Fixed Fractional vs Volatility-Based Position Sizing
Fixed-fractional position sizing links planned price-distance risk to a fraction of account equity, while volatility-based methods scale exposure using a measure such as ATR or historical return volatility. The two approaches can be combined, but neither guarantees maximum losses or makes a weak trading strategy profitable.
Fixed-fractional sizing in practice
A simplified rule sets a planned loss budget as account equity multiplied by a selected fraction. Dividing that budget by the estimated loss per unit between entry and an intended exit gives a position size. For example, £20,000 equity and a hypothetical 1% planned risk budget imply £200. If estimated price-distance loss is £2 per share, the arithmetic result is 100 shares before fees, gaps and size constraints. A 1% fraction is an illustration, not an appropriate recommendation for everyone.
The problem with a fixed stop distance
A £2 price distance may be wide for a share that normally moves £0.50 per day and narrow for one that often moves £5. A size calculation can be mathematically consistent yet based on an economically arbitrary stop. Transaction costs, spreads and overnight gaps can increase realised losses beyond the planned amount. Margin and leverage exposures require separate checks.
ATR-based volatility sizing
A volatility-based approach may define hypothetical stop distance as a multiple of ATR. If ATR is £1.50 and a two-ATR distance is selected, planned loss distance is £3. With a £200 budget, the unrounded arithmetic position is 66.67 shares, which must be rounded according to the product’s units. This does not prove that price is likely to remain within two ATR or that an exit will execute precisely at the threshold.
Variance-based volatility targeting
A different technique sizes positions so a target portfolio volatility is approached using estimated asset volatilities and correlations. For a single unleveraged asset estimated at 20% annual volatility, targeting a 10% annual contribution under simplistic assumptions suggests approximately 50% notional exposure. But the actual portfolio result depends on other holdings, volatility regimes, estimation error and derivatives’ nonlinear exposures.
When fixed-fractional and volatility rules conflict
A fixed-fractional budget based on planned stop distance may suggest a large notional position when recent volatility is low. A separate volatility or notional cap could require a smaller size. The safer analytical approach is to recognise the constraints rather than optimising one number to maximise exposure. Market depth and concentration can constrain a theoretical size even if the computed budget is not exceeded.
The danger of volatility targeting after calm periods
Historical volatility can decline during extended stable markets, encouraging an algorithm to increase leverage. If volatility then jumps suddenly, the position may be larger than the new conditions justify. Deleveraging across many strategies can amplify market stress. Explicit maximum leverage and liquidity assumptions are important because risk statistics are backward-looking.
Futures and Forex unit conversion
Futures use contract-specific multipliers and tick values; currency positions use pip values and exchange-rate conversion. A £200 risk budget divided by five index points is meaningless until the currency value per point is specified. Likewise, a Forex pip distance multiplied by a standard lot can produce exposure far larger than a retail account might expect. Verify the real instrument before interpreting the arithmetic.
Evaluate any sizing rule as a portfolio process
Backtest with real costs, gaps, margin constraints and changing correlations. Track realised loss relative to planned loss, drawdowns and periods of forced liquidation. A smaller estimated risk number may reflect underestimation rather than improved safety. Sizing manages exposure conditional on assumptions; it does not create forecasting skill.
Worked example: risk-based share quantities
The following original example uses hypothetical inputs to illustrate the method; it is not an actual market quotation.
| Method | Assumption | Result |
|---|---|---|
| Account value | £10,000 | Starting equity |
| Fixed risk budget | 2% of equity | £200 maximum planned loss before slippage |
| Price-based stop width | £2.00 per share | 100 shares for £200 intended price risk |
| Volatility-based wider stop | £4.00 per share | 50 shares for the same £200 intended price risk |
| Overnight gap | Price jumps beyond stop | Actual loss can exceed £200 |
The quantity in this example is risk budget divided by per-share price distance from entry to stop, with a separate check that notional exposure, liquidity and account constraints permit the trade. A stop price is not an execution guarantee; financing costs, spreads, gaps, commissions and currency conversion also matter. Volatility-based stops change with the market estimate, so position quantity should be recalculated rather than assuming the same number of shares remains equally risky.
Frequently asked questions
Does fixed fractional mean a fixed number of shares?
No. Position quantity varies with equity and assumed per-unit loss.
Does ATR guarantee an exit within its multiple?
No. ATR is historical range information, not a loss cap.
Can both sizing methods be combined?
Yes. A process can impose separate budget, volatility, notional and leverage constraints.