Trading · Risk management
Position sizing: how many shares to buy for a set risk
How fixed-fractional position sizing turns a risk budget and a stop distance into a share count, with worked examples, ATR-based sizing and the common traps.
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Position sizing answers one question before every trade: how many shares? It sounds like a detail, yet it decides how much a single bad trade can cost, how deep a losing streak can cut, and whether an account survives long enough for any edge to show. Two traders with identical entries and exits can end the year in very different places purely because one sized positions consistently and the other did not.
This page explains the most common method, fixed-fractional sizing, in which you risk the same small percentage of your account on each trade and let the distance to your stop decide the share count. You'll see the formula, several worked examples, how to size with the Average True Range so volatile stocks get smaller positions, and the traps that make real losses larger than planned. It is an educational outline of how the arithmetic works, not a recommendation of any risk level or strategy.
What position sizing is
A position size is the number of shares (or contracts) you hold in one trade. Sizing is the rule you use to choose it. There are three broad families of rules:
- Fixed dollar amount. Every trade buys, say, $10,000 of stock. Simple, but the risk varies wildly: $10,000 of a calm utility and $10,000 of a volatile biotech are very different bets.
- Fixed share count. Every trade buys 100 shares. Even more arbitrary, because 100 shares of a $20 stock and 100 shares of a $900 stock have nothing in common.
- Fixed fractional (risk-based). Every trade risks the same percentage of the account, measured as the loss you take if the stop is hit. The share count falls out of the arithmetic.
The third family is the subject of this page, because it is the only one of the three that ties the size of a position to how much you could actually lose on it.
The fixed-fractional formula
You need three numbers: your account equity, the percentage you are willing to lose on this trade, and the per-share distance between your entry and your stop.
For a short position the per-share risk is stop price − entry price, because the stop sits above the entry. Always round the share count down. Rounding up quietly lifts your risk above the budget.
Notice the difference between the two percentages. The position is one-eighth of the account, but the planned loss is one-hundredth. Beginners often confuse the two and either take far too much risk ("I only put 10% in it") or far too little ("I can't risk 1%, that's only a $500 position").
Why the stop distance drives everything
Hold the risk budget fixed and change only the stop, and the share count swings by an order of magnitude. A tight stop permits a large position; a wide stop forces a small one. Neither is better in itself. A stop must sit where the trade idea is genuinely wrong, and the size must then adapt to that distance, never the other way round.
The chart also shows a trap. With a $1 stop, 500 shares cost $25,000, half of a $50,000 account. Squeeze the stop to $0.50 and the formula asks for 1,000 shares, the entire account, on a stop so tight that ordinary noise is likely to hit it. That is why most written sizing rules add a cap on position value (for example, no single position above a set percentage of equity) alongside the risk rule. The smaller of the two answers wins.
How losing streaks interact with the risk percentage
The purpose of a small fixed fraction is survival through bad runs, which every trading method eventually has. Because each loss is a percentage of the current balance, losses shrink in dollars as the account falls.
Ten losses in a row sounds unlikely, but it isn't for a method that wins 40% of the time. The chance that any particular run of ten trades loses every time is 0.6¹⁰ ≈ 0.6%, but over roughly 300 trades it is about an even bet that such a run appears somewhere (a simple simulation of independent trades gives about 51%). The risk percentage is the dial that decides whether such a streak is an annoyance or the end of the account.
Common risk percentages, and what they mean
| Risk per trade | Loss on a $50,000 account | Full losses to lose 20% | Typical description |
|---|---|---|---|
| 0.25% | $125 | about 89 | Very conservative |
| 0.5% | $250 | about 45 | Conservative |
| 1% | $500 | about 22 | The figure most often quoted |
| 2% | $1,000 | about 11 | Upper end of common guidance |
| 5% | $2,500 | about 4 | Aggressive; streaks hurt quickly |
The middle column is the first trade's loss. The third column counts full-stop losses in a row, each a percentage of the shrinking balance, until the account is down 20%. None of these is "correct". They are conventions, and the right level depends on the method's win rate, how often it trades, and how much drawdown a person can sit through without abandoning the plan.
ATR-based sizing: let volatility set the stop
A fixed percentage stop (say, 5% below entry) treats every stock alike. A stock that routinely moves 4% a day will hit a 5% stop on noise, while a stock that moves 0.5% a day barely notices it. Volatility-based sizing fixes this by expressing the stop in units of the stock's own typical range.
The usual yardstick is the Average True Range (ATR), introduced by J. Welles Wilder in 1978. The true range of a day is the largest of: high minus low, the distance from the previous close up to the high, and the distance from the previous close down to the low. Including the previous close means overnight gaps count as range. ATR is a smoothed average of true range, usually over 14 days, quoted in dollars.
