Position Size Versus Stop Loss Tug of War

A couple of weeks ago, I had a Skype consultation with “Bill.”

He’s a new trader who started over the past few months but not a stranger to markets, having actively invested in mutual funds and his retirement for a long time.

On our call, we covered a wide range but there was one topic in particular that turned into a real aha moment for Bill: position sizing and stop losses.

Position Size Versus Stop Loss Tug of War- Image of sticky notes with question mark

Bill’s been studying so he knows that risk management and cutting losses is critical to surviving over the long haul. That in itself is a great thing because most new traders only pay lip service to risk management, but don’t actually appreciate its significance until it’s too late.

So while Bill has the right intentions focusing on risk management and stopping out of positions that are going against him, his actual execution has been backwards.

How Bill Was Reacting to Risk

When Bill found a stock that fit his entry criteria he would proceed to buy a few hundred shares. If it went in his direction, fantastic, no problems.

But if the stock moved against him and he began losing money — let’s use -$300 for round numbers — he knew it was time to exit and get out.

While we were walking through his thought process in some of his past trades, I asked him why he got out of a certain position that he was describing to me.

Bill said, “Well, I was down $300 dollars so I knew I had to exit.”

And then he paused and added something very important.

“But I really didn’t want to because I still think the stock is going to head higher.”

Bill was exiting his trades when the dollar amount he was down on the trade got too unbearable to stand. What I proposed to Bill is that his risk management process start with position sizing.

I told Bill to consider “backing into” his positions by first clearly defining his exit point and then computing the number of shares to purchase based on that.

The takeaway being:

“Your exit should be at the point which invalidates your rationale for entering the trade in the first place and not at the dollar loss you can no longer stomach.”

In other words, instead of taking an arbitrary several hundred shares with a tight stop loss, a trader should figure out where exactly they’ll be proven wrong in their entry thesis, use that as where they will exit for a loss on the trade, and determine their shares accordingly. The equation should look something like this:

Position size = (Account size * Risk per trade) / (Entry – Stop loss)

Even though this way of doing things will likely lead to wider stop loss placement, you’re not increasing the risk in the trade, you’re actually lowering it.

Ok, so you may be asking yourself, did Evan really just say wider stops result in less risk?

Yes! But let’s first qualify the statement by saying I am referring to overnight holds.

Imagine you socked away 50% of your account in small cap company, ticker symbol RISK, with the plan that you were going to use a 2% stop loss on the trade. If your stop loss got hit, you would only lose (50% * 2%) = 1% of your overall account, a fairly reasonable amount for most strategies.

The problem is ticker RISK is a small cap volatile name that can easily gap up or down 5% without any newsworthy catalysts. The well-intentioned 2% stop loss is great in theory, but the overnight risks and past behavior of RISK raises the odds greatly you’re actually going to take a loss bigger than you had planned — or potentially take something very devastating if a large overnight move occurs.

Instead, imagine you looked over RISK’s prior trade history and recognized that a 10% stop loss is much more appropriate given the stock’s volatility. If you used a 10% stop loss you would only need to invest 10% of your account in this trade (as opposed to 50%) and if the trade does end up hitting your stop loss you’ll still only lose that same (10% * 10%) = 1% of your overall account value.

The somewhat hidden but very important benefit, however, is that if you position size the second way, it will greatly reduce the impact a large overnight gap down will have on your account.

It clicked instantly for Bill. He understood exactly what he had been doing wrong and now he knew what he needed to do going forward. I sent him a copy of the Trade Risk’s position size calculator which is designed to solve this exact problem by incorporating multiple risk constraints, for a meaningful yet responsible amount of exposure in each individual trade.

An important note, this all assumes that Bill has a positive expectancy system and that his discretion and decision making is assumed to be true (value-add). But that’s a topic for another blog post, or in Bill’s case another consulting session.

Position Size Versus Stop Loss Tug of War-Image of Trust the Process text

Position Size Versus Stop Loss Tug of War – Conclusion

Assuming you are not a systems trader:

If one of your positions is selling off but you still think the stock should head higher (which we’ll assume is the reason you bought in the first place) then you should not be exiting the trade at that point

Exiting a position you are bullish on means you have a position sizing problem, or one of several possible behavioral issues you need to straighten out.

The solution is to take less shares, widen out the stop loss to whatever is appropriate for your trade setup, and only exit when the market has convinced you that you were wrong in your entry rational.

This is an important lesson and one that took me years to learn the hard way. Does this make sense to you? Do you have any follow up comments? Leave them below, I’d love to read them.

If you think you could benefit from a one-on-one review of your process, check out our consulting sessions.

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Evan Medeiros

Evan is the founder of the Trade Risk. With 25 years of coding experience and a B.S. in computer science, Evan brings a systematic discipline to investing in the stock market.

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  1. Todd Katz on 7:26 pm February 1, 2020 at 7:26 pm

    This makes sense to me. Thanks very much.

  2. Hasham on 4:57 am January 29, 2022 at 4:57 am

    So position sizing tells me to buy $180 worth shares on a capital of $1000, so that I can risk 1% of my capital.

    Suppose I buy shares worth 180. Now if the price goes up, ill be booking profit on those 180 dollars, right?

    Now suppose I use my own strategy instead of position sizing. I buy shares worth 1000 on a capital of 1000 and book loss when the price hits 980. But if the prices goes up, ill be booking profit in accordance with 1000, right?

    So when it comes to profit booking, isnt my formula better than position sizing?

    • Evan Medeiros on 11:10 am January 29, 2022 at 11:10 am

      I am not sure I fully understand the question, but this might help: the more you risk, aka the more shares you purchase, the bigger your rewards will be and the bigger your losses will be.

      We focus on position size in order to manage risk and avoid the risk of ruin.

      • Billw on 5:47 am October 6, 2022 at 5:47 am

        With position size based solely on risk as in this article, aren’t we ignoring the profit potential in the equation for our overall trading? How would reward:risk enter into the picture? Also the chance of winning such as %win / % loss? Would position sizing based on Expectancy make (more?) sense? If so, how should this be determined? Thank you for any explanation.

        • patrick on 8:19 pm April 12, 2023 at 8:19 pm

          Position sizing is used as a risk management tool. The downside of it is it limits your profit potential on your opening trade. But, if one averages down, the eventual payoff is higher. It really depends on your conviction whether eventually the trade will go your way. I definitely believe position sizing is a better risk management tool than stop losses. We all know the heartache that comes with stop losses.

  3. Evan Medeiros on 8:14 am October 7, 2022 at 8:14 am

    The problem with position sizing based on expected reward, is what happens if you don’t achieve your target/reward? How much will you lose? When will you exit?

    The only way traders survive long-term is by always having a firm handle on risk, position size, and knowing where they are wrong and have to exit. We can’t control the direction of the market, therefore we optimize on damage control and survivability.

    % win/loss ratios and expectancy can be used to inform risk management decisions but it is a more advanced approach and can lead into dangerous risk of ruin scenarios if not considered carefully.

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