Position Size Calculator for Stocks
Name a stock you'd hate to be wrong about.
Volatility isn't risk. Ruin is. Pick the company. Choose the price you'd pay. Decide how much of your portfolio can be wrong. That — not a chart — sets the size.
Start with the name, not the chart
Which company?
You supply the price you would pay — this site never publishes one, so nothing here goes stale between visits.
How to calculate position size for a stock
Work backward from the loss you would accept if you are wrong:
Shares = (Account × Risk %) ÷ (Entry price − Stop price)
A $50,000 account risking 1% puts $500 on the line. Buy at $50 with a stop at $45 and you risk $5 a share: 100 shares, $5,000, 10% of the account.
The weak point is the stop — a number picked off a chart. This calculator replaces it with a floor modeled from the company's filings:
Maximum position = Loss you can live with ÷ Fall to the floor
Pay $100 with a floor at $60 and accept losing 2% of your portfolio: the fall is 40%, so 2% ÷ 40% = a 5% position.
Why this position size calculator ignores volatility
A position is not a prediction. It is a promise about what happens when you are wrong.
Textbook sizing divides by variance: the more the price wobbles, the smaller the bet. That makes wobble the definition of risk. But a falling stock is not automatically a risky stock. A stock with no credible floor is. Variance cannot tell those apart.
So two things set the size here: how far the price can fall before it reaches the modeled floor, and how predictable the cash flow underneath is. Nothing else.
We regard volatility as a measure of risk to be nuts [...] If somebody starts talking to you about β (beta), you know, zip up your pocketbook - W. Buffett, 2001. BRK
Where every floor comes from
The same three anchors for every name, all off the filings. Banks and insurers are held to book value; every other business to its earnings through a full cycle. A floor is a model, not a promise: net cash can be pledged, and the next bad year can be worse than any before it.
| Downside anchor | Built from |
|---|---|
| Earnings Power FloorAssumes the cycle repeats, not that it worsens | EPS × its lowest P/E |
| Book Value FloorThe price-to-book = 1.0 line | Equity ÷ shares |
| Recoverable FloorAssumes the cash is unpledged | Net cash, after all debt |
Pick a company above and you get its numbers, the one that binds, and the filing each came from. A name gets a page only when its filings support a floor we can point at — when they do not, there is no honest page to publish, which is itself worth knowing before sizing anything.
What this position size calculator does not do
- It won't tell you what to buy. It makes sure one mistake cannot own you.
- It does not value the business. Whether your price is a good one is your work.
- It does not know your other positions — ten names at 3% each can impair 30% of a portfolio in one correlated repricing.
- It is not advice, and it does not know your circumstances.
Position size calculator FAQ
How do I calculate position size for a stock?
Divide the amount you are willing to lose by the loss per share if you are wrong. The classic formula is (account × risk %) ÷ (entry price − stop price). This calculator uses the distance to a filing-based value floor instead of a stop.
What percentage of my portfolio should go into one stock?
There is no single right number: it depends on how much you can afford to lose and how far the stock can fall. Risking 1–2% per position is a common rule of thumb — that is the loss, not the position size. This calculator turns your loss limit into a maximum position.
Can I do this in Excel or Google Sheets?
Yes. With account size in B1, risk % in B2, entry in B3 and stop in B4: =ROUNDDOWN(B1*B2/(B3-B4),0) gives the share count.
Which stocks does it cover?
US-listed companies whose filings support a floor we can point at. When they do not, there is no honest page to publish.
Is it free?
Yes. Every covered stock, no account.
Does it tell me what to buy?
No. It does not value the business. It makes sure one mistake cannot own you.
Further reading
- Position sizing for value investorsWhy a risk budget built on the fall to a value floor beats gut feel.
- Business risk vs price riskThe two ways to lose money, and which one a floor measures.
- What business quality meansThe idea behind the quality score that caps every position.
- Seven steps before you buy a stockWhere sizing fits: it is the last of the seven.