Tools
Compound Interest Calculator
Most compound calculators show you a big final number. That is the least interesting part. The number that matters is the split — how much of that balance you paid in, and how much the market handed you for free.
Final balance
€0
You paid in
€0
Compounding paid you
€0
The crossover
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Illustrative arithmetic at a constant rate, compounded monthly. Excludes tax, inflation, fees and sequence risk — real markets deliver −18%, then +26%, then +4%, not a steady 7%. Not financial advice.
Why the crossover is the whole point
Compounding is famously described as a snowball, which is a nice image and slightly misleading — it suggests smooth, visible acceleration. What actually happens is that almost nothing seems to happen for a long time, and then the arithmetic quietly takes over.
There is a specific year in which the growth on your portfolio first exceeds everything you have ever contributed to it. Before that year, you are the engine. After it, your money is. Move the sliders above and watch where it lands.
This is why the single most valuable variable is not how much you invest — it is how early. Ten years of extra compounding at the end of a horizon is worth more than a decade of doubled contributions at the start, because those final years are operating on the largest balance you will ever have.
What this deliberately ignores
- Sequence risk. Real markets do not deliver 7% every year. They deliver −18%, then +26%, then +4%. The average can be identical and the experience completely different — particularly if you need to withdraw during a bad stretch.
- Fees. They come straight off the return. A 1.5% fee turns a 7% assumption into 5.5%, and over 30 years that is not a rounding error.
- Inflation and tax. Both reduce what the final number is actually worth to you.
None of that makes the exercise useless. It makes it a floor for your thinking, not a forecast of your future.