Of every skill in trading, position sizing is the one that decides whether you survive long enough for the others to matter. You can have the best entry, the cleanest setup, and the steeliest psychology — and still blow up an account in a week if your size is wrong. This guide walks through the exact formula professional traders use, with worked examples for forex, equities, and crypto, plus five sizing models compared side-by-side.
What Is Position Sizing?
Position sizing answers one question: how many units of this instrument do I buy or sell so that, if my stop is hit, I lose only what I'm willing to lose? It is the bridge between your risk policy ("I risk 1% per trade") and the broker order ticket ("1.43 lots", "228 shares", "0.085 BTC").
Without a sizing rule, every trade becomes an emotional negotiation: more conviction means bigger size, fear means smaller size, and the account equity curve becomes a mirror of the trader's mood. With a sizing rule, conviction is irrelevant — math does the work.
The Universal Position Sizing Formula
Every position sizing calculation reduces to the same fixed-risk equation:
Position size = (Account equity × Risk %) ÷ (Entry price − Stop-loss price)
Three inputs, one output. Internalize this and every market becomes the same problem.
- Account equity — current trading capital, not deposit. Recalculate at the start of each session.
- Risk % — your maximum loss per trade. Most full-time traders use 0.5% to 1.0%. Above 2% is reckless on individual trades.
- Stop distance — the absolute price gap between entry and stop. Always quoted in the instrument's own units.
Worked Example #1 — Equities
Account: $50,000. Risk: 1% = $500. Entry: $185.40. Stop: $182.20. Stop distance: $3.20.
Position size = $500 ÷ $3.20 = 156 shares. If the stop hits, you lose 156 × $3.20 = $499.20. Total exposure: 156 × $185.40 = $28,922. Margin required varies by broker.
Worked Example #2 — Forex (EUR/USD)
Account: $25,000. Risk: 0.75% = $187.50. Entry: 1.0842. Stop: 1.0817. Stop distance: 25 pips.
For a standard lot (100,000 units), 1 pip = $10. Risk per pip = $187.50 ÷ 25 = $7.50. Position size = $7.50 ÷ $10 = 0.75 standard lots (75,000 units).
Worked Example #3 — Crypto (BTC/USDT)
Account: $10,000. Risk: 1.5% = $150. Entry: $67,400. Stop: $66,200. Stop distance: $1,200.
Position size = $150 ÷ $1,200 = 0.125 BTC. Notional exposure: 0.125 × $67,400 = $8,425 — well under account equity, so no leverage required.
Five Position Sizing Models Compared
1. Fixed Percentage Risk (most common)
Risk a fixed percentage of current equity per trade. Scales naturally with the account. Used by nearly every prop firm.
Strength: Compounding wins automatically increase size; losses automatically decrease it. Weakness: Requires accurate equity tracking.
2. Fixed Dollar Risk
Always risk the same dollar amount per trade (e.g., $250). Simple, predictable.
Strength: Mental math is trivial. Weakness: Doesn't scale — small after a big drawdown, oversized after winning streaks.
3. Volatility-Based (ATR sizing)
Stop distance = N × Average True Range. Position size adjusts so per-trade risk stays constant even as volatility changes.
Strength: Adapts to changing market regimes. Weakness: Requires ATR data and discipline to recalculate.
4. Kelly Criterion
Optimal sizing based on your historical win rate and average win/loss ratio: K = W − (1 − W) ÷ R. Almost no discretionary trader uses full Kelly — drawdowns are brutal.
Strength: Mathematically maximizes long-term growth. Weakness: Assumes your edge is stable; most traders use ¼ or ½ Kelly to soften drawdowns.
5. Fixed Fractional (Ralph Vince)
Variation of fixed-percentage that ties size to the largest expected loss rather than stop distance.
Strength: Protects against tail risk. Weakness: Often produces overly conservative size for shorter-term traders.
Common Position Sizing Mistakes
- Sizing to the entry, not the stop. "I'll buy $5,000 worth" ignores risk entirely. Always size to the stop distance.
- Moving the stop to fit the size. If your math says 50 shares, don't tighten the stop to allow 200. Wider stop, smaller size — every time.
- Forgetting fees and slippage. Build a 10–15% buffer into your risk number to absorb execution costs.
- Not recalculating after drawdowns. A 20% drawdown means your dollar risk per trade should fall 20% too, automatically.
- Pyramiding without rules. Adding to a winner is fine — but only if total open risk stays inside your daily cap.
How Alpha Charts Automates Position Sizing
Alpha Charts ships with a built-in position-size calculator that lives inside the trade-entry flow:
- Pulls your account equity automatically from past journal entries.
- Accepts risk % per trade and converts to position size in shares, lots, or coins.
- Toggles between USD and BTC sizing for crypto traders.
- Saves the exact risk per trade alongside the trade record, so the journal can later answer questions like "what was my average risk per trade in October?" or "did I over-size on losing days?"
Sizing decisions become data, and data becomes the input for the AI coach — which is how patterns like "you over-size in the first 20 minutes of London session" get surfaced.
Step-by-Step Action Plan
- Pick a fixed risk percentage between 0.5% and 1.0% and commit to it for 30 days. Write it on your monitor.
- Before every trade, calculate: position size = (equity × risk %) ÷ stop distance. Use the Alpha Charts calculator if it's available; otherwise pen and paper.
- Log the actual risk (in $ and %) on every trade. Most journals skip this — Alpha Charts captures it automatically.
- At the end of each week, review your sizing log. Flag any trade that violated your rule. Note the emotion that drove the override.
- After 30 days, compute the standard deviation of your per-trade risk. A consistent trader's number is small. A reactive trader's number is large. Aim to shrink yours over time.
Conclusion
Position sizing is the most boring part of trading and the most important. The traders who survive aren't the ones with the sharpest entries — they're the ones whose risk-per-trade barely changes from setup to setup, regardless of mood, market, or news cycle.
CTA: Open Alpha Charts, set your default risk percentage in settings, and let the calculator make the sizing decision before emotion does.