Direct Answer
Position-sizing methods answer different risk questions and should not be treated as interchangeable. Fixed-risk sizing limits the planned loss on one position based on entry and stop distance. Volatility-adjusted sizing scales exposure to how much an asset typically moves. Liquidity-adjusted sizing limits a position by how difficult it may be to exit. Portfolio heat measures the combined open risk across multiple positions. A robust process can use all four as separate constraints and adopt the smallest position allowed by the relevant limits.
Why Is One Sizing Formula Not Enough?
Position size looks like a single number, but the number can be constrained by several different failure modes.
A trade can be acceptable under a fixed 1% risk budget and still be too large relative to daily volume. Two positions can each risk the same number of dollars while one is several times more volatile. Eight individually small positions can collectively create a large portfolio loss if their stops are triggered together.
The methods below solve different problems. See risk-based position sizing for the core formula and worked examples.
Comparison at a Glance
| Method | Main question | Typical input | What it controls | Main limitation |
|---|---|---|---|---|
| Fixed-risk / stop-distance | How many units fit inside a planned dollar loss? | Account value, risk budget, entry, stop | Planned loss on one trade | Assumes the stop can execute near the planned price |
| Volatility-adjusted | How should size change when assets move by different amounts? | ATR or return volatility, risk target | Unequal volatility contribution | Historical volatility can change abruptly |
| Liquidity-adjusted | Can the position be exited without requiring an unrealistic share of market volume? | Position size, average volume, participation assumption | Exit capacity and market-impact risk | Average volume can overstate crisis liquidity |
| Portfolio heat | What is the combined open risk if multiple positions fail? | Per-position risk and account equity | Aggregate open risk | Correlation and gap risk can make realized loss larger |
How Does Fixed-Risk Sizing Work?
Fixed-risk sizing starts with a maximum dollar loss. If an account is $100,000 and the investor's internal risk budget for a trade is 1%, the dollar risk budget is $1,000. If the entry is $50 and the planned stop is $48, the planned risk is $2 per share. Ignoring costs and slippage, $1,000 divided by $2 produces 500 shares.
This method is intuitive because the result is tied directly to a planned loss.
Its weakness is that the stop is an assumption, not a guarantee. A market can gap through the stop. A thin security can fill far from the requested price. A position can also be too large for the available liquidity even when the arithmetic is correct.
The exact formula inputs and rounding conventions are documented in the position-size formula and assumptions registry.
How Does Volatility-Adjusted Sizing Work?
Volatility-adjusted sizing asks whether equal dollar positions really represent equal risk. Suppose one stock typically moves 1% per day and another moves 4%. Equal $10,000 allocations expose the portfolio to very different normal price variation.
A volatility method uses a measure such as Average True Range or historical return volatility to scale the more volatile position down.
This can create more balanced risk contributions, but the calculation is backward-looking. Volatility often rises during stress. A position sized during a quiet regime may be larger than desired after conditions change.
For a full explanation, see volatility-adjusted sizing.
How Does Liquidity-Adjusted Sizing Work?
Liquidity-adjusted sizing asks whether the position can realistically be exited. A 10,000-share position means little by itself. If the security trades 20 million shares per day, it is a small fraction of normal activity. If the security trades 25,000 shares per day, the same position is enormous relative to the market.
A useful diagnostic is days to liquidate:
Days to liquidate = Position size / (assumed participation rate × average daily volume)
The participation rate is an assumption, not a law. Lower assumptions are more conservative. Liquidity can also disappear during the exact market conditions in which an exit becomes urgent.
For a deeper treatment, see liquidity-adjusted position sizing.
What Is Portfolio Heat?
Portfolio heat moves from one position to the whole account. For each open position, estimate the planned loss between the current or entry price and the stop. Sum those dollar risks and divide by account equity.
Eight positions each carrying 1% planned risk can represent approximately 8% total planned open risk if all stops are reached, before considering gaps, slippage or correlation.
Portfolio heat is particularly important when positions share a common driver. Five semiconductor stocks are not five independent risk events during a sector shock.
The portfolio heat calculator measures combined open risk across all positions and groups correlated names.
Using the Methods Together
The methods can be applied as constraints rather than competitors:
- Calculate fixed-risk size.
- Calculate any volatility-based cap used by the strategy.
- Calculate the liquidity cap.
- Check the effect on portfolio heat and correlated exposures.
- Use the smallest size allowed by the constraints that actually apply.
- Recalculate when price, volatility, liquidity or portfolio composition changes.
This approach prevents one elegant formula from hiding another form of risk.
Worked Example
Assume a $100,000 account with a $1,000 trade-risk budget.
A stock is entered at $50 with a stop at $48. Fixed-risk sizing permits 500 shares.
Now assume the stock's liquidity policy permits only 300 shares at the chosen participation rate. The liquidity constraint is tighter, so 300 becomes the maximum before portfolio-level checks.
If adding 300 shares would push total portfolio heat above the account's internal limit, the portfolio constraint may reduce the size again.
The result is not "the correct number of shares." It is the largest size that satisfies the chosen risk constraints under the assumptions supplied.
Failure Modes
False Precision
A calculator can produce an exact integer even when the assumptions are uncertain. Precision in the output does not make the stop price, volatility estimate or liquidity assumption certain.
Correlation Blindness
Individually acceptable positions can share the same economic driver.
Gap Risk
Stops do not guarantee execution at the stop price.
Regime Change
Historical volatility and volume can change sharply.
Ignoring Costs
Spreads, slippage, financing and taxes can affect the realized outcome even when commissions are small.
Frequently Asked Questions
Why Is One Sizing Formula Not Enough?
Position size can be constrained by several different failure modes. A trade can be acceptable under a fixed 1% risk budget and still be too large relative to daily volume. Two positions can each risk the same number of dollars while one is several times more volatile. Eight individually small positions can collectively create a large portfolio loss if their stops are triggered together. Fixed-risk, volatility, liquidity and portfolio heat each solve a different problem.
How Does Fixed-Risk Sizing Work?
Fixed-risk sizing starts with a maximum dollar loss. If an account is $100,000 and the risk budget is 1%, the dollar risk is $1,000. If the entry is $50 and the stop is $48, the planned risk is $2 per share. $1,000 divided by $2 produces 500 shares. The weakness is that the stop is an assumption, not a guarantee. A market can gap through the stop, and a position can be too large for available liquidity even when the arithmetic is correct.
What Is Portfolio Heat?
Portfolio heat measures combined open risk across all positions. For each open position, estimate the planned loss between the current price and the stop, sum those dollar risks and divide by account equity. Eight positions each carrying 1% planned risk can represent approximately 8% total planned open risk if all stops are reached. Portfolio heat is particularly important when positions share a common driver, because correlated positions are not independent risk events during a sector shock.