What is compound interest?
Compound interest is interest earned on both the money you originally invested and the interest that money has already earned. Where simple interest only ever pays you on your initial deposit, compound interest reinvests every payout so each new period earns slightly more than the last. Given enough time, that small reinvestment effect produces dramatic, exponential growth.
Albert Einstein is often quoted as calling it "the eighth wonder of the world." Whether or not he ever actually said that, the underlying math is real — and it's the single most important concept to understand if you're trying to build long-term wealth.
The formula behind it
The standard compound interest formula is:
A = P × (1 + r/n)n·t
- A — the future value of the investment
- P — the principal (your starting amount)
- r — the annual interest rate, written as a decimal (7% = 0.07)
- n — the number of times interest is compounded per year
- t — the number of years
When you also add money each month (which most savers do), the calculator above accounts for those contributions period by period — each new deposit then has its own compounding runway.
Why time matters more than amount
The single biggest lever in compound growth isn't the size of your contribution. It's how long the money has to compound. A small amount invested early almost always outperforms a much larger amount invested later, because the early money earns interest on interest for decades longer.
Consider two savers, both aiming to retire at 65:
- Anna invests €250/month from age 25 to 35 (10 years, €30,000 total), then stops contributing — but leaves the money invested.
- Ben waits until age 35, then invests €250/month every month until 65 (30 years, €90,000 total).
At a 7% annual return, Anna ends up with more money than Ben at age 65, despite contributing one-third as much. The 10-year head start is worth more than 20 extra years of contributions later. That's compounding doing the heavy lifting.
The "Rule of 72"
A useful shortcut: divide 72 by your annual return rate to estimate how many years it takes your money to double. At 7%, money roughly doubles every 10.3 years (72 ÷ 7). At 10%, every 7.2 years. At 4%, every 18 years. It's not perfectly precise, but it's accurate enough to do quick mental math at the dinner table.
If you have a specific target in mind — a house deposit, a sabbatical, a kid's tuition — and want to work backwards from the goal to find the required monthly contribution, use our savings goal calculator instead. It solves the same compound interest formula in reverse.
What's a realistic return rate?
The numbers you plug into a compound interest calculator only matter if they reflect what you can actually earn. Here's a rough guide to long-term historical returns, before tax and inflation:
- Savings account — 0% to 4% depending on the rate environment. Safe, liquid, but rarely keeps up with inflation.
- Government bonds (long-term) — 2% to 5% historically. Lower volatility than stocks.
- Broad stock market index (e.g. MSCI World, S&P 500) — around 7% annualised after inflation over the past century. Higher volatility, but the most consistent long-term wealth-builder.
- Real estate — 3% to 6% on capital appreciation alone, more with rental yield, but with significant transaction costs and concentration risk.
If you're modelling a long-term scenario, 7% is a defensible middle estimate for a diversified stock portfolio. If you want to be more conservative, try 5%. If you want to stress-test against poor markets, try 3% — you'll see how much more contribution you'd need to compensate.
Don't forget inflation
A euro in 30 years won't buy what a euro buys today. At 2.5% average inflation, prices roughly double every 29 years. The "Adjust for inflation" toggle in the calculator shows your future balance expressed in today's purchasing power — usually a much smaller number than the nominal figure, but a more honest one.
How to use this calculator
- Starting amount — whatever you already have set aside today.
- Monthly contribution — what you can realistically add each month. Be honest, not aspirational.
- Years invested — how long until you'd want to start drawing on the money. For retirement, this is usually your retirement age minus your current age.
- Annual return — your expected long-term return. 7% is reasonable for a diversified portfolio; pick lower for safer assets.
- Compounding frequency — for most investment accounts, monthly is the right default. Some savings accounts compound quarterly or annually.
- Inflation toggle — turn this on if you want to see results in today's purchasing power instead of the nominal future amount.
Try running three scenarios: a pessimistic case (3% return, lower contribution), a baseline (your realistic plan), and an optimistic case (8% return, higher contribution). The spread between them tells you how sensitive your plan is to assumptions — and where it's worth focusing.
Common mistakes to avoid
1. Starting late
The biggest mistake is waiting. Every year you delay isn't just one fewer year of contributions — it's one fewer year of compounding on every euro that came before. If you can only afford €50 a month right now, start with €50. The amount is less important than the start date.
2. Stopping contributions during downturns
When markets fall, the instinct is to stop investing. But a downturn is exactly when each contribution buys the most shares. Investors who kept contributing through 2008, 2020, and 2022 generally came out far ahead of those who paused — because they bought cheaply when others were selling.
3. Underestimating fees
A 1% annual fee on a portfolio that would have returned 7% net effectively cuts your real growth by around 25% over 30 years. Use low-cost index funds (typically 0.05%–0.30% expense ratios) wherever possible, especially for the core of a long-horizon portfolio.
4. Cashing out early
Compound interest is exponential, which means the biggest gains happen in the last years of the investment horizon, not the first. Cashing out at year 15 of a 30-year plan means you missed not just half the time, but well over half the growth. This is also why our retirement calculator often shows that delaying retirement by just 3 years can have a larger impact on your final outcome than doubling your contributions — those last few compounding years are doing the heavy lifting.
Compound vs. simple interest
Simple interest pays you a flat percentage of your original deposit, every year, forever. €10,000 at 7% simple interest pays €700 every year — no more, no less. After 30 years you've made €21,000 in interest and you'd have €31,000 total.
Compound interest, by contrast, reinvests each year's interest into the principal. The same €10,000 at 7% compounded annually becomes around €76,000 after 30 years — more than three and a half times the simple-interest result. Same starting amount, same rate, dramatically different outcome.
Frequently asked questions
Is the result before or after tax?
The calculator shows pre-tax returns. Tax treatment depends heavily on the account type (pension wrappers, ISAs, taxable brokerage, etc.) and your country of residence. For a realistic after-tax estimate, multiply the "interest earned" portion of your result by your expected marginal capital gains or dividend tax rate, and subtract.
Does it matter when in the month I contribute?
For long horizons (10+ years), the impact of contributing on the 1st versus the 28th of the month is negligible — typically less than a fraction of a percent of the final result. Pick a date and stick with it; automating contributions is far more important than timing them precisely.
What return rate should I use if I'm conservative?
For a diversified portfolio with a heavier bond allocation, 4%–5% nominal is a defensible long-term assumption. For an all-cash savings strategy, use the current interest rate on your account — typically far lower than long-term inflation, which means real (inflation-adjusted) returns are often negative.
What if my contributions increase over time?
This calculator assumes a fixed monthly contribution. In practice, most people increase their contributions as their income grows. A useful rule of thumb: increase your contribution by 3% each year to keep pace with typical wage inflation. Your final balance will be meaningfully higher than the static-contribution result the calculator shows.
Why does the curve get steeper over time?
That's compounding in action. Early on, the absolute amount of interest earned each year is small, because the underlying balance is small. As the balance grows, each year's interest is larger — and that larger interest itself starts earning interest. The result is the upward-curving line you see in the chart: linear contributions on top of an exponentially growing interest layer.