Four numbers determine whether a trading account survives long enough to compound: position size, maximum dollar loss, total portfolio heat, and correlation-adjusted exposure. This calculator handles all four simultaneously, using the core formula Risk $ ÷ (Entry − Stop) = Shares to size each trade, then aggregating open risk to flag when total exposure approaches dangerous levels.
How to Use
| Input | What to Enter | Example |
|---|---|---|
| Account Size | Total account equity, not buying power | $30,000 |
| Risk Per Trade | Percent of equity to lose if stopped out | 1% |
| Entry Price | Planned fill price | $880 (NVDA) |
| Stop Loss Price | Price at which the trade is exited as a loss | $862 (NVDA) |
| Open Position Risks | Combined risk % of all currently open trades | 2.9% |
The calculator outputs share count, dollar risk, raw portfolio heat, and correlation-adjusted heat. If adjusted heat exceeds your chosen threshold (typically 5-6%), reduce the new trade to half size or skip it entirely.
Formula Explained
Position Size = (Account Size × Risk %) / (Entry Price − Stop Loss Price)
Portfolio Heat = (Sum of all open trade risks / Account Size) × 100
R-Multiple = Trade P&L / Initial Risk $
Position size keeps every trade loss equal in dollar terms regardless of share price. A $30,000 account risking 1% loses $300 whether the trade is a $20 stock or an $880 stock — the share count adjusts accordingly.
Portfolio heat is the number most traders never track. It answers the question: “If my stops trigger on every open position simultaneously, what percentage of my account is gone?” FTMO and major prop firms cap daily drawdown at 5% and maximum drawdown at 10%, which in practice means never carrying more than 5-6% total heat.
R-multiples normalize outcomes so a $500 win on a $250 risk trade (2R) is directly comparable to a $1,000 win on a $500 risk trade (also 2R). According to the Van Tharp Institute, traders with documented position sizing rules outperform unstructured traders by 2-3x over 12-month rolling periods. Tracking R-multiples is what makes that documentation actionable.
For options, delta-adjust the position: if you buy a 0.50-delta call on 100 shares of SPY, the effective share exposure is 50 shares, and your risk is the premium paid, not a stop-based calculation. For futures, substitute point value × contract count for share count (e.g., one ES contract = $50 per point, so a 10-point stop = $500 risk per contract).
Example Calculations
Scenario 1: SPY Swing Trade
- Account: $25,000
- Risk: 1% ($250)
- Entry: SPY at $520, Stop: $515 (risk $5/share)
- Position Size: $250 ÷ $5 = 50 shares ($26,000 notional)
- Portfolio Heat: 1.0% (assuming no other open positions)
A $5 stop on SPY is roughly equal to the 14-day ATR in normal market conditions ($3-5), making this a logically placed stop rather than an arbitrary round number.
Scenario 2: NVDA Growth Trade
- Account: $30,000
- Risk: 1% ($300)
- Entry: NVDA at $880, Stop: $862 (risk $18/share)
- Position Size: $300 ÷ $18 = 16 shares ($14,080 notional)
Despite the high share price, the position size formula keeps dollar risk identical to any other 1% trade. Notional value is irrelevant — risk per share is what matters.
Scenario 3: Monday Morning Portfolio Heat Check
A swing trader begins the week with a $30,000 account and three open positions:
- AAPL long from $210, stop $205 → 60 shares × $5 risk = $300 risk (1.0%)
- TSLA long from $175, stop $169 → 50 shares × $6 risk = $300 risk (1.0%)
- SPY call spread, max loss = $270 (0.9%)
- Raw heat: 2.9%
Adding NVDA at 1% ($300 risk, 16 shares) brings raw heat to 3.9% — well under the 5% limit. However, AAPL, TSLA, and NVDA all carry 0.82+ correlation. JournalPlus flags the correlation-adjusted heat at approximately 4.8%, nudging the trader toward a half-size NVDA entry of 8 shares ($144 risk) to keep effective exposure under 4%.
This is the calculation most standalone calculators cannot perform — it requires knowing what positions are already open, which JournalPlus pulls automatically from broker-imported data.
When to Use This Calculator
- Before every new trade entry: Confirm the new position does not push portfolio heat past your limit before placing the order.
- After a gap open: Recalculate heat if overnight news moved positions through logical stop levels, requiring mental stop adjustments.
- When adding to a winner: Scaling into a position increases heat even if the original stop is moved up — recalculate the new risk dollar amount.
- During sector concentration review: When three or more open positions share a sector or high correlation (above 0.80), use correlation-adjusted heat, not raw heat, as the binding constraint.
- For system development: Run historical trades through R-multiple tracking to measure expectancy. A system with 40% win rate and 2.5R average winner has +0.50R expectancy per trade — that is a viable edge worth trading.
Research by Brad Barber and Terrance Odean (UC Davis, 2000) found that 70% of active retail day traders lose money over a six-month horizon, with poor risk sizing cited as a primary factor. The consistent application of a position sizing formula, tracked across every trade, is one of the few mechanical edges available to retail traders.
Related Tools
- Position Size Calculator — Focused single-trade position sizing with equities, options, and futures modes. Use it when you need a quick calculation without the full portfolio heat view.
- Risk of Ruin Calculator — Calculates the probability that a sequence of losses wipes a defined percentage of account equity given your win rate, average R, and risk per trade. Run this quarterly to verify your risk parameters are sustainable.
- R-Multiple Calculator — Converts raw P&L into R-multiples for any trade. Useful for reviewing historical trades and building expectancy statistics.
- Portfolio Heat — In-depth explanation of the portfolio heat metric, how prop firms apply it, and how to monitor it inside JournalPlus.
Frequently Asked Questions
What is a good portfolio heat percentage for active traders?
Professional prop firms such as FTMO cap total open risk at 5-6% of account equity. Most retail traders unknowingly run 10-15% heat by sizing positions based on round lot numbers rather than a fixed risk percentage. Keeping heat under 6% means a single correlated sector selloff cannot wipe a significant portion of the account.
How do I calculate position size for a stock trade?
Divide your maximum dollar risk by the distance from entry to stop loss. If you risk $300 on a trade and your stop is $15 below entry, you buy 20 shares ($300 ÷ $15). This keeps every trade loss bounded to the same dollar amount regardless of stock price.
What is an R-multiple in trading?
An R-multiple expresses a trade’s profit or loss as a multiple of the initial risk. If you risked $250 and made $500, the trade returned 2R. Van Tharp’s research shows that a system with a 40% win rate and a 2.5R average winner has a positive expectancy of +0.50R per trade, meaning it is profitable even though it loses more often than it wins.
How does correlation affect portfolio risk?
Holding multiple positions in highly correlated assets multiplies your effective exposure. SPY, QQQ, and IWM have averaged 0.85-0.92 correlation over rolling 90-day windows, meaning a broad market selloff hits all three simultaneously. During the August 2024 volatility spike, S&P 500, Nasdaq, and Russell 2000 all fell 3-5% in a single session, turning what appeared to be diversified risk into a concentrated bet.
What is the Kelly Criterion and should I use it for position sizing?
The Kelly Criterion formula (f* = (bp − q) / b) calculates the theoretically optimal fraction of capital to risk per trade, where b is the win/loss ratio and p is the win rate. At a 45% win rate with a 2R average winner, full Kelly suggests risking roughly 17.5% per trade — far too aggressive for most traders. Half-Kelly (about 8-9%) is a common practical compromise that reduces drawdown volatility while preserving most of the compounding benefit.
