Compound Interest Calculator
Project how a starting amount plus monthly contributions grows over time, and see how much of the final balance comes from interest alone. Free and private — nothing leaves your browser.
Compound Interest Calculator
Watch contributions snowball over time.
How it works
FV = P(1 + r/12)^(12t) + PMT × [((1 + r/12)^(12t) − 1) / (r/12)]The projection has two engines running side by side. Your starting amount P compounds monthly at the annual rate r — each month's interest is added to the balance, and next month's interest is calculated on that bigger balance. Meanwhile every monthly contribution PMT starts its own compounding clock the moment it lands. Early contributions do far more work than late ones, which is why the interest share of the final balance keeps accelerating: the calculator shows you exactly when the money you earned starts out-earning the money you deposited.
Worked example
Start with €10,000, add €300 a month, and assume 7% a year for 25 years. The lump sum alone grows to about €57,300. The €90,000 of contributions grow to roughly €243,100. Together that's about €300,400 from €100,000 actually paid in — over €200,000 of it is compounding, not saving. Cut the horizon to 10 years and the interest share drops from two-thirds to about a quarter: time is doing most of the lifting.
Frequently asked questions
How is compound interest calculated?→
Interest is added to your balance, and from then on that interest earns interest too. With monthly compounding, the formula is FV = P(1 + r/12)^(12t), plus the future value of any monthly contributions. This calculator handles both parts for you.
What rate of return should I assume?→
The S&P 500 has returned roughly 10% per year on average over the long run, or about 7% after inflation. For a conservative plan, many people model 5-7%. The rate you pick is an assumption, not a guarantee.
Why does starting early matter so much?→
Because growth compounds on itself, the last few doubling periods create most of the wealth. €200/month at 7% grows to about €52,000 in 15 years — but €244,000 in 40 years. Time in the market beats almost everything else.
No black boxes — the exact formula is shown above · Last reviewed July 2026