Compound interest calculator - deposit growth

Example: 5,000 EUR to start and 100 EUR a month for ten years

The inputs are a starting amount of 5,000 EUR, a monthly contribution of 100 EUR, an annual rate of 5% and a term of 10 years. The calculator works month by month: the monthly rate is 5 ÷ 12 = about 0.4167% and there are 120 months.

The starting amount grows by a factor of (1 + 0.05 ÷ 12) to the power of 120, which is about 1.647. So 5,000 EUR becomes 8,235.05 EUR, of which 3,235.05 EUR is interest. The monthly contributions are a separate calculation: each 100 EUR grows only for the months that remain, so the first one earns interest for almost 10 years and the last one earns nothing. Together the 120 contributions reach 15,528.23 EUR, of which 3,528.23 EUR is interest.

The future value is the sum of the two: 8,235.05 + 15,528.23 = 23,763.28 EUR. You paid in 5,000 + 120 × 100 = 17,000 EUR in total and the interest earned is 6,763.28 EUR. Notice that the 12,000 EUR of monthly contributions earned more interest over the decade than the starting amount did, even though each one was small.

Time does more of the work than the amount

In the next comparison the starting amount is zero, the contribution 200 EUR a month and the rate 5%. Only the term changes.

• 10 years: 24,000 EUR contributed, future value 31,056.46 EUR, interest 7,056.46 EUR.
• 20 years: 48,000 EUR contributed, future value 82,206.73 EUR, interest 34,206.73 EUR.
• 30 years: 72,000 EUR contributed, future value 166,451.73 EUR, interest 94,451.73 EUR.

What the comparison shows

Stretching the term from 10 to 20 years doubles the contributions, but the final sum grows 2.6 times. After 30 years interest is 57% of the total, more than everything you paid in yourself. Compounding looks modest in the early years and becomes visible only over longer periods.

The same goes for the rate. A one-off 10,000 EUR over 15 years grows to 18,203.02 EUR at 4%, 24,540.94 EUR at 6% and 28,489.47 EUR at 7%. The single point between 6% and 7% is worth 3,948.53 EUR over 15 years. That is why, over a long horizon, it matters how much of the return goes on fees: if costs take 1 percentage point out of a 7% return, you should run the numbers at 6%.

For a quick mental estimate use the rule of 72: divide 72 by the annual rate to get roughly the number of years it takes to double. At 6% that is about 12 years. The calculator agrees: 10,000 EUR at 6% grows to about 20,500 EUR in 12 years.

How to read the result and what the calculator assumes

If a bank pays deposit interest once a year or at the end of the term, the real result at the same nominal rate will be a little lower than shown here, because interest starts earning interest later.

• Interest is added every month. A nominal 5% therefore yields slightly more than 5% in a year: 10,000 EUR grows to 10,511.62 EUR, an effective annual rate of about 5.12%.
• The monthly contribution is added at the end of each month, so the first one starts earning interest only in the following month.
• The rate is the same for the whole term. In reality investment returns swing from year to year and deposit rates can change.
• The term is rounded to whole months: 2.5 years means 30 months.
• Total contributions are the starting amount plus all monthly payments; interest earned is the gap between the future value and those contributions.

What the result does not show

The future value is a nominal euro amount, not purchasing power. If prices rise by an average of 2% a year over ten years, 23,763.28 EUR is worth roughly 19,494 EUR in today's money. To estimate real growth, subtract expected inflation from the rate and run the calculation again.

The calculator ignores taxes and fund or account fees and makes no distinction between a deposit and an investment. A deposit rate is fixed by contract; an investment return is only an assumption and can turn negative. Use the result to compare scenarios, not as a promise.