What Is Compound Interest?
Compound interest is one of the most powerful concepts in personal finance. Unlike simple interest, which is calculated only on the initial principal, compound interest is calculated on the principal plus all previously accumulated interest. In other words, you earn interest on your interest.
Albert Einstein reportedly called compound interest "the eighth wonder of the world." Whether or not the attribution is accurate, the sentiment is spot-on: over long periods, compound growth can transform modest, regular contributions into substantial wealth.
The mathematical formula is straightforward: A = P(1 + r/n)^(nt), where A is the future value, P is the principal, r is the annual interest rate, n is the number of times interest compounds per year, and t is the number of years. Our calculator uses monthly compounding (n = 12), which is the most common frequency for investment accounts.
How This Calculator Works
This compound interest calculator lets you model a realistic investment scenario. You set four inputs:
- Initial investment — the lump sum you start with today.
- Monthly contribution — the amount you add every month on top of your initial investment.
- Expected annual return — the yearly rate of return you expect from your investments.
- Investment period — how many years you plan to stay invested.
The calculator compounds your returns monthly and adds your contribution at the end of each month. The chart shows two stacked areas: the purple area represents your total contributions (money you put in), while the green area shows the interest earned (money your money made). As time goes on, the green area typically grows much larger than the purple one — that is the power of compounding.
What Return Rate Should I Use?
The return rate you choose depends on how you invest. Here are some historical benchmarks to guide you:
- Global stock index funds (e.g., MSCI World) — have historically returned around 7-10% per year before inflation, averaged over decades. A 7% assumption is considered conservative; 10% is more optimistic.
- Bonds and fixed income — typically return 2-5% annually, depending on maturity and credit quality.
- Savings accounts — in the current European environment, high-yield savings offer roughly 2-4% per year.
- Balanced portfolio (60/40 stocks/bonds) — historically returns around 5-7% per year.
Keep in mind that these are nominal returns. Inflation (typically 2-3% per year in Europe) erodes purchasing power. To estimate real returns, subtract inflation from the nominal rate. For example, 7% nominal minus 2% inflation gives roughly 5% real return.
No one can predict future returns with certainty. Using a range of scenarios (optimistic, moderate, conservative) gives you a more realistic picture of possible outcomes.
The Power of Time
The single most important variable in compound interest is time. Starting early — even with small amounts — gives your money more years to compound. Consider two investors:
- Investor A starts at age 25, invests €200/month for 35 years at 7% annual return. By age 60 they have approximately €346,000 — of which only €84,000 was money they contributed. The rest is compound growth.
- Investor B starts at age 35, invests €200/month for 25 years at the same 7%. By age 60 they have approximately €162,000 — less than half of Investor A, despite only contributing €24,000 less.
The difference is dramatic: those extra 10 years of compounding more than doubled the outcome. This is why financial advisors universally recommend starting to invest as early as possible, regardless of the amount.
Use this calculator to experiment with different time horizons and see for yourself how a few extra years can transform your financial future. When you are ready to track your real investments and see actual compound growth in action, try WonderMoney for free.