Key Takeaways
- The stop comes first and the share count follows from it. Choosing the size first and then hunting for a stop that justifies it inverts the logic and usually produces a stop placed where the trader can afford it rather than where the idea is wrong.
- Risk per share is the gap between entry and stop. Dollars at risk divided by that gap, rounded down, is the position size.
- A reward-to-risk ratio is meaningless alone. Its break-even win rate, one divided by one plus the ratio, is the number that makes it comparable to a strategy's real hit rate.
- A wider stop is not more risk if the share count shrinks to match. It is the same dollars spread over fewer shares, which is the entire point of sizing from the stop.
- A stop order becomes a market order when triggered, so the realized loss can exceed the planned loss. Gaps, halts, and thin books all break the assumption every calculator on this topic makes.
- Costs push the real break-even win rate above the theoretical one. A 2:1 trade needs more than 33.3% wins once commission, spread, and slippage are counted.
What Is a Stop Loss and How Does It Work?
A stop loss is a resting order that becomes live when the market trades at or through a price the trader chose in advance. The SEC's investor education describes the mechanic plainly: a stop order, also called a stop-loss order, becomes a market order once the stop price is reached. That second half is the part traders internalize last. The stop price is a trigger, not a fill price.
FINRA's order-type guidance describes the same mechanism and the variation built to address it, the stop-limit order, which triggers a limit order rather than a market order. A stop-limit caps the price but reintroduces the risk of no fill at all, which in a fast decline is the worse of the two failures. Neither version makes the loss deterministic.
The reason a stop is worth placing anyway is not that it caps the loss precisely. It is that it converts an open-ended position into a bounded one before the position is opened, which is what makes position sizing possible at all. Without a defined exit there is no risk per share, and without risk per share there is no way to answer how many shares are appropriate.
The Formula: From Stop Distance to Share Count
Four steps, in order. Each one depends on the one before it.
- Choose the stop price. By percentage, by ATR multiple, or by a level on the chart.
- Risk per share = |entry price - stop price|. Direction does not change the arithmetic, only which side the stop sits on.
- Dollars at risk = account balance x risk percent. This is the budget for the trade, not a target.
- Shares = floor(dollars at risk / risk per share). Rounding down keeps the planned loss at or below the budget rather than just over it.
Shares = floor((account balance x risk percent) / |entry price - stop price|)
Worked example
A $25,000 account, 1% risk per trade, a long entry at $80.00, and a 5% stop.
Dollars at risk = $25,000 × 1% = $250.00
Stop price = $80.00 × (1 − 5%) = $76.00
Risk per share = $80.00 − $76.00 = $4.00
Shares = floor($250 / $4) = 62
Planned loss = 62 × $4.00 = $248.00
Position value = 62 × $80.00 = $4,960.00Two figures are worth reading together. The planned loss is $248, or 0.99% of the account, comfortably inside the 1% budget because of the rounding down. The position value is $4,960, or 19.84% of the account. A position occupying nearly a fifth of the account still risks under 1% of it, and that gap between exposure and risk is exactly what a stop buys. It also disappears the moment the stop fails to hold, which is why the exposure figure is worth watching alongside the risk figure.
Adding the target
Take the same trade with a $92.00 target. Reward per share is $12.00, risk per share is $4.00, so the reward-to-risk ratio is 3:1, and the break-even win rate is 1 / (1 + 3) = 25%. A strategy taking this trade shape needs to win more than a quarter of the time, before costs, to make money.
Percentage, ATR, or Chart Level: Which Stop Method?
The three methods answer the same question with different information. None of them is universally correct, and the differences matter most across a portfolio rather than on a single trade.
| Method | How the stop is set | Strength | Weakness |
|---|---|---|---|
| Percentage | A fixed percent of the entry price | Consistent and instantly comparable across holdings | Ignores how much the instrument actually moves, so the same percent is loose on a quiet name and tight on a volatile one |
| ATR multiple | A multiple of Average True Range, subtracted from entry on a long | Scales the distance to the instrument's own recent range, equalizing the chance of a noise stop-out | ATR is backward looking, so a regime shift makes yesterday's multiple the wrong one |
| Chart level | A specific price beneath support, a moving average, or a swing low | Placed where the trade idea would actually be wrong, not where the arithmetic is convenient | Levels are discretionary, and a level that many traders can see is a level many stops cluster beneath |
Common mistake: tightening the stop to afford a larger position. This is sizing backwards. A tighter stop does allow more shares at the same dollar risk, but it also raises the probability of being stopped out by ordinary volatility, which converts a well-sized loser into a stream of small losses that add up to the same money with none of the upside.
