position-sizer

Position Sizer

Overview

"How many shares should I buy?" only has a defensible answer once you know how much you're willing to lose if the stop is hit. This skill does the arithmetic — fixed-fractional risk sizing, ATR-based stop distance, and Kelly criterion — on numbers the user supplies. It does not fetch prices or recommend a trade.

Not financial advice. This calculates how many shares fit a chosen risk budget; it does not evaluate whether the trade itself is a good idea.

When to use

• คำนวณจำนวนหุ้นที่ควรซื้อ ตามความเสี่ยงที่รับได้ต่อไม้
• Sizing a position given account equity, entry price, and a stop-loss level
• Converting ATR (Average True Range) into a volatility-scaled stop and matching position size
• Estimating a Kelly-criterion fraction from historical win-rate/win-loss stats (as an upper bound, not a recommendation — see Known limitations)

When NOT to use

• No stop-loss level or risk percentage has been specified — ask for these first; sizing "how many shares fit my account" without a risk budget is really just "how much can I afford," a different question
• The user wants a signal on whether to take the trade at all — this skill only sizes a trade already decided on

Core knowledge

Fixed-fractional sizing: risk a fixed percentage of account equity per trade.

dollar_risk_per_share = |entry_price - stop_price|
total_risk_amount = account_equity × risk_pct
shares = floor(total_risk_amount / dollar_risk_per_share)

Shares are always rounded down so actual dollar risk never exceeds the budget. Common risk_pct defaults discussed in trading literature range roughly 0.5%-2% per trade — this skill doesn't assert a "correct" number; ask the user or use their stated tolerance.

ATR-based stop: instead of a fixed percentage stop, scale the stop distance to the security's own recent volatility: stop_price = entry_price - (ATR × multiplier), commonly multiplier 1.5-3×. Then size with the same fixed-fractional formula using this derived stop distance.