Level 5 · Flexibility
Compound-interest calculator: what your savings become
Compounding is the engine of wealth building: returns earn returns of their own. See how savings and time turn into a portfolio.
Nominal growth before tax and inflation. Rule of thumb: 72 ÷ rate ≈ doubling time in years.
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How to go about it
- Enter starting amount and contribution. Enter what you have invested today and how much you add each month.
- Choose rate and duration. Set the expected interest rate per year and how long you stay invested.
- Read the split. The calculator shows the final amount, the sum of your contributions and the interest earned — and how the two shift over time.
Frequently asked questions
What is compound interest?
Returns are reinvested and earn returns themselves. Over long periods that interest often makes up the larger share of the final amount — not the money you paid in.
What is the rule of 72?
72 divided by the interest rate gives roughly the doubling time in years. At 7 percent, capital doubles about every ten years.
Are tax and inflation included?
No. The calculator shows nominal growth before tax. For real purchasing power, subtract inflation from the rate; for a net view, apply German capital-gains tax to the returns.
What does starting five years later cost?
More than the five missing contributions – because the earliest years compound the longest. The calculator shows the difference in final amount if you start five years later.
Related
From projection to real number
A projection shows what is possible. Whether you actually keep up the contribution only shows over time. In sum you set goals with a savings rate and watch your progress and your net worth develop. Today you bring your transactions in via CSV import; the automatic bank connection (PSD2) is in the works.
sum is in early access.
The first step toward your financial goals is the overview — that's exactly what we're building sum for. Secure your access early.