How to Calculate Position Size (the 1% Rule Explained)
By William · Swift Penguin · Educational content · Not financial advice
Most beginners fall in love with entries and treat position size like an afterthought. That's backwards. Sizing won't invent an edge — but it can keep you in the game long enough to find one. Get this wrong and one ugly trade can erase months of careful wins.
Learn the position size formula, walk through a real 1% rule example, and see the mistakes that turn a normal stop-out into an account-wrecking loss. Pair it with our free calculator.
What is position sizing?
Position sizing answers one question: how many shares should I buy? It's not about picking winners — it's about limiting damage when you're wrong.
The goal: cap every loss to a small percentage of your account. Many discretionary traders aim for about 1–2% risk per trade.
If you don't define risk before entry, you're guessing. Guessing feels fine in a bull market and catastrophic the first time volatility expands against you.
The position size formula
Position size = (Account size × Risk %) ÷ (Entry price − Stop-loss price).
Example: $10,000 account, risking 1% ($100). Entry at $50, stop at $48. Risk per share = $2. Buy $100 ÷ $2 = 50 shares.
If it hits your stop, you lose exactly $100. Not $150 because you got emotional. Not "let me give it more room." $100. Done.
Volatile names need fewer shares for the same dollar risk. Quiet names can use more shares. The formula adjusts automatically — your ego doesn’t get a vote.
Why the 1% rule works
Risking 1% per trade means a losing streak hurts — but it doesn’t wipe you out overnight. After 100 consecutive 1% losses you’d still have roughly 37% of the account left (losses compound). Halving the account takes on the order of ~70 straight full-risk losses.
More importantly, it keeps you sane. When the downside is tiny and fixed, you stop panic-selling.
Consistency compounds. Traders who survive long enough to learn usually sized small while they were still wrong a lot. Sizing alone doesn’t make you profitable — you still need a process with positive expectancy over time.
Common mistakes
Sizing by feel instead of using a stop-loss. If you don't know where you're wrong, you can't calculate risk.
Going bigger on "sure things." Every trade gets the same risk budget. No exceptions.
Forgetting commissions and slippage. On small accounts, fees and bad fills can meaningfully increase your real risk.
Moving the stop farther after entry. That silently increases risk — recalculate size instead of “giving it room.”
Practice before you scale
Use a free position size calculator every time until the habit is automatic. Then practice the same risk rules in a paper trading simulator before increasing real size.
Journal the planned risk next to the outcome. If your average loss is bigger than your planned risk, you’re breaking the rule somehow — find where.
Next steps
Concepts stick when you apply them. Open a related calculator, run your own numbers, and — if you’re still learning execution — practice with paper trades before increasing real size. Browse more guides or the full set of free trading tools.
Free tool
Position Size Calculator
Turn account size, risk %, entry, and stop into an exact share count — with 1% rule presets, optional target for R:R, and a live formula walkthrough. Size the loss before you click buy.
Other free tools
- Stock Market Simulator — Practice trading with fake money.
- Trading 212 Portfolio Charts — Connect T212 and flex your P&L.
- Crypto Prices & Charts — Live quotes + historic charts.
Put this into practice
Run the numbers on our free tools, or paper trade before you risk real money.
This is educational content, not financial advice. Investing carries risk — you can lose money. Do your own research and consider a qualified advisor for personal decisions.