Position Sizing Basics

Position sizing determines how much capital each trade risks — often more important than entry timing for long-term survival. Proper sizing limits damage during drawdowns and prevents a single decision from dominating account outcomes. Size is a risk decision first and a return decision second.

Fixed fractional sizing

Fixed fractional method risks a constant percentage of account equity per trade — typically one to two percent. A one hundred thousand dollar account risking one percent allocates one thousand dollars of risk per trade.

Position size in units equals dollar risk divided by stop distance. Risking one thousand dollars with a fifty dollar stop per unit means twenty units.

As account equity grows, position sizes grow proportionally. As equity shrinks during drawdowns, sizes shrink automatically — a built-in de-risking mechanism.

The trade-off is slower compounding during winning streaks. Conservative fractions sacrifice peak growth in exchange for shallower declines.

Volatility-based sizing

Volatility targeting adjusts position size inversely to current market volatility. Higher volatility implies smaller positions for the same dollar risk budget.

Average True Range based sizing sets stop distances and unit counts from recent range expansion. Wide ranges automatically reduce exposure.

This approach normalizes risk across calm and stressed environments instead of treating every day as identical.

Volatility estimates lag. Sudden regime shifts can leave sizes temporarily too large until the measurement window catches up, which is why hard notional caps should sit alongside any volatility formula.

• Fixed fractional — constant percentage of equity at risk
• Fixed dollar — same dollar risk amount every trade
• Volatility-targeted — size inversely to ATR or realized vol
• Kelly criterion — mathematically optimal but aggressive in full form

Kelly criterion and optimal sizing

Kelly formula estimates optimal bet size from win rate and payoff ratio. Full Kelly maximizes long-run growth theory but produces drawdowns most participants cannot tolerate.

Fractional Kelly — half or quarter of the calculated size — retains much of the growth benefit with materially smaller peak declines.

Kelly requires accurate edge estimates. Overestimated win rate or payoff leads to oversized positions and faster capital erosion.

Use Kelly as an upper bound sanity check, not a default live setting. Real accounts need buffers for estimation error and correlated losses.

Common sizing mistakes

Oversizing after wins concentrates risk at moments when confidence is highest and discipline often slips.

Martingale-style doubling after losses accelerates ruin. Each doubled bet assumes unlimited capital and emotional control that real accounts lack.

Ignoring correlation between concurrent positions inflates effective risk. Five one-percent bets on correlated assets may behave like one five-percent bet.

Static sizing through changing volatility leaves accounts overexposed when ranges expand. Review size rules when realized volatility doubles, and recalculate units from risk dollars after any manual stop adjustment instead of rounding to convenient lot sizes.

Integrating sizing into your process

Write sizing rules before live trading: percent at risk, max concurrent exposure, and volatility adjustment formula.

Max heat limits total open risk across all positions. Breaching heat caps should block new entries until exposure clears.

Reconcile planned size with available liquidity. Theoretical size that cannot fill without moving price is not executable size.

Review sizing monthly against realized drawdown and slippage. Rules that looked conservative on paper may prove aggressive in thin markets.

• Fixed fractional — risk a set percentage of equity per trade
• Volatility targeting — scale size inversely with realised volatility
• Kelly criterion — mathematically optimal fraction under strict assumptions
• Max heat — total open risk across all positions combined