Position sizing explained: how much to put into one idea

Two investors can buy the same stock on the same day and have completely different outcomes, simply because one put 5% of their account into it and the other put 50%. Position sizing is the decision about how much of your money goes into a single idea. It is the most direct control you have over how much a mistake can cost you.

Start from what you are willing to lose

A common method begins with risk, not with how attractive the idea looks. You decide in advance the most you are prepared to lose on any one position, as a percentage of your account. Then you work backward to the number of shares, using the price at which you would admit the idea is wrong and exit.

Amount at risk = account value × risk percentage

Risk per share = entry price − exit (stop) price

Number of shares = amount at risk ÷ risk per share

A worked example

The numbers below are illustrative. An investor has a $10,000 account and a rule of risking no more than 1% of the account on one idea. They plan to buy at $50 and decide that if the price falls to $46, their reason for owning it is no longer valid.

If the price falls to $46 and the investor exits as planned, the loss is about $100, roughly 1% of the account, before costs. The position is 12.5% of the account, but the planned risk is 1%.

Add a concentration limit

Risk-based sizing can produce very large positions when the stop is close to the entry price. With a $1 stop distance in the example above, the same $100 risk would mean 100 shares, a $5,000 position, half the account. That is why most investors pair the risk rule with a cap on any single position, for example a maximum share of the account set in your written rules. Use whichever rule produces the smaller position.

Our guide to diversification versus concentration explores how many positions and what limits make sense for different goals.

Where should the exit level go?

The exit price should come from your thesis, not from how much you want to lose. Ask what price, or what event, would show your reason for buying was wrong. That might be below a recent low, or it might not be a price at all: a falling margin in the next earnings report, a lost contract or a guidance cut. For a long-term investor, a fundamental invalidation condition is often more meaningful than a price level. Our research guide on building an investment thesis covers writing that condition.

Volatility changes the picture

A stock that routinely moves 4% a day needs a wider exit level than one that moves 1%, or ordinary noise will trigger it. A wider stop at the same risk amount means fewer shares. That is the point: more volatile holdings get smaller positions, so each idea carries a similar planned risk. Measure recent daily price moves before setting the stop, rather than guessing.

Common sizing mistakes

• Sizing by conviction alone. Feeling sure is not evidence. Some of the most confident ideas are the most crowded.
• Moving the stop after buying. Widening it when the price approaches turns a planned small loss into an unplanned large one.
• Adding to losing positions without a rule. Averaging down can be part of a plan, but only if it was in the plan.
• Ignoring correlation. Five positions that all depend on the same factor behave more like one big position.

Sizing a fund versus a single stock

The same thinking applies to funds, with different inputs. A broad, diversified fund usually carries less single-company risk than one stock, so investors often allow it a larger share of the account. A narrow sector or thematic fund can behave more like a concentrated position. Judge each holding by what could make it fall sharply, not by its label.

Write it on a ticket

Before each purchase, fill in a simple sizing ticket: account value, risk percentage, entry, exit, shares, position value, share of account, and the reason for the exit level. It takes two minutes and prevents most impulsive sizing errors.

Our ebook How to Choose Investments, Manage Risk, and Build a Portfolio includes a position-sizing ticket worksheet, worked sizing examples and a chapter on adding to, trimming and exiting positions.