How to Calculate Position Size: A Trader's Guide
Position sizing is the single most important skill in trading — more important than entries, indicators, or which market you trade. It decides how much you lose when you're wrong, and since every trader is wrong often, it's what keeps your account alive long enough for your edge to play out.
This guide walks through exactly how to calculate position size for any trade, the simple formula behind it, and worked examples for both stocks and forex.
Why position size matters more than your entry
Two traders can take the exact same trade and have completely different outcomes — not because of where they entered, but because of how much they risked. A trader who risks 1% of their account per trade can be wrong ten times in a row and still have roughly 90% of their capital. A trader who risks 10% per trade can be wiped out by a single bad streak.
Position sizing converts an abstract idea — 'manage your risk' — into a precise number of shares or lots. It anchors every trade to a fixed, pre-decided loss, so no single trade can do serious damage.
The position size formula
The core formula is simple: Position Size = (Account × Risk %) ÷ Risk per unit. The 'risk per unit' is the distance between your entry price and your stop-loss. In other words, you first decide how many dollars you're willing to lose, then divide by how many dollars you'll lose per share if the stop is hit.
Say you have a $10,000 account and risk 1% — that's $100 at risk. If your entry is $50 and your stop-loss is $48, you lose $2 per share. So your position size is $100 ÷ $2 = 50 shares. If the stop is hit, you lose exactly $100, your planned 1%.
The 1% (and 2%) rule
Most professional traders risk between 0.5% and 2% of their account on any single trade. The '1% rule' is the most common starting point: never risk more than 1% of your capital on one position. This isn't about being timid — it's about survival math. At 1% risk, even a brutal 20-trade losing streak only draws your account down by roughly 18%, which is fully recoverable.
At 2% risk, the same streak costs you about 33%. Above 2% the math turns hostile fast: large drawdowns require exponentially larger gains just to break even, so disciplined sizing is what separates traders who last from those who don't.
Position sizing in forex
Forex works the same way, but position size is measured in lots and your stop is measured in pips. The formula becomes: Lots = (Account × Risk %) ÷ (Stop in pips × Pip value). A standard lot is 100,000 units, a mini lot 10,000, and a micro lot 1,000.
For example, on a $10,000 account risking 1% ($100) with a 25-pip stop and a pip value of $10 per standard lot: Lots = $100 ÷ (25 × $10) = 0.40 lots. The principle is identical to stocks — fix the dollar risk first, then size the position to fit it.
Key takeaways
- →Decide your dollar risk first (account × risk %), then size the position to fit it.
- →Position Size = Risk amount ÷ (distance from entry to stop-loss).
- →Risk 1–2% per trade so a losing streak can't end your account.
- →Forex uses the same logic, expressed in lots and pips.
Frequently asked questions
What percentage should I risk per trade?+
Most professionals risk 1–2% of their account per trade. Beginners are often best served by 0.5–1% while they build consistency, because smaller risk makes a losing streak far easier to recover from.
Does position sizing work for crypto and futures?+
Yes. The formula — risk amount divided by the distance to your stop — works in any market. Only the unit changes (shares, contracts, coins, or lots).
Should position size change with conviction?+
Some traders scale risk slightly with setup quality, but the safest approach is a fixed percentage. Increasing size on 'high-conviction' trades is how disciplined accounts blow up, because conviction and outcome are weakly correlated.
Calculators in this guide
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For educational purposes only. Not financial advice.