How fixed-fractional sizing works
Fixed-fractional position sizing starts with a single parameter: the percentage of current account equity to risk on each trade. Risk in this context means the maximum dollar loss the investor is willing to accept if the trade goes entirely wrong (from entry to the predefined stop-loss or maximum acceptable drawdown level). Risking 2% on a $100,000 account means no single trade can lose more than $2,000 under normal conditions.
The calculation of share size follows directly from the dollar risk and the per-share risk. Per-share risk is the difference between the entry price and the stop-loss price. If a stock is entered at $50 and the stop-loss is set at $45, the per-share risk is $5. With a $2,000 dollar risk budget, the position size is $2,000 / $5 = 400 shares. The dollar value of this position is 400 shares x $50 = $20,000, which is 20% of the $100,000 account. The key insight is that position weight in the portfolio (here 20%) is a function of where the stop-loss is placed relative to entry; a tighter stop on the same stock would allow a larger position, and a wider stop would require a smaller one.
Position size scales with account equity automatically. After a winning period where equity grows to $120,000, the same 2% risk means a $2,400 dollar risk per trade, allowing slightly larger positions. After a losing period where equity falls to $85,000, the dollar risk drops to $1,700, forcing smaller positions. This automatic scaling has two valuable properties: it prevents the investor from taking oversized positions after a losing streak (when confidence and judgment may be impaired) and it compounds gains by allowing larger absolute positions as the account grows.
The choice of risk percentage matters enormously over time due to the arithmetic of drawdowns. At 1% risk per trade, a losing streak of 20 consecutive losses would reduce equity to approximately 82% of its starting value (compounded). At 3% risk, the same 20-loss streak would reduce equity to approximately 54%. At 5%, equity would fall to approximately 36%. Because recovery from a 64% drawdown requires a 178% gain to break even, the choice of risk percentage is one of the most important long-run decisions in position sizing.
Stop-loss placement and its effect on position size
The stop-loss price is not arbitrary in a fixed-fractional system; it is the price at which the original trade thesis is disproven, not the price at which the investor would feel uncomfortable with the loss. A stop-loss should be placed at a level that would not be touched if the thesis is correct (below a key support level, below the most recent swing low, below a fundamental valuation floor) but would be touched if the thesis is wrong. Placing stops too close to entry increases the number of stop-outs on correct positions due to normal volatility; placing them too far away forces very small position sizes that do not contribute meaningfully to portfolio performance.
Volatility-adjusted stop placement acknowledges that different stocks have different normal price ranges. A stock with a 30-day average true range of $2 should not have a $1 stop-loss, because the normal daily price movement would trigger the stop before the thesis has time to develop. A common rule of thumb is to place stops at 1-2 times the average true range below entry, which keeps the stop outside the normal noise range while still being close enough to the level that would disprove the thesis.
The position size calculation then adjusts automatically to the stop placement. A wider stop (necessary for a volatile stock with large normal price swings) will produce a smaller position in dollar and share terms. An investor risking $2,000 per trade who enters a volatile stock with a $10 per-share stop will hold 200 shares; the same investor entering a stable stock with a $2 per-share stop will hold 1,000 shares. The fixed-fractional method naturally leads to larger positions in lower-volatility assets, which is consistent with risk parity principles at the position level.
Stop-loss placement must also account for the bid-ask spread and realistic execution slippage. In liquid large-cap stocks, the stop can be placed very precisely. In less liquid stocks or during volatile market conditions, actual exit prices may be significantly worse than the nominal stop price, especially on gap-down opens where the market opens below the stop level. Sizing for the worst reasonable slippage scenario (rather than the nominal stop price) provides an additional margin of safety in the dollar risk calculation.
Advantages and limitations of fixed-fractional sizing
Fixed-fractional sizing has several practical advantages over alternatives. It is simple to implement and produces consistent behavior regardless of market conditions: the investor always risks the same fraction of equity, never more in good markets or less in bad ones. It prevents the overconfidence-driven over-betting that typically follows a run of successful trades. And it provides a clear, objective rule for position size that eliminates the subjective judgment that otherwise allows emotions to drive allocation decisions.
The method also has inherent limitations. It treats all trades as having equal expected value, which ignores conviction and estimated edge. An investor with high conviction in one position and low conviction in another would size them identically under strict fixed-fractional rules, which may not reflect the actual return opportunity. Conviction-weighted sizing (varying the risk percentage based on the strength of the thesis) is a common modification that retains the fixed-fractional framework while allowing some differentiation between positions.
Fixed-fractional sizing also does not account for correlation between simultaneous positions. If all open trades are correlated (same sector, same macro factor), the combined drawdown potential is the sum of individual risks rather than a diversified blend. Portfolio heat monitoring (the total percentage of equity at risk across all open positions simultaneously) is the corrective: capping total heat at 10-20% of equity ensures that even if all stops trigger simultaneously on correlated positions, the portfolio drawdown remains manageable.
Frequently asked questions
What is fixed-fractional position sizing?
Fixed-fractional position sizing risks a constant percentage of current account equity on each trade, typically 1-3%. The percentage is applied to the current equity balance to calculate the maximum dollar loss acceptable per trade. Share size is then calculated by dividing the dollar risk by the per-share risk (entry price minus stop-loss price). The method automatically scales position sizes with account growth and declines, compounding gains and limiting drawdowns from any single trade.
How do you calculate position size using fixed-fractional sizing?
The calculation has three steps: (1) multiply account equity by the risk percentage to find the dollar risk per trade (e.g., $100,000 x 2% = $2,000); (2) calculate the per-share risk by subtracting the stop-loss price from the entry price (e.g., entry $50, stop $45, per-share risk = $5); (3) divide dollar risk by per-share risk to find the number of shares (e.g., $2,000 / $5 = 400 shares). The dollar value of the position is 400 x $50 = $20,000, which is 20% of the account.
What percentage should I risk per trade with fixed-fractional sizing?
Most professional traders risk 1-3% of account equity per trade. The right percentage depends on the strategy's win rate and average win-to-loss ratio, the investor's risk tolerance, and the typical number of simultaneous positions. Riskier strategies with lower win rates require smaller per-trade risk percentages to withstand losing streaks. Conservative investors often start at 1% or less. Risking more than 3-5% per trade materially increases the probability of a catastrophic drawdown from a normal losing streak.
How does a wider stop-loss affect position size in fixed-fractional sizing?
A wider stop-loss (more distance between entry price and stop price) reduces position size in shares, because the same dollar risk budget is spread over a larger per-share risk. If the dollar risk is $2,000 and the per-share risk is $10 (wide stop), the position is 200 shares. If the per-share risk is $2 (tight stop), the position is 1,000 shares. Wider stops are appropriate for more volatile stocks but force smaller positions; tight stops allow larger positions but risk being triggered by normal price volatility.
What is portfolio heat and how does it relate to fixed-fractional sizing?
Portfolio heat is the total percentage of account equity at risk across all open positions simultaneously (the sum of each position's dollar risk divided by total equity). If five positions are each sized at 2% risk, total heat is 10%, meaning the portfolio would lose approximately 10% if all five stops triggered at once. Fixed-fractional sizing controls individual position risk but not portfolio heat; monitoring total heat and capping it (typically at 10-20% of equity) ensures correlated losses across multiple positions do not produce a catastrophic combined drawdown.