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How much your money grows over time

Adjust the values and see, month by month, how much comes from your pocket and how much from interest. Compare against a benchmark and see your real buying power at the end.

Educational simulation. Not investment advice. Results are estimates based on the data you enter and do not replace advice from a qualified professional.

Reviewed on Nov 1, 2026

Used to show results in today’s money and for the inflation lines in the comparison chart.

Main scenarioReference rate

Type the yearly rate of the alternative you want to compare, such as a savings account or a government bill. Rates stay constant over the whole period.

Your inputs stay only on this browser and are not sent anywhere. Untick to delete them.

Final balance, main scenario
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Milestones of your simulation

How your investment grows

Hover or tap the chart to see each year.

Tap the chart to see the values.

Your scenario against the reference rate

Same contributions, different returns. Tick other benchmarks to add them.

Tap the chart to see the values.

What if the rate or the period change?

Final balance of the main scenario. Your simulation is in the center; tap any cell to simulate that case.

below your simulationabove

Year-by-year table

Next step

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The summary lists your inputs, results, formulas, assumptions and sources. The full report also includes the charts and interpretations, in color. In the print dialog, choose Save as PDF.

Method, assumptions and sources

Formulas

Monthly rate: i = (1 + yearly rate)^(1/12) − 1
Monthly balance: B(m) = B(m−1) × (1 + i) + C (contribution C at month end; raised every 12 months)
Tax at withdrawal = tax rate × (balance − total put in)
In today’s money = net balance ÷ (1 + inflation)^years
Fee impact (optional): net return = (1 + gross return) × (1 − management fee − custody fee) − 1; cost of fees = balance without fees − balance with fees

Assumptions and limits

  • Rates stay constant over the whole period; past or projected returns do not guarantee future results.
  • Monthly compounding, contributions at the end of each month and a yearly contribution increase.
  • Tax is simplified: one rate on the gain, at withdrawal; fees must be taken out of the rate you type.
  • The reference rate is typed by you and kept constant.

Official sources and references

  • Brealey, Myers & Allen. Principles of Corporate Finance, ch. 2 (time value of money). McGraw-Hill.
  • U.S. SEC, Investor.gov compound interest (concept reference).

Frequently asked questions

What is compound interest, in plain words?

It is interest earned on your money plus on the interest you have already earned. Each month the balance gets a little bigger, so the next month’s interest is bigger too. Over long periods this snowball effect, not your deposits, becomes most of the final balance.

What is the compound interest formula?

FV = P × (1 + i)^n, with the rate and the period in the same unit. With equal monthly contributions C at the end of each month, add C × [(1 + i)^n − 1] ÷ i. This calculator applies the same logic month by month, which also lets the contribution grow each year.

How much does $500 a month grow in 20 years?

At 10% a year, about $359,000: $120,000 from your deposits and about $239,000 from interest. In the default example on this page ($5,000 upfront, $500 a month rising 3% a year, 7% return), the balance after 20 years is about $338,700, of which $166,200 is your own money.

How do I turn a yearly rate into a monthly rate?

Use (1 + yearly rate)^(1/12) − 1. For example, 10% a year equals 0.797% a month. Simply dividing by 12 overstates the monthly rate, because monthly interest compounds within the year.

What does “in today’s money” mean?

It is the future amount with inflation taken out: what that money would buy today. In the default example, the $338,700 balance in 20 years, with 3% yearly inflation, buys what about $187,500 buys today. That is still more than the $166,200 you put in, so your money grew faster than prices.

Which rates should I type in?

For the main scenario, use the yearly return your investment realistically offers after fees. For the reference rate, use a safe alternative you can actually get, such as a high-yield savings account, a certificate of deposit or a government bill. For long periods, be conservative: the calculator keeps every rate constant from start to finish.

Does it include taxes and fees?

Fees should be taken out of the rate you type. Taxes are applied only if you enter a tax rate on gains: it is charged once, at the end, on the gains only. Tax rules differ a lot by country and by account type, so check the rules that apply to you or ask a tax professional.

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How compound interest works

With compound interest, the return earned each month starts earning interest in the following months as well. That is the famous “interest on interest”. At first the difference from simple interest is small, but over time it becomes the largest part of your savings. The chart above shows exactly this: the band of what you put in grows in a straight line, while the interest band speeds up.

The formula

Without contributions, the future value is FV = P × (1 + i)n, where P is the starting amount, i the rate per period and n the number of periods. With equal monthly contributions (C) at the end of each month, add C × [(1 + i)n − 1] ÷ i. Rate and period must use the same unit: to turn a yearly rate into a monthly one, use (1 + yearly rate)1/12 − 1. So 10% a year equals 0.797% a month, not 0.833%.

Example: $500 a month for 20 years

Investing $500 every month at 10% a year, you put in $120,000 over 20 years and end up with about $359,130. Almost two thirds of the result comes from interest. Starting with $10,000 already invested, the total rises to about $426,405.

PeriodTotal put inBalance at 10%/yrShare from interest
5 years$30,000$38,28122%
10 years$60,000$99,93240%
20 years$120,000$359,13067%
30 years$180,000$1,031,42283%

The rule of 72

To estimate how long money takes to double, divide 72 by the yearly rate. At 10% a year it doubles in about 7.2 years; at 6%, in 12 years. It is a handy shortcut to compare options in your head.

Real return: take inflation out

An investment earning 10% a year with 4% inflation has a real return of (1.10 ÷ 1.04) − 1 = 5.8% a year. For long-term goals such as retirement, think in real terms: the result in today’s money tells you whether it will be enough.

Compound interest also works against you

Credit card balances grow by the same logic, at much higher rates. A $1,000 balance at 2% a month becomes about $1,268 in a year if nothing is paid. That is why paying off expensive debt is often the best investment available.