Position Size Formula for Stocks: Worked Examples

Shares = (account × risk percent) ÷ (entry − stop). That is the whole of a basic position size calculator for stocks. Choose how much of the account you accept losing if the stop is hit, measure the distance from entry to stop, and divide. Round down. Then apply two limits most calculators leave out: a cap on how much of the account goes into one position, and a check that the share count is small next to the stock’s average daily volume. The worked examples below use hypothetical prices to show each step.

The basic sum

The risk in dollars is a fixed share of the account. The risk per share is the gap between where you buy and where the trade is wrong. Dividing one by the other gives the share count, and because the dollar risk is fixed first, the loss you take when the stop is hit stays the same whether the chart calls for a stop a few cents away or several dollars away.

If the stop is hit, the loss is 166 × 3 = 498, just under the 500 you chose. Rounding down is why it lands just under. The position size calculator does the same sum and shows each line.

Choosing the risk percent

The formula needs one number you choose yourself. Pick it with losing streaks in mind. The percent is a choice about those streaks, because every strategy has runs of consecutive losses, and the smaller the share you risk on each trade, the more of the account is left standing when a run like that ends and you can start trading normally again.

Ten losses in a row sounds rare. Over hundreds of trades with a modest win rate, it can happen. Size for it.

Write the percent down, apply it to every trade in the same way, and change it only on a schedule you set in advance, never in the middle of a run of losses or straight after a big win.

A short trade

For a short sale, the stop sits above the entry. The distance is the stop minus the entry. Everything else stays the same.

Same risk, same share count. The direction changes nothing in the sum. Shorting brings its own costs, such as borrow fees, which belong in the risk per share if they are material for the time you expect to hold.

Setting the stop with ATR

A stop placed inside the stock’s normal daily range gets hit by noise. The average true range measures that range, and a multiple of it gives a stop distance that sits outside it.

A volatile stock gets a wider stop and fewer shares. A quiet one gets a tighter stop and more shares. The dollar risk holds steady either way, which is the point of sizing this way.

When the cap on position value binds

A tight stop produces a large share count, and the formula alone will happily put most of the account into one stock. Set a maximum position value as a share of the account, and take whichever share count is smaller.

The cap cuts the risk on this trade to a quarter of what you allowed. That is fine. A stop half a point away can be jumped in seconds, and a position worth most of the account turns a gap or a bad fill into a much larger loss than the formula assumed. The 20% here is an illustration. Choose your own ceiling and write it down.

The liquidity check

Most calculators stop at the share count. Before placing the order, compare it with the stock’s average daily volume. If your order is a large slice of a normal day’s trading, you may move the price against yourself getting in, and again getting out, especially at a stop when everyone else is selling too.

Decide on a maximum share of average volume you accept, and check the bars on the day as well. Reading volume bars shows how to spot a session that is trading far below its average, which is when a normal-sized order can still move a thin stock.

The gap check

A stop order becomes a market order once the price reaches it. If the stock opens below your stop, the fill comes at the opening price.

Scheduled events make gaps more likely. Before earnings or a major economic release, work out the loss for a gap of a size you consider plausible and cut the share count if the answer is more than you could stand. Letting the economic calendar shape position size takes the same idea further.

Costs and adding to a position

Commissions and expected slippage widen the true risk per share. Add them to the distance before dividing. For a cheap stock with a wide spread, the difference can be large.

If you plan to buy in two or three stages, size the whole plan, not each piece. Add up the risk of every planned entry against the final stop and keep the total inside your dollar risk.

Measure what the sizing does

Record each trade’s planned risk, actual loss or gain, and the result as a multiple of the risk. Over enough trades, that log shows whether your stops are too tight, whether gaps keep breaking the plan, and whether the risk percent fits the strategy. Calculating portfolio returns covers measuring the account as a whole.

What goes wrong

• Rounding the share count up.
• Setting the stop after choosing the size.
• Ignoring the cap on position value.
• Forgetting the gap risk before earnings.
• Sizing each add-on separately.