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.
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Compound interest
What it measures
Projects how a starting amount plus regular monthly contributions grows when returns are reinvested. Interest starts earning interest of its own, so growth accelerates the longer the money stays invested.
When to use it
Use it to plan long-term saving and investing: retirement accounts, index fund plans, or a child's education fund. Try doubling the time horizon and watch what happens to the interest share.
Watch out for
The rate you enter is an assumption, not a promise. Markets don't return a smooth 7% every year, and inflation quietly eats into the real value of the final number.
Compound interest
How savings and regular contributions grow over time.
Most of the balance now comes from interest rather than deposits.Over 50% from interest
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, so 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.
Common 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.
Last reviewed July 2026