Stock Position Size Calculator

Work out how many shares to buy for a given risk and stop-loss.

Please note: For information only, not investment advice. Share prices can fall as well as rise and past performance does not predict future results.

What the Stock Position Size Calculator does

Position sizing sets the number of shares so that hitting your stop-loss costs a fixed, predetermined amount. A concentration cap then prevents any single holding dominating the portfolio, even when a tight stop would technically allow it.

Formula

Risk amount = Portfolio × Risk % ÷ 100
Risk per share = Entry − Stop-loss
Shares = Risk amount ÷ Risk per share
Capped at: Portfolio × Max position % ÷ Entry price

Inputs explained

Input	Unit	Required	Notes
Portfolio value	selected currency	Yes	Accepts 0 or more.
Risk per position	%	Yes	—
Entry price	selected currency	Yes	Accepts 0 or more.
Stop-loss price	selected currency	Yes	Accepts 0 or more.
Maximum position size	%	Optional	Cap on any single holding as a share of the portfolio.
Currency	one of 10 options	Optional	—

How to use it

Choose Currency.
Enter Portfolio value, Risk per position, Entry price and Stop-loss price.
Optionally add Maximum position size.
Select Calculate.

Worked example

A $100,000 portfolio risking 1%, entering at $50 with a stop at $46.

Portfolio: 100000
Risk: 1
Entry: 50
Stop: 46

$1,000 ÷ $4 = 250 shares, a $12,500 position — 12.5% of the portfolio.

Reading the result

"Risk if stopped out" is the loss if the stop fills exactly at the stop price — the notes' point about gaps means the real loss on a bad day can exceed this figure, particularly around earnings.
The "shares at other risk levels" table is also a way to check whether a higher risk tolerance would trigger the concentration cap even though your current setting does not.

Assumptions and limitations

The concentration cap is fully user-editable and defaults to 20% — the "position capped" note does not distinguish an intentional policy from an unchanged default, so it is worth confirming the entered percentage matches what you actually intend.
This sizes one position in isolation — it has no visibility into other open positions, so "position as % of portfolio" does not reflect total concentration across everything you hold.

Common mistakes

Confusing "risk per position" (a percentage of the whole portfolio) with "stop distance" (the percentage price move to the stop) — they measure different things, and mixing them up produces a mis-sized position.
Assuming a position stays uncapped once checked — the cap is based on position value versus portfolio size, so raising the risk percentage later can trigger it without a proactive warning until you re-run the calculation.

Frequently asked questions

Why cap position size if the risk is already fixed?

A very tight stop can mathematically justify an enormous position. If the stock gaps through the stop overnight, that position size becomes the real risk.