Investing · General financial education

Long horizons and uncertainty

A 40-year model with a constant rate is a calculation assumption. It is not a forecast of what your money will do.

In more detail

Time amplifies reinvested returns in the model, and also the effect of costs, gaps and swings. A smooth curve assumes a constant return that real paths do not deliver that way.

Background

K(n) = K(0)×(1+r)^n with fixed r ignores sequence risk, inflation, tax, contributions and lifespan. Two paths with the same average return can finish very differently. Finmon calculators remain scenarios, not a forecast.

An example

The same 5% model over 40 years looks large. Lower the assumption or insert a weak first decade and the picture changes sharply, without the formula being “wrong”.

A common misconception

A 40-year model with a large ending value is a forecast.

It is a calculation under constant assumptions, not a view of your path.

Source: Investor.gov · Compound growth. Last reviewed: 2026-11-01. Editorially reviewed on 2026-11-01. Next review 2027-11-01. Version 0.2.0. Finmon explains. It does not advise. Not investment, tax or legal advice.