Compound Interest Calculator
Calculate how your money grows over time — through the power of compound interest.
Tip
The earlier you start, the stronger compound interest works. Just 10 years difference can double the result.
But: where does the X €/Monat come from?
This is where most savings plans fail — not at discipline, but at the lack of overview in the family budget.
- ✓ What's left at the end of the month (before the month begins)
- ✓ Where you can free up X €
- ✓ How your savings plan flows automatically into your budget
What compound interest really means for your family
Most people think of compound interest linearly: 7% per year — that's not much. True, in the first year. But 7% over 30 years doesn't triple your money. It nearly multiplies it by eight. €10,000 becomes around €76,000. €50,000 becomes €380,000. The trick isn't to save a lot. The trick is to start early — and then leave the money alone.
3 examples from everyday family life
1. Child savings plan: €100/month from birth
Total deposits over 18 years: €21,600. Final amount at 6% return: €38,700. Of that, compound interest gain: €17,100. By the age of majority, your child has a cushion for studies, time abroad, or a first apartment.
2. Your own retirement: €300/month over 30 years
Total deposits: €108,000. Final amount at 7%: around €366,000. Of that, compound interest gain: €258,000. That's more than three times your deposits.
3. The early-start effect
Father A saves €200/month from age 25 to 35 (10 years, €24,000 deposited) and has around €280,000 at age 65. Father B saves €200/month from 35 to 65 (30 years, €72,000 deposited) and has around €244,000 at 65. Father A deposits a third — and ends up with more. Every year earlier counts more than every additional euro.
The 4 most common mistakes in compound interest calculations
- Forgetting inflation. €100,000 in 20 years at 2% inflation is only worth €67,300.
- Forgetting taxes. In Germany and Austria, capital gains tax applies. The tax-free allowance helps, but doesn't cover everything.
- Linear expectations. The last 10 years of a 30-year savings plan often earn more than the first 20.
- Underestimating costs. 1% higher TER sounds harmless — over 30 years it costs you around 25% of your final amount.
How to start — in 3 steps
- 1
Determine your savings rate. Use the calculator above. Realistic start: €50–200/month per child, €200–500/month for your own retirement.
- 2
Anchor it in your budget. Make room for the savings rate in your monthly budget. Otherwise it works for one month — and then no longer.
Done in 5 minutes with BudgetHeld → - 3
Open an ETF savings plan. With a broker with TER below 0.3%, on a broadly diversified index like MSCI World.
Frequently asked questions about compound interest
What is compound interest?
Compound interest means you earn interest not only on your deposited money, but also on previously earned interest. Your money grows exponentially rather than linearly.
What return is realistic?
A broadly diversified ETF (e.g. MSCI World) has historically achieved about 7% p.a. before inflation. Conservative estimates use 5–6% p.a.
How often is interest calculated?
This calculator compounds monthly — your savings rate is invested each month and returns are credited monthly.
What is the Rule of 72?
Divide 72 by your interest rate and you'll know how many years it takes to double your money. At 7% = about 10 years.
What's better — a large lump sum or small monthly amounts?
Both work, but starting early matters more than the amount. €200/month from age 25 often beats €400/month from age 35 — thanks to compound interest.
What's the difference between interest and compound interest?
Simple interest: you only earn on your deposited capital. Compound interest: you also earn on previously earned interest. The difference becomes enormous over the years.
Are taxes included?
No. Depending on which country you live in, taxes on capital gains may apply. The actual result may therefore be lower.
How do I start an ETF savings plan?
The easiest way is an app like Revolut — ETF savings plan from €1/month, set up in minutes.
Is a savings plan for the child worth it, or better to save for yourself?
If your emergency fund is in place and your own retirement is running: yes, a child custody account makes sense. Save for yourself first — your child can borrow money, you can't borrow a pension.
When should I start the child savings plan?
As early as possible — ideally in the birth month. Over 18 years, the difference between starting at birth and at age 3 already adds up to €5,000–7,000.
How much should a family save per month?
Rule of thumb: 10–15% of net income for wealth building together. At €4,000 net that's €400–600. Plus €50–100/month per child.
What happens to the savings plan during parental leave or a job change?
Savings plans are flexible — you can pause, reduce, or increase them. A 12-month pause over 30 years costs less than a permanent stop.