How to Calculate Position Size

12 min read · Trading Basics

What is position size?

Position size is the number of coins, shares, or contracts you buy in a single trade. Correct calculation ensures that when your stop-loss triggers, you lose exactly what you planned — no more.

The Formula

Where Risk ($) is the maximum loss amount you are willing to accept. Typically this is 1–2% of your account balance.

Why position sizing matters more than entry timing

Most retail traders focus on finding the "perfect" entry — but research consistently shows that a mediocre strategy with disciplined position sizing outperforms a great strategy with bad sizing. The math is simple: with a 50% win rate and 1:1 risk-reward, risking 1% per trade gives you a 99.9% chance of surviving a 10-trade losing streak with your account intact (down 10%). Risking 10% per trade? One bad week and half your account is gone, and you need a 100% gain to recover.

Crypto Example

Balance: $10,000. Risk: 1% = $100. BTC entry: $65,000. Stop-loss: $63,500.

If price drops to $63,500 — you lose exactly $100 (1% of balance). If it rises to $68,000 (R:R 1:2) — you make $200.

Stock Example

Balance: $5,000. Risk: $50 (1%). AAPL price: $185. Stop-loss: $181.

Detailed example: sizing from scratch

You want to long ETH. You have $20,000 in your account. Your rule: risk 1% per trade. Step 1 — determine dollar risk: $20,000 × 1% = $200. Step 2 — find entry and stop using technical analysis. You see ETH at $3,200 with support at $3,120; you enter at $3,200 and stop at $3,110 (just below support). Step 3 — calculate distance: $3,200 − $3,110 = $90 per coin. Step 4 — calculate size: $200 ÷ $90 = 2.222 ETH. Step 5 — notional: 2.222 × $3,200 = $7,111. Step 6 — if using 5× leverage, margin required: $7,111 ÷ 5 = $1,422. Your account can afford this. If the trade hits the stop: 2.222 × $90 = $200 loss, as planned. If it hits a 2:1 target at $3,380: 2.222 × $180 = $400 profit.

Fixed-dollar vs percentage risk

Two main approaches to sizing: (1) Fixed-dollar risk — same $ amount on every trade regardless of account size. Easier mentally, but becomes too aggressive as the account shrinks and too conservative as it grows. (2) Percentage risk — a fixed % of current balance. Self-corrects: losses shrink future positions, wins grow them. Almost all professional traders use percentage risk. The calculator supports both — use the toggle to switch between "$ risk" and "% risk" input modes.

Accounting for fees and slippage

Exchange fees eat into your planned risk. Example: you want to risk $100 on a BTC trade, calculator shows you need 0.0667 BTC. On Binance Futures with 0.06% taker fee on entry and exit, fees on a $4,334 notional position are about $5.20 total — over 5% of your risk amount. The calculator has a "fee adjustment" option: toggle it on and provide your fee rate, and the tool will slightly reduce position size so net risk (after fees) matches what you planned. For stocks with per-share commissions (Interactive Brokers at $0.005/share, minimum $1), use the per-share mode.

Leverage does not change your risk

A common mistake: thinking 10× leverage means 10× the risk. It does not. Leverage only changes how much margin (collateral) is locked up and where the liquidation price sits. Your dollar risk is determined by two things alone: stop-loss distance, and position size. If you size a position to risk $100 with a stop 2% away, it does not matter whether you used 1× or 20× leverage — when the stop hits, you lose $100. Leverage becomes dangerous only when traders skip sizing and just put in "max leverage" — then stop-loss distance becomes larger than liquidation distance, and you get wiped out instead.

Rounding and minimum position sizes

Crypto: most exchanges allow fractions down to 6–8 decimal places. Binance has a "tick size" per pair — for BTC/USDT, minimum increment is 0.00001 BTC. The calculator output is usable as-is; just truncate to the exchange's precision. Stocks: most exchanges require whole shares. If the calculator returns 12.5 shares, round down to 12 — this reduces your actual risk slightly. Rounding up (to 13) increases risk above plan, which defeats the purpose. A few US brokers (Fidelity, Robinhood) offer fractional shares; there you can use the exact number.

Common Mistakes

• Using a fixed lot size instead of calculating from risk
• Ignoring exchange fees in the calculation
• Using a too-tight stop-loss to increase position size
• Risking more than 2% per trade even on "high-confidence" setups
• Adjusting position size by intuition after a losing streak ("I'll double down to make it back")
• Using percentage of price for stop-loss instead of the technical structure (arbitrary stops)
• Forgetting to recalculate position size when the account balance changes significantly
• Running the same risk % on highly correlated trades (e.g., three long crypto positions = one bigger position)