The test
Before buying, ask two questions about the next three to five years:
- What are the odds this company's stock doubles?
- What are the odds it loses half its value?
Buy only when the first is clearly larger than the second. If they feel about equal, you are guessing.
Why it works
Most bad investments are not bad because the upside was small. They are bad because the downside was ignored. The test forces you to imagine the failure case as clearly as the success case.
It also forces a time frame. "Could this double?" is meaningless without "by when?". Three to five years is long enough for a business to grow and short enough to reason about.
What pushes the odds each way
| Makes doubling more likely | Makes halving more likely |
|---|---|
| Revenue growing 20%+ and holding | Growth slowing quarter after quarter |
| Expanding margins | Shrinking margins |
| More cash than debt | Heavy debt, especially floating-rate |
| A moat that is widening | Competitors catching up |
| Reasonable valuation (PEG ratio near or below 1) | Priced for perfection |
| Little public attention yet | Peak media coverage and hype |
An example
A company grows revenue 30% a year, earnings faster still, has net cash, and trades at a forward P/E of 25. If earnings double in three years and the P/E simply holds, the stock doubles. For it to halve, growth would have to collapse and the market's valuation fall at the same time. The odds favour doubling.
Compare a company growing 4% a year at a P/E of 40, with debt. Doubling needs the valuation to rise even further; halving only needs the valuation to fall back to normal. The odds favour halving.
Use it with the rest of your research
The test is a judgment call built on facts. Do the research first, then use the test to decide.
See also
- How to research a company A step-by-step way to research a stock: understand the business, run a SWOT, apply the double-or-halve test, then check the numbers and valuation.
- Economic moats What an economic moat is, the five main types of competitive advantage, and how to tell whether a company's moat is widening or shrinking.
- PEG ratio The PEG ratio divides the P/E by earnings growth, so it judges valuation and growth together. What the bands mean and how to use it.
Pages that link here: Fundamentals checklist
Last updated September 30, 2026. Education only, not investment advice.