# What position size should I trade?

Works out how many shares to buy or sell short so that a loss at your stop price is the share of your account you choose to risk, with the position value and the reward-to-risk ratio.

- Page: https://www.acalculator.org/finance/position-size-calculator
- JSON spec: https://www.acalculator.org/finance/position-size-calculator.json
- Version: 65ba9758f68c

## Default answer

Example with the default inputs (Account size $25,000.00, Risk per trade 1%, Entry price $50.00, Stop-loss price $47.50): Trade 100 shares to risk $250.00 at a stop of $47.50.

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| account | Account size | The value of the trading account. |
| risk | Risk per trade | The share of the account you accept to lose if the stop price is hit. |
| entry | Entry price | The price per share you buy at (or sell short at). |
| stop | Stop-loss price | The price per share where you would exit at a loss. Below the entry for a buy, above it for a short. |
| target | Target price | Optional: the price per share where you plan to take the profit, for the reward-to-risk ratio. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| shares | Shares to trade | Risk amount ÷ risk per share, rounded down to a whole share. |
| side | Trade | A buy (long) when the stop is below the entry, a short sale when it is above. |
| riskAmount | Money at risk | Account size × risk per trade: the most you plan to lose. |
| perShare | Risk per share | The gap between the entry price and the stop-loss price. |
| value | Position value | Shares × entry price: the money the trade ties up. |
| share | Share of the account | Position value as a percent of the account. Over 100% needs margin. |
| loss | Loss at the stop | Shares × risk per share: the loss if the stop fills at its price. Never above the money at risk. |
| reward | Profit at the target | Shares × the gap between the entry and the target. |
| ratio | Reward-to-risk ratio | The gap to the target ÷ the gap to the stop. |

## Method

money at risk = account × risk% ÷ 100; risk per share = |entry − stop|; shares = ⌊money at risk ÷ risk per share⌋; position value = shares × entry; ratio = |target − entry| ÷ |entry − stop|.

## Assumptions

- The stop order fills at the stop price. A stop becomes a market order, so a fast market or a price gap can fill it lower (higher for a short) and lose more.
- Commissions, fees and slippage are not included.
- Shares are whole shares, rounded down so the loss at the stop is never above the money at risk.
- This is a planning tool, not investment advice.

## Worked examples

1. account = $25,000.00, risk = 1%, entry = $50.00, stop = $47.50 gives shares = 100, riskAmount = $250.00, perShare = $2.50, value = $5,000.00, share = 20%, loss = $250.00. Source: FINRA, Stop Orders: Factors to Consider During Volatile Markets (a stop order becomes a market order and may fill well past the stop price), https://www.finra.org/investors/insights/stop-orders-factors-consider-during-volatile-markets (retrieved 2026-10-03).
2. account = $10,000.00, risk = 2%, entry = $23.45, stop = $21.90, target = $28.00 gives shares = 129, riskAmount = $200.00, perShare = $1.55, loss = $199.95, value = $3,025.05, reward = $586.95, ratio = 2.935484. Source: FINRA, Stop Orders: Factors to Consider During Volatile Markets (a stop order becomes a market order and may fill well past the stop price), https://www.finra.org/investors/insights/stop-orders-factors-consider-during-volatile-markets (retrieved 2026-10-03).
3. account = $50,000.00, risk = 0.5%, entry = $80.00, stop = $84.00, target = $70.00 gives shares = 62, riskAmount = $250.00, perShare = $4.00, value = $4,960.00, loss = $248.00, reward = $620.00, ratio = 2.5. Source: FINRA, Stop Orders: Factors to Consider During Volatile Markets (a stop order becomes a market order and may fill well past the stop price), https://www.finra.org/investors/insights/stop-orders-factors-consider-during-volatile-markets (retrieved 2026-10-03).
4. account = $1,000.00, risk = 1%, entry = $0.30, stop = $0.10 gives shares = 50, riskAmount = $10.00, perShare = $0.20, value = $15.00.

## FAQ

### How do I calculate position size?

Multiply the account by the risk percent to get the money at risk, then divide by the gap between the entry and the stop. With $25,000, 1% risk, an entry at $50 and a stop at $47.50: 250 ÷ 2.50 = 100 shares.

### Why is the share count rounded down?

You can only trade whole shares here, and rounding up would make the loss at the stop larger than the risk you chose. $200 of risk at $1.55 a share is 129.03 shares, so the page says 129, which loses $199.95 at the stop.

### Can I lose more than the money at risk?

Yes. A stop order becomes a market order when the stop price is reached, and FINRA warns it can fill well below the stop price in a fast market or after a price gap. Fees and commissions are not included either.

### Does it work for short sales?

Yes. Put the stop above the entry price. The risk per share is still the gap between the two, and a target must be below the entry.

### What does the reward-to-risk ratio mean?

It is the gap from the entry to the target divided by the gap from the entry to the stop. A ratio of 2 means the planned profit is twice the planned loss.

### Why can the position be worth more than my account?

A tight stop gives a large share count. If the position value is over 100% of the account you would need margin, so many traders also cap the position size.

## Sources

- FINRA, Stop Orders: Factors to Consider During Volatile Markets (a stop order becomes a market order and may fill well past the stop price). https://www.finra.org/investors/insights/stop-orders-factors-consider-during-volatile-markets (retrieved 2026-10-03)
- FINRA, Order Types (market, limit, stop and stop-limit orders). https://www.finra.org/investors/investing/investment-products/stocks/order-types (retrieved 2026-10-03)