Compound Interest Calculator

What Is the Compound Interest Calculator?

The Compound Interest Calculator is a free online tool that shows how money grows when the interest it earns is added back to the balance and starts earning interest too. That snowball effect is called compounding, and over long periods it is often the biggest part of a savings or investment balance. You enter a starting amount, how much you plan to add each month, an annual rate and a time span, and the tool projects the balance month by month until the end of the period.

The result is not just one number. You get the future balance, the total you deposited, the total interest earned, the effective yearly rate, an estimate of how long it takes money to double, a stacked growth chart and a schedule with one row per year. The page recalculates the moment you change any field, so you can compare scenarios quickly without pressing a button.

What the Calculator Can Do

• Starting deposit plus monthly contributions. Model a lump sum, a regular saving plan, or both together.
• Deposit timing. Choose whether each monthly deposit lands at the start or the end of the month. Start-of-month deposits earn one extra month of interest each time.
• Five compounding frequencies. Yearly, half-yearly, quarterly, monthly and daily (365 times a year).
• Yearly raise in deposits. Increase your monthly contribution by a fixed percentage every year, the way many people raise their saving when their salary grows.
• Inflation adjustment. Enter an inflation rate to see the future balance in today's money, plus an extra line on the chart.
• Effective yearly rate. The tool converts the nominal rate and compounding frequency into the effective annual rate, often called APY.
• Doubling time. An exact estimate of how many years it takes a balance to double at the effective rate, without extra deposits.
• Breakdown bar. A coloured bar splits the final balance into initial deposit, later deposits and interest, with percentages.
• Year-by-year schedule. Deposits and interest for each year, running totals and the closing balance, with a one-click CSV download.
• Eight currencies. USD, EUR, GBP, INR, BDT, CAD, AUD and JPY formatting. The currency only changes how numbers are shown, not the maths.

Why Compound Interest Matters

Simple interest pays you only on the money you put in. Compound interest pays you on your money and on the interest already earned. In the early years the difference looks small, which is why many people underestimate it. Over 20 or 30 years the gap becomes large. With the example values on this page, a 10,000 starting deposit plus 200 a month at 7 percent for 20 years reaches roughly 144,573. Of that, 58,000 is money you put in and the rest, more than 86,000, is interest. The chart makes this visible: the green interest band starts thin and ends up larger than the blue deposit band.

The same maths works against you on debt. A credit card balance that compounds monthly grows the same way if it is not paid down. Understanding compounding helps with both sides of personal finance: saving earlier and paying off high-interest debt sooner.

How the Calculation Works

The calculator runs a month-by-month simulation instead of a single formula, which lets it handle deposit timing and yearly raises correctly. For each month it does three things in order:

1. If deposits are set to the start of the month, it adds the monthly contribution.
2. It applies one month of growth. The monthly growth factor is (1 + r / n) ^ (n / 12), where r is the annual rate as a decimal and n is the number of compounding periods per year. This gives exactly the same yearly result as compounding n times a year.
3. If deposits are set to the end of the month, it adds the monthly contribution after the growth.

After each 12 months the monthly contribution is increased by the yearly raise percentage, if you set one. The effective yearly rate is (1 + r / n) ^ n - 1. Doubling time is ln(2) / ln(1 + effective rate), which is more accurate than the rule of 72 shortcut. When inflation is set, the balance for each year is divided by (1 + inflation) ^ years to express it in today's money.

All of this happens in your browser with JavaScript. Nothing you type is sent to a server or stored.

How to Use the Calculator

1. Pick the Currency you want numbers shown in.
2. Enter your Initial Deposit. Use 0 if you are starting from nothing.
3. Enter a Monthly Contribution and choose whether it is paid at the month start or month end.
4. Type the Annual Rate you expect, for example the rate on a savings account or a long-term average return you are planning around.
5. Set the number of Years, from 1 to 80.
6. Choose the Compounding frequency. Bank accounts usually compound daily or monthly. If you do not know, monthly is a sensible default.
7. Optionally add a Yearly Raise in Deposit and an Inflation rate.
8. Read the results: the large future balance at the top, the four summary cards, the breakdown bar and the chart. Scroll down for the yearly schedule and click Download CSV to keep a copy.

Use Reset to example to return to the starting values at any time.

Practical Ways to Use It

• Retirement planning. See how a monthly pension or index fund contribution could grow over several decades.
• Comparing savings accounts. Enter two rates and frequencies one after the other and compare the effective yearly rate.
• Starting early versus starting late. Run 30 years and then 20 years with the same deposit to see what a ten-year delay costs.
• Testing a saving habit. Check what an extra 50 or 100 a month adds by the end of the period.
• Education or house deposit goals. Adjust years and contribution until the future balance reaches your target.
• Teaching. The chart and schedule make the idea of interest on interest easy to explain to students or family.

Limitations

This calculator uses one fixed rate for the whole period. Real investments such as shares and funds go up and down from year to year, so the actual balance will differ from a smooth projection even if the long-term average matches. Treat the result as a planning estimate, not a promise.

Taxes, account fees, fund charges and withdrawals are not included. If your interest is taxed every year, or a fund charges an annual fee, your real growth rate is lower than the headline rate. A simple way to allow for this is to enter a slightly lower rate. The tool also assumes every deposit is made on time; skipped months are not modelled. Negative rates are accepted, down to just above minus 100 percent, so you can see how a balance shrinks, but doubling time is only shown when the effective rate is above zero.

What to Do and What Not to Do

Do use realistic rates. For a savings account, use the rate the bank actually offers. For long-term investing, use a cautious average rather than the best year you can remember. Do add an inflation rate when planning for something decades away, because a large future number can buy much less than it sounds. Do download the CSV if you want to build on the numbers in a spreadsheet.

Do not treat the projection as financial advice or as a guaranteed outcome. Do not compare a nominal rate from one product with an effective rate from another; use the Effective Yearly Rate card to compare like with like. Do not forget fees and taxes when the gap between two options looks small. If you are making a major decision, talk to a qualified financial adviser.

Frequently Asked Questions