Stock Position Sizing Calculator
Compute max position size from account size, max risk %, entry price, and stop-loss. Risk-per-trade math for active traders.
Stock Position Sizing Calculator
Compute the maximum number of shares you can buy while limiting your loss to a fixed percentage of account size if the stop-loss hits. The standard risk-per-trade formula every disciplined active trader uses.
Risk-per-trade table (same entry + stop, different risk %)
| Risk % | $ at risk | Shares | Position $ |
|---|
How to Use the Position Sizing Calculator
Enter your account size
Your total trading capital. Use the equity number from your broker's daily statement.
Set max risk per trade
Typical professional active traders risk 0.5-1% per trade. Beginners often start at 0.25-0.5%. Aggressive risks (3%+) require a high win-rate edge to compound positively. The 2% Rule (Van Tharp) is the most-cited canonical maximum for retail traders.
Enter entry and stop-loss prices
Entry is where you plan to buy (or short). Stop-loss is the price at which you exit for a loss — set this based on technical analysis (below support, above resistance) or volatility-based (1-2× ATR). The distance between entry and stop is your "risk per share".
Optional: enter target price for reward:risk
Target is where you plan to take profit. Reward:Risk ratio compares the profit potential to the risk. Disciplined trading requires R:R of at least 1.5:1, ideally 2:1 or higher. Below 1:1 means you need very high win-rate to be profitable.
Position Sizing — The Single Most Important Skill in Active Trading
Why Position Sizing Beats Stock Picking
The vast majority of retail trading losses come not from picking bad stocks but from over-sizing positions on bad stocks. A trader with a 60% win rate on stock selection can still go broke if they risk 25% of account per trade — a 3-trade losing streak (16% probability) would wipe out roughly half the account. The same trader risking 1% per trade would lose 3% on the same losing streak — survivable, recoverable. Van Tharp's Trade Your Way to Financial Freedom built an entire framework around this insight: position sizing matters more than entry signal quality for long-term capital survival.
The risk-per-trade formula is universal: shares = (account × risk%) / (entry − stop). At a USD 100,000 account, 1% risk = USD 1,000 max loss; if entry is USD 50 and stop is USD 47, risk per share is USD 3, so max shares = 1,000 ÷ 3 = 333 shares. Position value is 333 × 50 = USD 16,650, or 16.65% of account. The position-percentage of account is incidental to the formula — the discipline is around dollar-risk-per-trade, not percentage-of-account-deployed.
The 1% Rule, 2% Rule, and Kelly Criterion
The 1% Rule is the canonical retail-trader guideline: risk no more than 1% of account on any single trade. The math: even on a 5-trade losing streak (which happens roughly 3% of the time on a 50%-win-rate strategy), you lose 5% of account, recoverable in a few good trades. The 2% Rule (Van Tharp's original framework) is the more aggressive cap — used by experienced traders with proven edge and operational discipline. Risk above 2% per trade requires either very high win-rate (60%+) or very high reward:risk (3:1+) to compound positively.
The Kelly Criterion derives the mathematically-optimal position size given win rate and reward:risk ratio. Formula: f* = (p × b − q) ÷ b, where p = win probability, b = reward:risk, q = 1 − p. A 55% win rate with 2:1 R:R suggests Kelly fraction of 32.5% — far above what any real trader uses, because Kelly's assumption of perfect win-probability knowledge is unrealistic. Most professionals use "fractional Kelly" of 0.25-0.5× — meaning the Kelly-suggested 32.5% becomes 8-16% in practice. The 1% rule corresponds to a Kelly fraction implied by a very conservative win-rate estimate, which is the right error direction for retail traders.
"A USD 100K account at 1% risk per trade, with an entry at USD 50 and stop at USD 47, sizes to 333 shares — risking USD 1,000 to potentially gain USD 2,000 at a 2:1 reward:risk target. Survivable losing streaks. Compoundable wins."
Stop-Loss Placement — The Other Half of the Equation
Position sizing is only as good as the stop-loss it's based on. Three common approaches: technical stops (below recent support, above recent resistance, below moving averages) — appropriate for swing trading and trend-following; volatility-based stops (1-2× ATR or Average True Range) — appropriate when entry is near consolidation breakouts where price-action structure is unclear; fixed-percentage stops (5%, 7%, 10% below entry) — simplest, but doesn't adapt to per-stock volatility. The position-sizing math works for any stop strategy as long as the stop is mechanical and respected.
The trap most retail traders fall into: moving the stop-loss after entering. Once a position is in red, the temptation is to widen the stop to "give it more room" — which converts a controlled-risk trade into an uncontrolled-risk position. The discipline of position sizing depends on respecting the stop. Use a hard market-order stop in your broker; don't manage it manually. Modern brokers (Interactive Brokers, Tiger, Moomoo, Fidelity Active Trader Pro) all support OCO (One-Cancels-Other) orders that automatically place both stop and target at entry — set them once, walk away.
10 Facts About Position Sizing
The risk-per-trade formula: shares = (account × risk%) ÷ (entry − stop). Universal across all active trading styles.
1% Rule: risk no more than 1% of account on any single trade. Canonical retail-trader guideline.
2% Rule (Van Tharp): the maximum risk per trade for experienced traders. Higher requires extraordinary edge or discipline.
Kelly Criterion: f* = (p × b − q) ÷ b — mathematically optimal but always reduced to fractional Kelly (0.25-0.5×) in practice.
Most professional traders use 0.25-1% risk per trade — much lower than retail traders typically assume.
A 5-trade losing streak at 1% risk = 5% drawdown — easily recoverable.
