AlphaTRADER Academy
Position Sizing Methods
Same setup, same stop, same target. Five different sizing methods give you five different lot sizes — and five different long-run outcomes. Pick the wrong one and your edge can't compound.
"Position sizing is not a risk problem. It is the risk problem." — Van Tharp
Sizing Calculator — Five Methods Side-by-Side
Enter your trade specs. Each method produces its own lot size.
Reading the calculator: the larger the lot, the larger your absolute P&L per trade — but also the larger your drawdowns. Fixed-Fractional is the safest practical default. Kelly is for traders who've measured their edge precisely and can stomach the drawdowns.
When to Use Each Method
1. Fixed Lot — Beginner default
Same lot every trade. Simple, predictable. Risk fluctuates wildly with stop distance — a 10-pip stop and a 100-pip stop on the same lot risk wildly different amounts.
Use when: learning, paper trading, or running a single-instrument strategy with consistent stop distances.
2. Fixed % of Balance
Lot size = (Balance × X%) / pip value. Scales with account size — good. Still ignores stop distance, so risk per trade still varies.
Use when: account-size-aware sizing matters but stop distances are uniform (e.g., single-strategy bots).
3. Fixed-Fractional (Risk-Based) — Industry standard ⭐
Lot = (Account × Risk%) / (Stop Pips × Pip Value). Dollar-risk is identical on every trade regardless of stop distance. The "1% risk per trade" method everyone references.
Use when: trading multiple setups with varying stop distances. This should be your default unless you have a specific reason to deviate.
4. ATR-Based
Stop distance computed from ATR (e.g., 1.5 × ATR). Lot size auto-adjusts to volatility. In low-vol regimes, larger position. In chaotic regimes, smaller. Self-defensive in market shifts.
Use when: trading across instruments with very different volatility profiles, or operating in regimes where vol shifts dramatically (news, earnings, macro events).
5. Kelly Criterion — Advanced
Optimal fraction: f* = (WinRate × R) − LossRate, where R = AvgWin / AvgLoss. Mathematically maximizes long-run geometric growth. Full Kelly drawdowns are brutal — 30-50% is normal.
Practical use: Half Kelly (0.5×) or Quarter Kelly (0.25×) captures 75-95% of the growth advantage with much softer variance.
Use when: edge is well-measured (1000+ historical trades), drawdowns are emotionally tolerable, prop firm rules don't apply. Never with full Kelly on prop firm accounts (instant max-DD breach).
Prop Firm Trader? Skip Kelly.
Most prop firms (GOAT, FTMO, Fintokei, etc.) have max-drawdown rules in the 5-10% range. Full Kelly produces drawdowns 3-5× that as a normal feature of the math — guaranteed account closure.
For funded accounts: use Fixed-Fractional 0.5% per trade as default. Daily loss cap: 2-3%. Maximum total open risk across positions: 2%. These numbers keep you well inside firm constraints with room for drawdown variance.
See the live GOAT $100K Trader Journey for real-world application — including the Soft Breach incident logged in Trader's Log.
Full breakdown of prop vs personal account constraints (max DD, daily caps, Kelly fraction, mindset): Prop Firm vs Personal Account lesson
Key Takeaways
- Fixed-Fractional (1% risk per trade) is the practical default for 90% of traders. Risk stays constant regardless of stop distance.
- ATR-based sizing shines when trading instruments with very different volatility profiles or in regime-shift environments.
- Full Kelly is for theorists and YouTube videos. Real traders use 0.25-0.5× Kelly — much milder drawdowns, almost all the growth.
- Prop firm accounts demand Fixed-Fractional 0.5% or smaller. Kelly (even half-Kelly) breaches max-DD rules.
- Pick one method, stick to it for a quarter, measure results. Switching sizing methods mid-stream destroys your ability to attribute results.
Test Your Understanding
4 questions — instant feedback, no scoring stored.