Position Sizing: How to Calculate the Right Trade Size Every Time

Why Position Sizing Matters More Than Entry Timing

You can have the best stock picks in the world, but if you size your positions wrong, you'll blow up. Position sizing is the single biggest factor separating profitable traders from broke ones.

A 50% drawdown requires a 100% gain to recover. A 20% drawdown only needs 25%. Proper position sizing keeps drawdowns manageable so your edge has time to play out.

Method 1: Fixed Percentage Risk

The simplest and most popular method. Risk a fixed percentage of your total account on each trade.

How It Works

1. Decide your risk per trade (1-2% of account for most traders)
2. Determine your stop-loss distance (in dollars or percentage)
3. Calculate: Position Size = (Account Size x Risk %) / Stop-Loss Distance

Example

• Account: $25,000
• Risk per trade: 1% = $250
• Stock price: $150
• Stop-loss: $142 (5.3% below entry)
• Stop distance: $8 per share
• Position size: $250 / $8 = 31 shares ($4,650 position)

If the stock hits your stop, you lose exactly $250 — 1% of your account. Whether the stock is $10 or $500, whether the stop is 2% or 10% away, the dollar risk stays constant.

Method 2: ATR-Based Sizing

Volatility-adjusted sizing uses Average True Range to account for how much a stock actually moves.

How It Works

1. Calculate the stock's 14-day ATR
2. Set stop-loss at 1.5-2x ATR below entry
3. Use the same fixed-percentage formula with the ATR-based stop

Example

• Account: $25,000, Risk: 1% = $250
• Stock A: $100, ATR = $2 (2% daily moves). Stop at 2x ATR = $4 below. Shares = $250 / $4 = 62 shares
• Stock B: $100, ATR = $6 (6% daily moves). Stop at 2x ATR = $12 below. Shares = $250 / $12 = 20 shares

Stock B is 3x more volatile, so you get 1/3 the position. Dollar risk is identical. This prevents volatile stocks from causing outsized losses.

Method 3: Kelly Criterion

The Kelly formula calculates mathematically optimal bet sizing based on your edge.

The Formula

Kelly % = (Win Rate x Average Win/Average Loss - Loss Rate) / (Average Win/Average Loss)

Example

• Win rate: 55%, Loss rate: 45%
• Average win: $400, Average loss: $200
• Win/Loss ratio: 2.0
• Kelly % = (0.55 x 2.0 - 0.45) / 2.0 = 0.325 = 32.5%

Full Kelly says risk 32.5% per trade. That's aggressive. In practice, use fractional Kelly:

• Half Kelly (16.25%): Good balance of growth and safety
• Quarter Kelly (8.1%): Conservative, lower variance
• Tradewink default: Uses the most conservative of fixed-percentage, ATR-based, and half-Kelly

Portfolio Heat: Total Risk Across All Positions

Individual position sizing isn't enough. You also need to manage total portfolio risk (portfolio heat).

• Portfolio heat = sum of risk across all open positions
• If you risk 1% per trade with 5 open positions, your heat is 5%
• Conservative: Max 3-5% portfolio heat
• Moderate: Max 6-8% portfolio heat
• Aggressive: Max 8-10% portfolio heat

If portfolio heat hits your limit, stop opening new positions until existing ones are closed or stops are trailed to breakeven.

Micro Account Sizing (Under $1,000)

Small accounts need special rules:

• Use fractional shares to size precisely (most brokers support this)
• Risk 2-3% per trade (slightly higher than normal to make commissions worthwhile)
• Maximum 25% of account in any single position
• Minimum order value: $1 (Tradewink enforces this automatically)
• Focus on fewer, higher-conviction trades rather than diversifying too thin

Common Sizing Mistakes

1. Sizing based on conviction: "I'm really confident, so I'll double the size." Your confidence is not calibrated well enough for this. Use the same sizing rules every time.
2. Averaging down: Adding to losers increases risk when the thesis is already failing. Only add to winners.
3. Ignoring correlation: Five tech stocks aren't five independent bets. Account for sector concentration.
4. Not adjusting for regime: Reduce position sizes by 25-50% in high-volatility market regimes.