Position Sizer

A precise command-line tool for calculating proper position sizes based on account risk percentage, entry price, stop loss, and current portfolio value.

Features

• Position Sizing - Calculate exact position size based on risk tolerance
• Long & Short - Support for both directions
• Risk/Reward - Calculate R:R ratios and break-even win rates
• Kelly Criterion - Optimal bet sizing based on historical performance
• Drawdown Simulator - See impact of consecutive losses
• Commission Aware - Factor in trading fees
• High Precision - Uses Decimal math for accuracy (important for crypto)
• Zero Dependencies - No external API calls, works completely offline

Installation

pip install position-sizer

Quick Start

# Basic position sizing - risk 1% of $10,000 account
position-sizer calc --account 10000 --entry 100 --stop 95 --risk-pct 1.0

# Short position with fixed dollar risk
position-sizer calc --account 10000 --entry 100 --stop 110 --risk-usd 500 --side short

# Include commission in calculation
position-sizer calc --account 10000 --entry 100 --stop 95 --risk-pct 1.0 --commission 0.04

# Calculate with R:R target
position-sizer calc --account 10000 --entry 100 --stop 95 --risk-pct 1.0 --target 115

Commands

calc - Position Size Calculator

The main command for calculating position sizes.

position-sizer calc \
  --account 48250.75 \
  --entry 4231.40 \
  --stop 4187.90 \
  --risk-pct 1.0

Output:

╭─────────────────────────────╮
│ Position Size Calculator    │
╰─────────────────────────────╯
Account size       $48,250.75
Risk %             1.00%
Risk amount        $482.51
Side               LONG
Entry price        4231.4
Stop price         4187.9
Distance to stop   43.5 (1.03%)

Position Size: 11.09195402 units
Position Value: $46,928.07

Options:

• --account, -a - Account size in USD (required)
• --entry, -e - Entry price (required)
• --stop, -s - Stop loss price (required)
• --risk-pct, -r - Risk as percentage (e.g., 1.0 for 1%)
• --risk-usd, -u - Risk as fixed USD amount
• --side - Trade direction: long or short (default: long)
• --commission, -c - Commission per unit traded
• --target, -t - Take profit target (calculates R:R)

rr - Risk/Reward Calculator

Calculate risk/reward ratio and required win rate.

position-sizer rr --entry 100 --stop 95 --target 115

Output:

╭─────────────────────────────╮
│ Risk/Reward Calculator      │
╰─────────────────────────────╯
Side               LONG
Entry              100
Stop               95
Target             115
Risk               5
Reward             15

Risk/Reward Ratio: 1:3.00
Break-even win rate needed: 25.0%

kelly - Kelly Criterion

Calculate optimal position sizing based on your historical performance.

position-sizer kelly \
  --account 10000 \
  --win-rate 0.55 \
  --avg-win 150 \
  --avg-loss 100

Output:

╭─────────────────────────────╮
│ Kelly Criterion Calculator  │
╰─────────────────────────────╯
Win rate           55.0%
Avg winner         $150.00
Avg loser          $100.00
Win/Loss ratio     1.50

Full Kelly: 18.3% of account
Half Kelly (safer): 9.2%
Quarter Kelly (conservative): 4.6%

For $10,000.00 account:
  Full Kelly risk: $1,833.33
  Half Kelly risk: $916.67

drawdown - Drawdown Simulator

See what happens after a streak of losses.

position-sizer drawdown --account 10000 --risk-pct 2.0 --trades 10

Output:

╭─────────────────────────────╮
│ Drawdown Simulator          │
╰─────────────────────────────╯
┏━━━━━━━━━┳━━━━━━━━━━━━┳━━━━━━━━━━━━┳━━━━━━━━━━┓
┃ Trade # ┃ Loss       ┃ Balance    ┃ Total DD ┃
┡━━━━━━━━━╇━━━━━━━━━━━━╇━━━━━━━━━━━━╇━━━━━━━━━━┩
│ 1       │ -$200.00   │ $9,800.00  │ -2.0%    │
│ 2       │ -$196.00   │ $9,604.00  │ -4.0%    │
│ 3       │ -$192.08   │ $9,411.92  │ -5.9%    │
│ ...     │ ...        │ ...        │ ...      │
│ 10      │ -$166.77   │ $8,171.02  │ -18.3%   │
└─────────┴────────────┴────────────┴──────────┘

After 10 consecutive losses at 2.0% risk:
  Starting: $10,000.00
  Ending: $8,171.02
  Total Drawdown: -18.3%
  Gain needed to recover: +22.4%

Use as Library

from position_sizer import PositionCalculator

calc = PositionCalculator()

# Calculate position size
result = calc.calculate(
    account_size=10000,
    entry_price=100,
    stop_price=95,
    risk_percent=1.0,
    side="long",
)

print(f"Position size: {result.position_size} units")
print(f"Position value: ${result.position_value}")
print(f"Risk amount: ${result.risk_amount}")

# Calculate risk/reward
rr = calc.calculate_risk_reward(
    entry_price=100,
    stop_price=95,
    target_price=115,
    side="long",
)
print(f"Risk/Reward: 1:{rr}")

# Kelly Criterion
kelly = calc.kelly_criterion(
    win_rate=0.55,
    avg_win=150,
    avg_loss=100,
)
print(f"Kelly optimal: {kelly * 100:.1f}%")

The Math

Position Size Formula

Position Size = Risk Amount / Risk Per Unit

Where:
- Risk Amount = Account Size × Risk Percentage
- Risk Per Unit = |Entry Price - Stop Price| + (2 × Commission)

Kelly Criterion Formula

f* = (p × b - q) / b

Where:
- f* = fraction of account to risk
- p = probability of winning
- q = probability of losing (1 - p)
- b = win/loss ratio (avg_win / avg_loss)

Why Use Half-Kelly?

Full Kelly maximizes long-term growth but causes large drawdowns. Most traders use:

• Half Kelly - Good balance of growth and drawdown
• Quarter Kelly - Conservative, smoother equity curve

Tips

1. Never risk more than you can afford to lose - The calculator doesn't know your situation
2. Account for slippage - Your actual stop might execute worse than planned
3. Use Half-Kelly or less - Full Kelly is mathematically optimal but emotionally brutal
4. Round down - The calculator rounds position sizes down, never up
5. Test with paper trading - Verify the math matches your broker's calculations

License

MIT License