How to Calculate Position Size the Right Way
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What position size actually means
Position size is simply how much of an asset you buy or sell in a single trade — expressed in shares, coins, contracts, or lots. It is one of the few variables in trading you control completely. You cannot control whether a trade wins or loses, but you can control exactly how much money is on the line when it does.
Two traders can take the identical entry, stop-loss and take-profit on the same asset and end up with completely different outcomes purely because they used different position sizes. Get the size wrong and even a strategy with a real statistical edge can produce account-damaging losses during a losing streak.
The position size formula
The calculator on this site uses a two-step, percentage-based formula that works the same way regardless of the instrument:
Risk Amount = Account Balance × Risk % ÷ 100Step 2 —
Position Size = Risk Amount ÷ |Entry Price − Stop-Loss Price|
The first step converts your risk tolerance — expressed as a percentage of your account — into a fixed dollar (or other currency) amount. The second step divides that dollar amount by the price distance between your entry and your stop-loss, giving you the number of units that will lose exactly that dollar amount if the stop-loss is hit.
Worked example
Say you have a $10,000 account and you never want to risk more than 1% on a single trade. You plan to buy an asset at $100 and place your stop-loss at $97.
- Risk Amount = $10,000 × 1% = $100
- Price distance = $100 − $97 = $3
- Position Size = $100 ÷ $3 = 33.33 units
If the price falls to $97 and your stop-loss triggers, you lose 33.33 × $3 = $100 — exactly 1% of your account, regardless of how far away the stop was placed in price terms. This is the core benefit of percentage-based sizing: the dollar risk stays constant even as your stop distance changes from trade to trade.
Applying it to forex, crypto and stocks
The formula above works in any market because it operates purely on price distance, not on market-specific units. A few practical notes for each market:
Forex position sizing
Forex is usually quoted in lots (a standard lot is typically 100,000 units of the base currency), so a forex position size calculator is really a forex lot size calculator by risk. Calculate your position size in units first using the formula above, then divide by the contract size — the Position Size Calculator does this automatically with the "units per lot" field.
Crypto position sizing
A crypto position size calculator works the same way as a crypto risk calculator: crypto exchanges usually let you enter fractional coin amounts directly, so the units figure from the formula can typically be used as-is. Be mindful of any minimum order size or step size your exchange enforces.
Stock and CFD position sizing
For shares, the "units" from the formula are simply the number of shares. Many brokers only allow whole shares, so round down to avoid slightly exceeding your intended risk.
Common mistakes
- Sizing by "gut feel" instead of the stop distance. A fixed number of shares or lots does not account for how far your stop actually is, so identical position sizes can carry very different risk from trade to trade.
- Ignoring fees and spread. The formula above measures price risk only — transaction costs add to your real-world loss and should be factored in separately for tight stops.
- Widening the stop to "fit" a desired position size. The stop-loss should be placed where the trade idea is invalidated, not adjusted to justify a bigger position.
- Recalculating rarely. As your account balance changes, the dollar value of a fixed risk percentage changes too — recalculate position size for every new trade.
Frequently asked questions
What is position size in trading?
Position size is the number of units, shares or lots you trade in a single position. It determines how much money moves for every tick the price moves, and therefore how much you gain or lose.
What is a safe position size?
There is no universal safe size — it depends on your stop-loss distance and how much of your account you are willing to risk. Many traders risk between 0.5% and 2% of their account per trade, then size the position so that if the stop-loss is hit, the loss equals that percentage.
Does position size affect risk if my stop-loss stays the same?
Yes. With a fixed stop-loss distance, a larger position size increases the dollar amount at risk, and a smaller position size decreases it, even though the stop-loss price itself has not changed.
Is position size the same as lot size?
Not quite. Position size is usually measured in raw units (shares, coins or base-currency units), while a lot is a fixed contract size — commonly 100,000 units for a standard forex lot. To convert, divide your position size in units by the lot size to get the equivalent number of lots.
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About the author
SOFTYTOOLS Editorial Team writes and maintains the calculators and guides on this site, focusing on clear, formula-based explanations of trading and investing concepts rather than opinion or speculation.