compound-interest-calculator

The difference between annual and daily compounding is much smaller than intuition suggests. The effective annual rate (EAR) formula is: EAR = (1 + r/n)^n − 1, where r is the nominal rate and n is the number of compounding periods per year. At 5% nominal: annual compounding = 5.00% EAR; monthly = 5.116% EAR; daily = 5.127% EAR. The difference between monthly and daily compounding is only 0.011 percentage points — on €10,000 over 10 years, this is approximately €71 extra. The difference between annual and monthly compounding (0.116 percentage points) yields about €650 extra over 10 years. For most investors, optimising compounding frequency matters far less than finding a slightly higher annual rate, investing consistently, avoiding high fees, and starting early.

Compound interest is exactly as powerful for debt as it is for savings — but now working against you. Consumer credit cards in Germany typically charge 12–25% APR, compounding monthly. A €5,000 credit card balance at 18% APR, making only minimum payments of 2% of balance per month: it takes approximately 30 years to repay and costs over €9,000 in interest — nearly double the original balance. BNPL (buy now, pay later) services often show 0% interest but charge retroactive high-rate compound interest if the balance is not cleared within the promotional period. The compound interest calculator works in reverse for debt: enter your loan balance as principal, your APR as the rate, and calculate the total interest cost — a sobering exercise that motivates faster repayment.

APR (Annual Percentage Rate, in German: effektiver Jahreszins) is the standardised cost of borrowing per year, including all mandatory fees — it is the legally required comparison metric for loans in Germany and the EU. For savings, the equivalent concept is AER (Annual Equivalent Rate) or in German: effektiver Jahreszins für Sparer — the actual annual return after compounding effects. The distinction: a savings account offering 3% nominal interest compounded monthly has an AER of 3.042%. A loan at 6% nominal compounded monthly has an APR of 6.168%. German law requires banks to show the effektiver Jahreszins on both loan offers and savings products, making comparison straightforward. Always compare using APR/AER rather than nominal rates when evaluating financial products.

Investing monthly (e.g., €500/month) vs. investing the same amount as a single annual lump sum (€6,000/year at year-end) has two effects. First, the timing effect: monthly contributions start earning returns earlier in the year — on average 6 months earlier than the year-end lump sum. At 7% annual return, this timing advantage is worth approximately 3.5% extra return on each year's contribution (half of 7%). Second, the psychological effect: monthly investing creates habit and removes the temptation to "time" the market by waiting for a dip that may not come. The calculator shows both scenarios side by side. For a 25-year investment horizon at 7% return, monthly investing of €500 generates approximately 2–3% more total return than equivalent annual lump-sum investing — a meaningful but not dramatic difference.