Stop Loss and Risk/Reward Calculator
Enter the account, the risk budget, the entry, and one stop-placement method. The target price is optional: supply one and the reward-to-risk ratio and break-even win rate appear alongside the sizing.
All calculations run in your browser. Values you enter are not sent to Swoopr Investment's servers, stored, or logged.
What Is a Good Risk Reward Ratio?
A reward-to-risk ratio on its own is a half-statement. It describes the shape of a winning trade relative to a losing one and says nothing about how often each occurs. Pairing it with its break-even win rate completes the sentence.
Break-even win rate = 1 / (1 + reward-to-risk ratio)
| Reward-to-risk | Break-even win rate | What it means |
|---|---|---|
| 0.5 : 1 | 66.7% | Two winners are needed for every loser just to stand still |
| 1 : 1 | 50.0% | A coin flip, before costs |
| 1.5 : 1 | 40.0% | Four wins in ten cover six losses |
| 2 : 1 | 33.3% | One win in three |
| 3 : 1 | 25.0% | One win in four |
| 5 : 1 | 16.7% | One win in six, which is roughly the trend-following profile |
Two strategies with very different ratios can both be sound. A mean-reversion approach taking 1:1 trades at a 58% win rate has a positive edge. A breakout approach taking 4:1 trades at a 28% win rate also has a positive edge, and it will feel dramatically worse to trade, because it loses roughly seven times in ten. The ratio is a design choice about which kind of losing streak the trader can actually sit through.
The break-even figure is a floor, not a goal. It is the win rate at which expected value is exactly zero before any cost. Commission, spread, and slippage all sit on the risk side of the calculation, so the real break-even rate is always higher than the table shows. Swoopr Investment's Trade Expectancy and R-Multiple Calculator models that cost drag directly, in R, across a range of assumptions.
What This Calculator Does Not Model
The arithmetic is exact. The assumptions behind it are not, and the gap between the two is where real losses live.
- Fill quality. Every risk figure assumes the stop fills at the stop price. A triggered stop becomes a market order, and a market order fills at whatever is available. Overnight gaps, halts, and thin order books all produce fills beyond the stop.
- Costs. Commission, spread, and slippage are excluded. They reduce reward, increase realized risk, and raise the break-even win rate above the theoretical figure.
- Correlation. Sizing one trade at 1% says nothing about what happens when six correlated positions all stop out on the same day. That is the question the Portfolio Heat Calculator exists to answer.
- Partial exits and trailing stops. The model assumes one entry, one stop, and one target. Scaling out changes the realized ratio, usually downward.
- Whole shares only. Share counts round down. Fractional-share brokers and instruments with contract multipliers will not match exactly.
- Margin and leverage. A position value larger than the cash available implies borrowing, which introduces maintenance requirements and forced-liquidation risk the calculator does not represent. FINRA Rule 4210 governs those requirements in the United States.
Nothing on this page is a recommendation to place any particular trade, use any particular stop distance, or risk any particular percentage of an account.
Frequently Asked Questions
How do you calculate a stop loss?
Pick the stop level first, then let it determine the share count. Three placement methods are common: a fixed percentage below the entry price, a multiple of Average True Range so the distance scales with the instrument's own volatility, or a specific price under a structural level on the chart. Whichever produces the stop, the distance between entry and stop is the risk per share. Divide the dollars you are willing to lose on the trade by that risk per share and round down, and the result is the largest position that keeps the loss inside the budget.
What is a good risk reward ratio?
