you intend to take profits. When you know your Capital Allocation, Position Size, Stop Loss, and Take Profit, you know the four basic components of the trade before you put your money at risk.
The objective isn't to eliminate losses. The objective is to make sure that no single loss has the power to prevent you from taking the next trade. Protect your capital, follow your plan, know your four components, and let the math make your position-sizing decisions for you.
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