Skip to content
Calculadora Capital

Compound Interest Calculator

Compound interest is interest that earns interest on the interest already accrued. By reinvesting what you earn, your money grows faster over time. Use the calculator to see how much you could accumulate, in euros, from an initial amount plus monthly contributions.

Future value
€17,175
Total invested
€13,000
Interest earned
€4,175

Year-by-year breakdown

YearInterestBalance
1€79€2,279
2€224€3,624
3€437€5,037
4€722€6,522
5€1,084€8,084
6€1,525€9,725
7€2,051€11,451
8€2,665€13,265
9€3,371€15,171
10€4,175€17,175

Educational estimate, not financial advice. Returns are not guaranteed.

Video: how to use the calculator

What compound interest is

Unlike simple interest (which is earned only on the initial capital), compound interest is also earned on the interest already credited. Each period the balance grows, and so does the base the next interest is calculated on. It is "interest on interest": over short horizons the difference is small, over long horizons it becomes enormous.

The formula and how to calculate it

With monthly compounding and regular contributions: FV = P·(1+i)^n + PMT·((1+i)^n − 1)/i, where P is the initial capital, PMT the monthly contribution, i the annual rate divided by 12 and n the number of months. The calculator applies this month by month: just enter the initial amount, the contribution, the rate and the term, with no manual maths.

What moves the result most

Time is the most powerful factor: the earlier you start, the more compounding cycles occur, and the final years earn the most because they act on the balance already built up. The interest rate and how regularly you contribute come next. Delaying the start costs more than it seems: you lose one of the most valuable cycles, the last one.

The rule of 72: how long until money doubles

A handy mental shortcut: divide 72 by the annual rate (as a percentage) to get the approximate number of years capital takes to double. At 4% a year it doubles in about 18 years, at 6% in 12 years, at 8% in 9 years. It is an approximation, not an exact calculation, but it shows how higher rates and longer horizons reinforce each other. You can confirm the exact figure in the calculator.

Where you meet compound interest in practice

In Portugal the effect shows up in term deposits that capitalise interest, in Certificados de Aforro (accrued interest is added to capital quarterly), in PPR retirement plans and in accumulating funds and ETFs, which automatically reinvest their income. The same mechanism also works against you in debt: on a credit card, unpaid interest itself accrues interest.

Gross interest, taxes and inflation

Two factors reduce the real result. First, taxes: in Portugal interest income is generally taxed at a flat 28% withholding rate, so what compounds year after year is the after-tax interest. Second, inflation: a balance growing 5% a year with 2% inflation only gains about 3% in purchasing power. Use a prudent rate and think in real terms.

Worked example

With €1,000 to start and €100 per month, at a 5% annual rate over 10 years, you invest €13,000 in total and end with about €17,175: over €4,000 comes from compound interest alone. Keeping the same plan, after 20 years you would have about €43,816 (€25,000 invested) and after 30 years about €87,694 (€37,000 invested). At 30 years, more than half of the final value is interest: that is the typical acceleration of compounding.

Frequently asked questions

How do you calculate compound interest?
Use the formula FV = P·(1+i)^n + PMT·((1+i)^n − 1)/i: initial capital P, monthly contribution PMT, annual rate divided by 12 (i) and number of months (n). In practice, just enter the initial amount, the monthly contribution, the rate and the term in the calculator and you get the final value and the interest earned, year by year.
What is the difference between simple and compound interest?
Simple interest is always earned only on the initial capital; compound interest is also earned on accumulated interest, so it grows faster over time. At 5% a year, €1,000 always earns €50/year in simple interest; with compounding, the interest in year 10 is already earned on a much larger balance.
What is the rule of 72?
A shortcut to estimate how many years money takes to double with compound interest: divide 72 by the annual rate as a percentage. At 6% a year, capital doubles in about 12 years; at 4%, in 18; at 8%, in 9. It is a useful approximation for comparing scenarios quickly.
How much does €10,000 earn with compound interest?
It depends on the rate and the term. At 4% a year with monthly compounding and no top-ups, €10,000 becomes about €14,908 in 10 years and about €22,226 in 20 years, before taxes. You can simulate your own amount, rate and term in the calculator.
How often is interest compounded?
This calculator uses monthly compounding, common in savings and investment products. The more frequent the compounding (annual, monthly, daily), the larger the effect, although the gap between monthly and annual is modest next to the impact of time and the rate.
Where do I get compound interest in practice?
In term deposits that capitalise interest, in Portuguese savings certificates (Certificados de Aforro), in PPR retirement plans and in accumulating funds and ETFs, which reinvest their income. Also in debt, but against you: unpaid credit-card interest is added to the balance and accrues interest itself.
Is compound interest taxed in Portugal?
Yes. Interest is capital income generally taxed at a flat 28% rate withheld at source. In practice, what compounds year after year is the after-tax interest, so it is worth projecting savings in net terms.
What happens if I start 5 years later?
You lose the most valuable compounding cycles. With €100 per month at 5% a year, 25 years of contributions build about €59,551; starting 5 years later (20 years), about €41,103. You contribute €6,000 less, but end with €18,400 less.
Are the results guaranteed?
No. It is an educational estimate using a fixed rate. Real returns vary and this is not financial advice.

Related calculators & reading

Embed this calculator

Paste this code on your site to show the calculator. It includes an attribution link.

Language
Theme
Colour

Preview

Free to use. The code auto-adjusts its height.

Sources

Author: Thorben Rasmus Idel · Reviewed by: Nahar Geva · Last reviewed: 2026-07-11