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Position Sizing

Difficulty advanced

Overview

Position sizing determines how much capital to allocate to each trade. It's arguably the most important aspect of risk management — it determines whether you survive long enough for your edge to play out.

Fixed Fractional

Position Size = (Account × Risk %) / (Entry - Stop Loss)

Example:
Account: $100,000
Risk per trade: 1% = $1,000
Entry: $50
Stop: $48
Risk per share: $2
Shares = $1,000 / $2 = 500 shares

where: Account total equity · Risk % fixed fraction of equity risked per trade · Entry entry price · Stop Loss predetermined exit price for losses does: derives share count from a fixed-risk budget given a defined stop distance — used as the default sizing rule for discretionary and systematic traders alike because it caps per-trade loss to a known fraction of equity regardless of asset price.

Kelly Criterion

Formula

f* = (bp - q) / b

where:
b = Average Win / Average Loss (odds)
p = Win probability
q = 1 - p (loss probability)

where: f* optimal fraction of equity per bet · b net win-to-loss ratio · p win probability · q = 1 − p does: the discrete-outcome Kelly formula — used to anchor maximum-growth sizing for binary trades; in practice cut by 50–75% because parameter estimates are noisy and full Kelly produces unmanageable drawdowns.

Practical Kelly

Full Kelly is too aggressive for most traders. Use:

Approach Fraction Risk Profile
Full Kelly 100% Aggressive, high drawdown
Half Kelly 50% Balanced, recommended
Quarter Kelly 25% Conservative
Tenth Kelly 10% Very conservative

Volatility-Based Sizing

Position Size = (Account × Risk %) / (ATR × Multiplier × Price)

Normalizes risk across assets with different volatilities

where: Account total equity · Risk % per-trade risk budget · ATR average true range (volatility proxy) · Multiplier stop distance in ATRs · Price current asset price does: sizes positions inversely to volatility so each position contributes the same expected risk — used in CTA, futures, and multi-asset systematic books to equalize position contribution across assets with very different volatilities.

Risk Parity

Weight_i = (1/σ_i) / Σ(1/σ_j)

Equalizes risk contribution across positions

where: Weight_i portfolio weight of asset i · σ_i volatility of asset i · Σ(1/σ_j) sum of inverse volatilities across all assets does: the inverse-vol weighting that gives each asset an equal marginal risk contribution — used as a robust allocation when expected returns can't be reliably estimated and as the building block inside multi-asset risk parity portfolios.

Optimal f (Ralph Vince)

Test different position sizes on historical trade sequence
Find f that maximizes Terminal Wealth Relative (TWR)

Portfolio-Level Sizing

Correlation-Adjusted

When positions are correlated, effective risk is higher

Adjusted Size = Base Size / √(1 + (n-1) × ρ)

where ρ = average correlation between positions

where: Base Size standalone position size · n number of correlated positions · ρ average pairwise correlation does: shrinks individual position sizes to keep portfolio variance constant as correlated positions accumulate — used so a basket of correlated trades doesn't quietly turn into a single oversized macro bet.

Maximum Portfolio Risk

Total Portfolio Risk ≤ 6% of capital
Individual Trade Risk ≤ 1-2% of capital
Maximum Correlated Risk ≤ 3% of capital

Position Sizing Decision Matrix

Account Size Risk/Trade Max Positions Notes
< $25,000 1% 3-5 Conservative
$25,000-100,000 1-2% 5-10 Standard
$100,000-1M 0.5-1% 10-20 Diversified
> $1M 0.25-0.5% 20+ Institutional

Scaling In/Out

Scaling In

Initial entry: 50% of target size
Add on confirmation: 25%
Add on further confirmation: 25%

Or: Equal thirds at predefined levels

Scaling Out

Take partial profit at Target 1: 33%
Take partial profit at Target 2: 33%
Let remainder run with trailing stop: 34%

Practical Guidelines

  1. Never Risk More Than 2% — Survival is paramount
  2. Size Before Entry — Know your position size before you trade
  3. Adjust for Conviction — Higher conviction = larger size (within limits)
  4. Correlation Matters — Correlated positions amplify risk
  5. Reduce After Losses — Drawdown recovery requires smaller risk
  6. Increase After Wins — Compound profits, but gradually
  7. Document Everything — Track position sizes and outcomes

Drawdown and Position Sizing

After 10% drawdown → reduce risk by 25%
After 20% drawdown → reduce risk by 50%
After 30% drawdown → reduce risk by 75%

Recovery table:
10% loss needs 11% gain
20% loss needs 25% gain
30% loss needs 43% gain
50% loss needs 100% gain

q&a

Why is position sizing more important than entry timing?

Entry timing affects expected return per trade. Position sizing affects whether you survive long enough for expected return to play out. A great edge with terrible sizing still blows up. A mediocre edge with disciplined sizing compounds. The variance of your account growth is dominated by how much you risk per position, not when you click buy.

Should I risk a fixed dollar amount or a fixed percentage?

Fixed percentage. Risking $1,000 per trade is fine at $100K, ruinous at $10K. Percentage of equity scales naturally with account size, automatically reducing risk after losses and increasing it after wins — the mechanics of geometric growth.

How do I size around correlated positions?

Treat correlated positions as if they were one larger position. If you're long 5 tech names with average correlation 0.8, effective risk is much closer to 1 position × √(1 + 4×0.8) ≈ 2× the single-position risk, not 5× independent risk. See Correlation Analysis for the math.

What's wrong with full Kelly?

Kelly assumes you know your win rate and odds with certainty. In practice you estimate both with noise — and the second derivative of geometric growth around full Kelly is steep and asymmetric. Slight overestimation of edge plus full Kelly sizing produces catastrophic drawdowns. Half-Kelly cuts variance ~75% for ~25% expected-return reduction.

When should I reduce size?

After material drawdowns (rules-based, e.g. 10% / 20% / 30% triggers), when your strategy is statistically out-of-regime (rolling Sharpe below threshold), or when you're emotionally compromised by recent losses. Never increase size in revenge for losses.

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