How do you tell an honest forecast from wishful thinking?

A forecast is easy to write and easy to fool yourself with. Before a single number goes into a DCF, run it past a few hard questions. This lesson is the smell test that separates a defensible model from a hopeful one.

Does the math imply an absurd company?

Growth compounds ruthlessly. A company earning €350 of free cash flow that grows 25% a year is producing over €3,250 a year within a decade — nearly ten times as much. Ask: is there a market big enough for that? If your assumptions quietly imply the company becomes larger than its entire industry, the assumptions are broken, not the industry.

Do margins defy gravity?

Very high margins attract competitors, who compete them down. A forecast that assumes margins rise year after year is betting against one of the most reliable forces in business. Sometimes that's justified — a genuine moat, a network effect — but the burden of proof is on you. Flat or gently fading margins are the safer default.

Does the story survive one sentence?

Every good forecast reduces to a sentence a skeptic could attack: "Sales grow 8% as the product expands into two new regions, margins hold near 20%, and growth fades to 3% by year ten." If you can't say your forecast in one plain sentence, you don't understand it — you're just moving numbers.

The two directions of self-deception

• Anchoring to the answer you want. If you already like the company, you'll nudge growth up until the model "proves" it's cheap. Guard against this by setting assumptions before you look at the price.
• False precision. €142.63 feels more trustworthy than "roughly €130–150," but it isn't. The decimals are noise. Think in ranges (the last unit builds this habit properly).

Try it now

1. Take the five-year forecast you sketched last lesson and extend the growth assumption to year 10. Is the company's implied size still believable? Year-10 free cash flow has to come out of year-10 revenue, so hold it against the revenue the company takes in today. The two companies this course has built free cash flow for:

Divide your year-10 figure by today's revenue. That is the share of today's sales the company would have to keep as free cash to deliver your forecast without its top line growing at all. For scale, Apple kept about 24% of its fiscal 2025 revenue as free cash flow ($98.8 billion of $416.2 billion, read 28 September 2026). If your ratio sits far above what the business has ever converted, the forecast needs sales several times today's, and the question becomes whether a market that large exists. If it does not, fade the growth sooner. 2. Now check the margin half against reality rather than against hope. Two companies, published on the same measure:

The operating margin in those two tables differs by an order of magnitude, and neither company is doing anything wrong. Which of the two is your forecast implicitly assuming? 3. Compare a company's history against a direct competitor's. Coca-Cola and PepsiCo, three fiscal years each on the same calendar:

Compute operating income ÷ revenue year by year for both. For your own company and its rival the same rows are on the Terminal's fundamentals tab; change the symbol there:

Open KO.US — fundamentals in the EODHD Terminal

Is your margin assumption in line with what both have actually achieved, or quietly optimistic? A forecast where margins rise every year is betting against one of the most reliable forces in business. 4. Write your forecast as a single sentence a critic could argue with. If it needs three sentences and a caveat, simplify until one honest sentence remains.