Position Size Calculator
Work out the share count implied by your account size, the percentage of it you are willing to risk, your entry price, and your stop-loss. Everything is computed from those four numbers alone — no market data is used and none is shown, so you can check every figure by hand.
Trade details
Total trading capital
Percent of the account to put at risk
Price per share at entry
Exit price. Below entry is a long, above entry is a short
Formula
dollar risk = account × (risk % ÷ 100)
risk per share = | entry − stop |
shares = floor(dollar risk ÷ risk per share)
position value = shares × entry
Position size
- Dollar risk if stopped
- $250.001.00% of account · limit $250.00
- Risk per share
- $2.004.00% from entry
- Position value
- $6,250.00
- Position as % of account
- 25.00%
Shares are rounded down, so the dollar risk shown is at or below the limit implied by your risk percentage. The figures assume a single fill at the entry price and an exit at exactly the stop price; commissions, slippage, gaps, and financing costs are not included.
How the calculation works
Position sizing inverts the usual question. Rather than picking a share count and seeing what it costs if the stop is hit, you fix the loss you are willing to take and let that determine the share count. Four steps, all of them arithmetic:
- Dollar risk = account size × (risk % ÷ 100)
- Risk per share = | entry price − stop-loss price |
- Shares = floor(dollar risk ÷ risk per share)
- Position value = shares × entry price
Worked through with the default inputs: a $25,000 account at 1% per trade gives $250 of dollar risk. An entry of $50 against a stop at $48 puts $2 of risk on each share, so $250 ÷ $2 = 125 shares. That is a $6,250 position, 25% of the account, and exactly $250 is lost if the stop fills at $48.
The absolute value in step two is what makes the calculation direction-agnostic. A stop at $52 against the same $50 entry is a short with the same $2 of risk per share and the same 125 shares; the page labels the result long or short from the position of the stop relative to the entry.
Two things the arithmetic cannot promise. The floor in step three means the realised risk is usually a little under the target rather than exactly on it. And the $250 figure holds only if the exit actually fills at the stop price — a gap through the stop is a larger loss than the calculation shows.
Frequently asked questions
How do you calculate position size?+
First find your dollar risk: account size multiplied by your risk percentage (e.g. $25,000 × 1% = $250). Then find your risk per share: the absolute difference between entry and stop-loss price. Divide dollar risk by risk per share and round down to get the number of shares to buy.
What does changing the risk percentage do to the numbers?+
The risk percentage scales the dollar risk linearly, and therefore the share count too. On a $25,000 account, 1% is $250 of dollar risk and 2% is $500, which doubles the shares for the same stop distance. The compounding effect of a losing run scales with it: ten consecutive full stop-outs at 1% leave about 90.4% of the starting balance, at 2% about 81.7%, and at 10% about 34.9%. This calculator does not suggest a percentage — enter the one you use.
Why round the number of shares down?+
Rounding down keeps the realised dollar risk at or below the amount implied by your risk percentage. Rounding up would push it slightly above the limit. Because of the rounding, the dollar risk shown is usually a little under the target, and the gap is largest when the stop distance is wide relative to the dollar risk.
Does it work for short trades?+
Yes. The calculator reads a stop-loss above the entry price as a short and one below the entry as a long, and labels the result accordingly. The risk per share is the absolute distance between entry and stop in both cases, so the share count is calculated the same way.
Does this calculator account for commissions or slippage?+
No. It computes the position size implied by your entry, stop, and risk inputs, assuming a single fill at the entry price and an exit at exactly the stop price. Commissions, slippage, gaps through the stop, and financing costs are outside the calculation and would each change the realised risk.
What if the position value comes out larger than my account?+
That happens whenever the stop is tight relative to your risk percentage, and the calculator flags it. The arithmetic is still correct — the share count risks exactly what you specified if the stop is hit — but holding that many shares would require margin, and a gap through a tight stop can cost more than the stated risk.
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This calculator performs and displays arithmetic on the inputs you provide. It is not investment advice, does not recommend a risk percentage, a share count, or any trade, and makes no claim about how a trade sized this way would perform.