position-sizer

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---
name: position-sizer
description: Calculate risk-based position sizes for long stock trades. Use when user asks about position sizing, how many shares to buy, risk per trade, Kelly criterion, ATR-based sizing, fractional-share sizing, or portfolio risk allocation. Supports stop-loss distance calculation, volatility scaling, and sector concentration checks.
---

# Position Sizer

## Overview

Calculate the optimal number of shares to buy for a long stock trade based on risk management principles. Supports three sizing methods:

- **Fixed Fractional**: Risk a fixed percentage of account equity per trade (default: 1%)
- **ATR-Based**: Use Average True Range to set volatility-adjusted stop distances
- **Kelly Criterion**: Calculate mathematically optimal risk allocation from historical win/loss statistics

All methods apply portfolio constraints (max position %, max sector %) and output a final recommended share count with full risk breakdown. The default output is whole shares. Use `--fractional` only when the user's broker supports fractional shares for the security and order type.

## When to Use

- User asks "how many shares should I buy?"
- User wants to calculate position size for a specific trade setup
- User mentions risk per trade, stop-loss sizing, or portfolio allocation
- User asks about Kelly Criterion or ATR-based position sizing
- User has a small account where whole-share rounding would under-deploy a defined risk budget
- User wants to check if a position fits within portfolio concentration limits

## Prerequisites

- No API keys required
- Python 3.9+ with standard library only

## Workflow

### Step 1: Gather Trade Parameters

Collect from the user:
- **Required**: Account size (total equity)
- **Mode A (Fixed Fractional)**: Entry price, stop price, risk percentage (default 1%)
- **Mode B (ATR-Based)**: Entry price, ATR value, ATR multiplier (default 2.0x), risk percentage
- **Mode C (Kelly Criterion)**: Win rate, average win, average loss; optionally entry and stop for share calculation
- **Optional constraints**: Max position % of account, max sector %, current sector exposure
- **Optional share mode**: Whole shares by default, or fractional shares with `--fractional --share-precision N` when supported by the broker

If the user provides a stock ticker but not specific prices, use available tools to look up the current price and suggest entry/stop levels based on technical analysis.

### Step 2: Execute Position Sizer Script

Run the position sizing calculation:

```bash
# Fixed Fractional (most common)
python3 skills/position-sizer/scripts/position_sizer.py \
  --account-size 100000 \
  --entry 155 \
  --stop 148.50 \
  --risk-pct 1.0 \
  --output-dir reports/

# Fractional shares for small accounts or high-priced stocks
python3 skills/position-sizer/scripts/position_sizer.py \
  --account-size 1000 \
  --entry 155 \
  --stop 148.50 \
  --risk-pct 1.0 \
  --fractional \
  --share-precision 4 \
  --output-dir reports/

# ATR-Based
python3 skills/position-sizer/scripts/position_sizer.py \
  --account-size 100000 \
  --entry 155 \
  --atr 3.20 \
  --atr-multiplier 2.0 \
  --risk-pct 1.0 \
  --output-dir reports/

# Kelly Criterion (budget mode - no entry)
python3 skills/position-sizer/scripts/position_sizer.py \
  --account-size 100000 \
  --win-rate 0.55 \
  --avg-win 2.5 \
  --avg-loss 1.0 \
  --output-dir reports/

# Kelly Criterion (shares mode - with entry/stop)
python3 skills/position-sizer/scripts/position_sizer.py \
  --account-size 100000 \
  --entry 155 \
  --stop 148.50 \
  --win-rate 0.55 \
  --avg-win 2.5 \
  --avg-loss 1.0 \
  --output-dir reports/
```

### Step 3: Load Methodology Reference

Read `references/sizing_methodologies.md` to provide context on the chosen method, risk guidelines, and portfolio constraint best practices.

### Step 4: Calculate Multiple Scenarios

If the user has not specified a single method, run multiple scenarios for comparison:
- Fixed Fractional at 0.5%, 1.0%, and