Compound interest: how small monthly amounts grow

Investing a little every month feels slow at first. The magic is that, given time, your growth starts to earn growth of its own.

🔎 Use a real ETF’s numbers (optional)

Starting amount € Added each month € Number of years Expected annual return % Adjust for inflation % (2% is a common example — shows the result in today’s money)

Projected value—

You put in—

Growth—

At year — · — — you’d have put in —, growth added —. Drag across the chart (or use ← → keys) to read any year.

Money you added Growth

See the key milestones (every 5 years)

Year	Put in	Growth	Balance

How this works: an educational scenario, not a forecast. We compound monthly and add your monthly amount each month. “Expected annual return” is your own assumption — pick a cautious one; real markets are bumpy and can fall. “Adjust for inflation” simply restates the result in today’s spending power. The fee figure includes the yearly fund fee (TER) and the growth those fees would otherwise have earned. The fund comparison repeats each fund’s last-12-months return every year — a rough illustration only, which real funds never do. Not advice.

Finance Hamster provides educational information about ETFs and investing. It is not investment, tax, or legal advice, and not a recommendation to buy or sell any security. Markets carry risk; do your own research or consult a licensed adviser.

👉 Change the numbers above — it’s your money, your assumptions.

Your money makes money - then that makes money

Start with €1,000, add €150 a month, and assume 6% a year. In the tool above, notice how the orange 'growth' part of each bar is small for the first years and then overtakes the money you actually paid in. That crossover is compounding doing the heavy lifting.

Time matters more than the amount

Drop the number of years from 30 to 15 and watch the projected value fall by far more than half. Then put it back and instead halve the monthly amount — the result barely moves by comparison. The lesson: how long you stay invested is usually a bigger lever than how much you add each month.

Honest caveat: returns are not a straight line

Real markets do not deliver a smooth 6% every year — some years are strongly positive, some are sharply negative, and your money can be worth less than you put in, especially over short periods. The calculator assumes a steady return to show the shape of compounding, not to predict your outcome. An fund reinvests dividends for you, which helps this snowball along.

Common questions

What return should I assume?

Nobody can tell you the future. Many people sanity-check with a modest long-run assumption and then test lower numbers too. The point of the tool is to explore scenarios, not to promise any of them.

What if I can only invest a small amount?

Small and regular still compounds. The tool lets you set any monthly amount, including very small ones, so you can see the long-run shape for your own situation.