Position Size Calculator

A position size calculator tells you exactly how many shares (or units) to buy or short on a trade so that if the stop-loss is hit, your account loses no more than a pre-set percentage — typically 1–2% — of your total trading capital. It removes guesswork and enforces consistent risk management on every trade.

Three steps: (1) Multiply your account size by your risk percentage to get the amount at risk (e.g., $10,000 × 2% = $200). (2) Subtract the stop-loss price from the entry price and take the absolute value to get the risk per share (e.g., |$50 − $48.50| = $1.50). (3) Divide the amount at risk by the risk per share and floor the result to whole shares (e.g., $200 / $1.50 = 133.33 → 133 shares).

The answer depends on three things: how much capital you have, how much of it you are willing to risk, and how far your stop-loss is from your entry price. Use the formula: shares = floor( (accountSize × riskPct / 100) / |entry − stop| ). Never size a trade on conviction alone — size it on the stop distance and your risk budget.

The 2% rule is a risk-management guideline that says you should never risk more than 2% of your total trading account on any single trade. For a $10,000 account, the maximum loss per trade is $200. Combined with a stop-loss, it keeps any one bad trade from doing serious damage to your capital. Some more conservative traders use 1%.

The stop-loss distance is the denominator in the position-size formula. A tighter stop (closer to entry) means a larger position size for the same dollar risk; a wider stop means a smaller position. At a 1% account risk: a $1 stop gives 100 shares, but a $5 stop gives only 20 shares. This is why professionals set the stop first and let position size follow, rather than the other way around.

Rounding to the nearest whole number can push shares above the exact quotient, which would make your actual loss exceed your risk budget. Flooring (rounding down) guarantees that shares × risk-per-share is always ≤ your risk limit. For example, 133.33 shares floored to 133 gives a max loss of $199.50, within a $200 budget; rounding up to 134 gives $201.00, which breaks the rule.

The formula is the same for long and short trades. The risk per share is always the absolute difference |entry − stop|. For a long trade the stop is below entry; for a short trade the stop is above entry. Either way, |entry − stop| gives the same positive number, and the shares calculation is identical.

"Capital required" (position value) is the total cash needed to open the trade: shares × entry price. "Max loss" is the most you can lose if the stop is triggered: shares × |entry − stop|. These are very different numbers. For example, 133 shares at $50 requires $6,650 in capital, but the max loss with a stop at $48.50 is only $199.50 — roughly 3% of the capital deployed.

This happens when the stop-loss is very tight relative to the entry price. The risk math is still correct — you are risking the right amount — but in a cash account you cannot spend more than your balance. In that case, apply an optional cap: shares = min(floor(riskAmount / riskPerShare), floor(accountSize / entryPrice)). With a margin account the broker’s leverage extends your buying power, but the risk calculation itself does not change.

"R" (also called "dollar at risk" or "risk amount") is the fixed monetary amount you risk on a single trade. It equals accountSize × riskPct / 100. All trades that get stopped out lose exactly 1R; a trade that hits a 2:1 reward-to-risk target gains 2R. Using consistent R multiples lets you evaluate a strategy’s expectancy — whether you make money on average even if you lose more than half your trades.

Yes. The core formula — shares = floor(riskAmount / riskPerShare) — applies to any instrument where you can express both entry and stop as a price per unit. For forex you might use pips and pip value per lot; for futures you use the contract’s point value. The calculator uses whole shares (no fractional units) for v1, which suits stocks; for fractional-share brokers or forex lot sizing the floor can be relaxed to the instrument’s minimum increment.

No. The calculation assumes your stop-loss fills exactly at the stop price with no friction. In practice, slippage and commissions add to your realized loss. As a conservative adjustment, some traders widen their effective stop by the expected slippage before computing position size, which reduces the share count slightly and builds in a buffer.