Simulkit

Compound interest

Compound interest makes your interest earn interest in turn. This calculator shows how savings grow from a starting balance and monthly contributions, after fees, tax and inflation.

Parameters

Savings

$
$/month
years

Return and fees

%
%

Your balance after 25 years

$430,971

For $160,000 contributed: your money grew 2.7 times.

Interest earned
$270,971
What interest added, before tax.
After tax
$390,325
Gains taxed at 15% when you withdraw.
In today's money
$210,538
Today's purchasing power, with 2.5% inflation a year.
Doubling time
11 years
At 6.79% a year, net of fees.

How it works

How the math works

Every month, the balance grows at the chosen return, net of yearly fees, then the month's contribution is added.

At the end, gains are taxed once. The value in today's money divides the result by cumulated inflation.

Why time matters so much

Growth is exponential: the last years add far more than the first ones. Starting 5 years earlier often matters more than contributing more.

Fees matter too: 1% a year can cost a large share of the final balance over 30 years. Compare two scenarios to see it.

Frequently asked questions

What is the difference between simple and compound interest?

With simple interest, only the starting balance earns interest. With compound interest, interest already earned earns interest too, which speeds up growth over time.

Which return should I use?

It depends on the investment. A savings account pays little but carries no risk. Stocks have historically returned more over long periods, with sometimes deep drops. Test several assumptions.

What is the rule of 72?

A shortcut for the doubling time: 72 divided by the annual return. At 6% a year, money doubles in about 12 years. The calculator shows the exact value.

Are my numbers sent anywhere?

No. The math runs in your browser. Nothing is sent until you click Save or Share.

Other simulators

Rent or buy?

Compare your net worth in both cases, year after year.

Open the simulator

SLA calculator

How much downtime does your SLA really allow?

Open the simulator