ATR sizing has a side benefit: it roughly equalizes the day-to-day swing of each position, so one volatile holding doesn't dominate the portfolio's daily P&L. Its weakness is that ATR looks backward. It measures the range the stock has had, and it rises only after a volatility shock, which is exactly when a stop is most likely to be gapped through.
Real-world frictions that enlarge the loss
The formula assumes the stop fills at its price. Several things break that assumption.
Gaps. A stop order becomes a market order once the stop price is reached. The SEC's investor bulletin on stop orders warns that in a fast-moving market the execution price can be significantly different from the stop price. If a stock closes at $50 and opens at $43 after bad news, a $46 stop sells near $43, and the 125-share position from the first example loses about $875, not $500.
Slippage and spreads. Even without a gap, a market order sells at the bid. In a thinly traded stock the bid can sit several cents, or several dimes, below the last trade.
Commissions and fees. Small per-trade costs are a meaningful slice of a small risk budget. Many traders subtract a cost allowance from the risk dollars before dividing.
Correlated positions. Five positions that each risk 1% are not five independent 1% risks if they are all semiconductor stocks. A sector-wide drop can hit every stop on the same day. A cap on total open risk (the sum of all positions' stop-loss amounts) addresses this.
Other sizing methods, compared
| Method | How size is set | Strength | Weakness |
|---|---|---|---|
| Fixed dollar | Same dollar amount every trade | Simplest possible rule | Risk varies with each stock's volatility |
| Fixed fractional | Constant % of equity at risk to the stop | Ties size to the actual planned loss | Depends on a stop that may not fill exactly |
| ATR / volatility based | Stop set in ATR units; size from the risk budget | Adapts to each stock's typical moves | Backward-looking; lags volatility shocks |
| Percent of equity | Position value = constant % of the account | Easy diversification rule | Ignores where the stop is |
| Kelly criterion | Fraction from estimated win rate and payoff | Theoretically maximizes growth | Very sensitive to estimation error; volatile |
The Kelly criterion, from John Kelly's 1956 paper on information theory, gives the fraction f = W − (1 − W) ÷ R, where W is the win rate and R is the average win divided by the average loss. With W = 45% and R = 2, f = 0.45 − 0.55 ÷ 2 = 17.5% of equity at risk per trade. That figure shows why the formula is mostly a theoretical benchmark in trading: real win rates and payoffs are estimates, and an overestimate pushes the fraction into ruinous territory. Practitioners who reference it typically scale it down sharply.
How Finabase helps
Finabase is a research app. It does not place orders or hold accounts, but it covers the inputs a sizing rule needs:
- ATR on any chart. The AAPL chart with ATR (14) adds Wilder's Average True Range as a study below the price, computed by the same indicator library as the rest of the chart. Read the latest value to estimate a volatility-based stop distance.
- Portfolios. Portfolios track your holdings, cost basis and gains at live prices, which lets you see each position's weight in the account.
- Alerts. Alerts can notify you when a price, a percentage change or a trailing-stop level is reached. An alert is a notification, not an order: you still act at your broker.
- Backtests. The backtester trades a fixed $10,000 per signal. That is fixed-dollar sizing, so remember that its results reflect that rule, not a risk-based one.
Frequently asked questions
What is the 1% rule in trading?
The 1% rule is a fixed-fractional sizing convention: risk no more than 1% of account equity on any single trade, measured from entry to stop. It does not mean putting 1% of the account into the position. With a $50,000 account and a $4 stop, it allows 125 shares of a $50 stock, a $6,250 position.
How do I calculate position size for a short sale?
Use the same formula with the stop above the entry: shares = risk dollars ÷ (stop − entry). Short positions carry extra gap risk because a stock can jump far above the stop, and they involve borrowing costs that the formula doesn't include.
Should I use the account balance or buying power?
Fixed-fractional rules use account equity, the value you would have if every position were closed. Buying power on a margin account can be larger than equity, but sizing from it multiplies the real percentage at risk.
Does position sizing guarantee a limited loss?
No. It limits the planned loss if the stop fills near its price. Overnight gaps, trading halts, fast markets and illiquid stocks can all produce fills well beyond the stop, which is why many rules also cap position value and total open risk.
Summary
Position sizing turns a trade idea into a number of shares. Fixed-fractional sizing starts from how much of the account you are prepared to lose, divides that by the distance to a stop placed where the idea is wrong, and rounds down. ATR-based stops extend the idea so volatile stocks automatically get smaller positions. The method controls planned losses and keeps losing streaks survivable, but it relies on stops that can be gapped through, so caps on position value, total open risk and event exposure complete the picture.
Sources and further reading
- Wilder, New Concepts in Technical Trading Systems (1978) — Average True Range
- SEC, Investor.gov — Investor Bulletin: Stop, Stop-Limit and Trailing Stop Orders
- Kelly, A New Interpretation of Information Rate, Bell System Technical Journal (1956)