Estimate savings growth with compounding

The job

You want a rough answer to a concrete question: if I put ¥30,000 a month into something returning 4% a year, what is it worth in 20 years? A compound interest estimate gives you that number in a few seconds — good enough for planning, not a forecast.

Reading the three outputs

Final balance is what the account would hold. Total paid in is your own money — the balance minus this is the growth. Interest earned is the part compounding produced; watch how it overtakes contributions in long scenarios.

Try the same inputs with the rate at 3% and 5%. The gap between those outcomes is usually larger than people expect, and it is the honest answer to “what does one point of return actually change.”

What the estimate leaves out

The tool assumes a constant rate and monthly compounding with contributions at month end. Real returns swing year to year, taxes and fees reduce gains, and inflation shrinks what the final number buys. Treat it as a comparison tool — scenario A versus scenario B — not a prediction of any account.