There is no single good ratio, because a ratio is only meaningful next to a win rate. A 3:1 trade breaks even at a 25% win rate and a 1:1 trade breaks even at 50%, so a strategy winning 60% of the time can be profitable at 1:1 while a strategy winning 20% of the time loses money at 3:1. The useful question is whether the win rate the strategy actually produces clears the break-even rate the ratio implies, with enough margin left to absorb commissions, spread, and slippage.
How do you calculate break-even win rate?
Break-even win rate equals one divided by one plus the reward-to-risk ratio. At 1:1 that is 1 / 2, or 50%. At 2:1 it is 1 / 3, or 33.3%. At 3:1 it is 1 / 4, or 25%. This is the win rate at which expected value is exactly zero before costs, so it is a floor rather than a target. Every dollar of commission, spread, and slippage raises the real break-even rate above the figure the formula returns.
Does a stop loss guarantee your maximum loss?
No. A stop order is an instruction to trade once a price is reached, not a promise about the price obtained. The SEC's Types of Orders guidance explains that a stop order becomes a market order when triggered, and a market order fills at whatever price is available. In a gap down, a fast selloff, or a thin book, the fill can land well below the stop, producing a larger loss than the calculator shows. Treat every risk figure here as the planned loss under an orderly fill, not a ceiling.
Should a stop be based on percentage or ATR?
A percentage stop is simple and consistent across a portfolio, but it ignores how much the individual instrument normally moves, so the same 5% stop is loose on a quiet utility and tight on a volatile small cap. An ATR-based stop scales the distance to the instrument's own recent range, which makes the odds of being stopped out by ordinary noise more comparable across holdings. Neither is inherently better. What matters is that the method is chosen before the trade and applied the same way every time.
What percentage of an account should be risked per trade?
This calculator takes the figure as an input rather than recommending one, because the right number depends on account size, strategy, win rate, and circumstances that no general page can know. What the arithmetic does show is the consequence of the choice: at 1% per trade, ten consecutive losses cost roughly 10% of the account; at 5% per trade, the same losing streak costs roughly 40%. The Drawdown and Recovery Calculator shows what each of those holes then requires to fill.
Does the calculation include the cost of entering and exiting the position?
No. The share count comes from the dollar risk divided by the stop distance, which treats the entry and the exit as free. Commissions, the spread paid on both sides and any financing on a held position all reduce the realized result without changing the calculation. On a wide stop those costs are a small share of the risk budget; on a tight stop they can be a meaningful fraction of it, which is why very tight stops need the cost added before the sizing is treated as accurate.
Why does the share count round down rather than to the nearest whole share?
Because rounding up would put more capital at risk than the stated limit allows, which defeats the purpose of the calculation. Rounding down keeps the realized risk at or below the intended figure, at the cost of a slightly smaller position. The difference is negligible on large positions and material on small accounts or high-priced instruments, where a single share can represent a meaningful share of the intended risk.
How does a gap through the stop change the realized risk and reward?
It changes the denominator of the ratio after the fact. The reward side was measured against an intended risk that the exit did not respect, so a trade planned at three to one becomes something worse once the fill lands below the stop. Over a series of trades this shows up as realized R-multiples clustering below minus one on losers. Assuming some slippage when setting the target ratio is what keeps the planned figure honest.
References
Order-mechanics claims on this page are sourced to U.S. regulator and self-regulatory-organization investor education, verified in August 2026. Every number in the worked example is computed from the inputs shown, not drawn from any external dataset.
- SEC Investor.gov: Types of Orders: the SEC's investor-education description of market, limit, stop, and stop-limit orders, and the source for the statement that a stop order becomes a market order once the stop price is reached.
- FINRA: Order Types: FINRA's parallel guidance on order types, including the stop-limit variation and the trade-off it makes between price certainty and fill certainty.
- SEC Investor.gov: How Stock Markets Work: the SEC's overview of market participants, order execution, and brokerage account structures that sit behind every figure this calculator produces.
- FINRA: Rule 4210, Margin Requirements: the rule text governing initial and maintenance margin for U.S. customer accounts, relevant whenever a calculated position value exceeds available cash.
Jurisdiction: United States. Last reviewed by the Swoopr Editorial Team in August 2026. This page is educational and is not personalized investment advice. Trading involves the risk of loss, including loss of the entire amount invested.