The same 5-trade losing streak at 5% risk = 23% drawdown — requires 30% gain to recover.
Reward:Risk of 2:1 is the canonical minimum for active trading. Below 1.5:1 needs win-rate above 60% to compound.
ATR (Average True Range) is a common volatility-based stop input — typically 1-2× daily ATR for swing trades.
FINRA Pattern Day Trader rule requires USD 25K minimum equity in margin accounts for traders making 4+ day trades in 5 business days.
Frequently Asked Questions
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Shares = (Account × Risk%) ÷ (Entry − Stop). The dollar amount you're willing to lose if the stop-loss hits, divided by the per-share risk. At USD 100K account, 1% risk = USD 1,000 max loss; if entry USD 50 and stop USD 47, risk per share is USD 3, so max shares = 1,000 ÷ 3 = 333. This is the universal active-trader formula — works for stocks, ETFs, options (delta-adjusted), forex, futures, and crypto.
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Most professional active traders use 0.25-1%. The "1% Rule" is the canonical retail-trader guideline. Risking 2% per trade (Van Tharp's stated maximum for experienced traders) is the aggressive end. Above 2% requires very high win-rate (60%+) and discipline. Below 0.5% is appropriate for beginners building confidence. Survivability matters more than per-trade gain — at 1% risk, a 10-trade losing streak (rare) costs 10% of account. At 5% risk, the same streak costs 40% — typically unrecoverable.
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Most retail traders should start at 0.5-1% and only increase after demonstrating consistent profitability over 50+ trades. The 2% Rule (Van Tharp) assumes proven edge and emotional discipline. The compounding math: at 1% risk and 60% win rate with 2:1 R:R, you compound ~10% per month — already aggressive. At 2% risk, you compound ~20% per month — extraordinary. Both require the assumed win rate and R:R to be real, not aspirational. Most retail traders overestimate their edge by 50%+.
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Three canonical approaches: (1) Technical — below recent support, below moving averages, below trend lines. Appropriate for swing trading. (2) Volatility-based — 1-2× ATR (Average True Range) below entry. Adapts to per-stock volatility. (3) Fixed-percentage — 5%, 7%, 10% below entry. Simplest. The wrong approach is "wherever feels comfortable" — that produces inconsistent risk and corrupts the position-sizing math. Use a single method consistently.
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Kelly's formula derives the mathematically optimal position size for compounding growth given a known win probability and reward:risk ratio: f* = (p × b − q) ÷ b. For 55% win rate at 2:1 R:R, Kelly suggests 32.5% of account — way too aggressive in practice because real win probabilities are uncertain. Most professionals use "fractional Kelly" of 0.25-0.5× the formula, which produces position sizes closer to the 1-2% rule. Kelly is most useful as a sanity check on aggressive sizing, not a sizing rule itself.
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2:1 is the canonical minimum for active trading. With 2:1 R:R, you only need 33%+ win rate to break even mathematically. At 3:1 R:R, 25% win rate breaks even. Below 1:1 R:R requires 60%+ win rate to be profitable — achievable in some scalping or arbitrage strategies but rare in swing trading. The temptation is to chase 5:1+ "home run" setups — those are real but rare; consistent 2-3:1 R:R compounds steadily over time.
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Yes, but with delta-adjustment. For buying options outright: risk per contract = premium paid (since options can go to zero); position sizing should treat max risk as the premium total. For selling defined-risk spreads (credit/debit spreads, iron condors): max loss is bounded, use that as risk-per-position. For naked options or futures: position sizing should treat the contract's notional exposure × stop-percentage as the risk. The 1-2% account-risk principle applies the same way — adapt the formula's "risk per share" denominator to the option's defined max loss.
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FINRA Rule 4210 designates traders making 4+ day trades (buy and sell same security same day) in 5 business days as "Pattern Day Traders" (PDT). PDT accounts must maintain USD 25,000 minimum equity in margin accounts. Falling below USD 25K triggers a trading restriction until the equity is restored. This is the single most-cited US regulation affecting active retail traders. Workarounds: use cash account (no PDT restriction but T+2 settlement limits trade frequency), trade futures (regulated differently), or trade in markets outside US jurisdiction.
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The math is universal but trading regulations and tax differ. Singapore SGX has no PDT rule — frequent trading allowed regardless of account size. Singapore stamp duty (0.2% on buys) and broker commissions (USD 5-15 per side) erode small positions; minimum profitable position size is typically SGD 10K+. Malaysia and Thailand have similar small-trade frictions. Hong Kong stamp duty is 0.1% per side. US markets are unique in having near-zero per-trade frictions thanks to commission-free brokers (Robinhood, Fidelity, Schwab, IBKR Lite) — which makes small position sizing viable in a way it isn't in most other markets. The risk-per-trade math is identical; the minimum economic position size differs by market.
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Three considerations. (1) PDT rule: applies if your account is USD-denominated even on IBKR Singapore — once you make 4 day trades in 5 days under USD 25K equity, you're restricted. Tiger Brokers and Moomoo Singapore use the same FINRA rule. (2) Withholding tax: dividends from US-listed stocks face 30% US withholding for non-US tax residents (vs 15% with treaty for Singapore residents who file W-8BEN). For day trading, capital gains aren't subject to US withholding for non-US residents — focus on dividend-payment dates. (3) Currency: USD trades from SGD/MYR accounts incur 1-3 pip FX spreads — material for short-term trading. Use the tool with USD account size and treat FX exposure as a separate position-sizing layer if you're not currency-hedging.
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