Compound Interest Calculator

Compound interest is the return you earn on your returns. Each period, your gains are added to the balance, and the next period’s growth is calculated on that larger base. The difference looks small in year one and decisive by year twenty. That is the entire idea, and it is why time in the market matters more than timing it.

The calculator below projects what a lump sum plus regular contributions could grow to at a steady annual return. Change any number and the result updates instantly — the point is to test assumptions, not to predict the future.

What it measures

The calculator answers one question: if you start with a given amount, add a fixed sum each month, and earn a constant annual return, what is the balance at the end of the period? It reports three numbers: the projected future value, the total you contributed out of pocket, and the difference — the interest earned. That split matters. It shows how much of the final number came from your deposits and how much came from compounding.

How it works

The math follows the standard future-value formula. Your initial investment grows at the periodic rate for every period. Your monthly contributions form an annuity — each deposit compounds for the months remaining after it lands. Contributions are treated as arriving at the end of each month.

The compounding-frequency setting changes how the annual rate is converted into a monthly rate. A nominal 7% compounded monthly is not identical to 7% compounded annually; the calculator converts the nominal rate to an equivalent monthly rate using the effective-rate relationship, so the frequency choice adjusts the result the way a real account would.

Each input, explained

Initial investment. The lump sum already available — an existing balance, a bonus, an inheritance. It starts compounding from day one, which gives it the heaviest weight per dollar in the final result.

Monthly contribution. The amount added every month. For most savers this is the number that matters most, because it is the input fully under their control. Small changes here compound for decades.

Expected annual return. The single most uncertain input. The 7% default is an editable example, not a forecast — try 4% and 9% side by side to see how wide the range of outcomes is. For a sense of what long-horizon market returns have actually looked like, see The Case for Boring Investing: What Index Data Shows Over Decades.

Years invested. The holding period. Compounding is back-loaded: the last five years of a twenty-year run typically add more than the first ten, because the base is larger.

Compounding frequency. How often the quoted rate is applied — monthly is standard for most savings and brokerage cash, annually for some bonds and CDs. The effect is small at low rates and larger at high ones.

Worked example

Take the defaults: $10,000 to start, $500 added each month, a 7% annual return, 20 years, monthly compounding.

The monthly rate is 0.07 / 12, about 0.5833%, applied over 240 months. The initial $10,000 grows to roughly $40,387. The 240 monthly deposits total $120,000 out of pocket ($130,000 including the starting lump sum); with each deposit compounding for its remaining months, the contribution stream grows to roughly $260,463. Combined future value: about $300,851. Subtract the $130,000 contributed and interest earned is about $170,851 — more than half the final balance came from compounding, not deposits.

Now change only the return to 5% and the total falls to roughly $233,000. Change it to 9% and it rises to roughly $394,000. The inputs you control — contribution and time — are the reliable levers; the return is the volatile one. If instead you have a fixed target in mind, work backward with the savings goal calculator.

Limitations

This is a projection, not a prediction. It assumes a constant return every month, which no real investment delivers — actual returns vary, and sequence matters. It ignores taxes, account fees, expense ratios, and inflation, all of which reduce real purchasing power. It assumes you contribute on schedule without interruption. Treat the output as a way to compare scenarios, not as a balance you can count on.

Frequently asked questions

What is the difference between simple and compound interest?
Simple interest pays only on the original principal. Compound interest pays on the principal plus all previously earned interest, so growth accelerates over time. Nearly every investment account compounds; simple interest mostly appears in short-term loans and some bonds.

Why does compounding frequency change the result?
A nominal annual rate can be applied in installments. Compounding monthly means each month’s interest starts earning its own return sooner than if the rate were applied once a year. The calculator converts your chosen frequency to an equivalent monthly rate so the comparison is exact.

Is the projected value guaranteed?
No. The return field is an assumption you type in. Real investments fluctuate, and a steady 7% never happens in practice — some years beat it, some years fall short. The tool shows what a given assumption implies, not what will occur.

Why does starting early matter so much?
Because every year of compounding multiplies the years before it. Money invested at 25 has roughly twice as long to compound as money invested at 35, and the gap widens at higher returns. The worked example above shows the same effect: most of the final $300,851 comes from growth, not deposits.

Does this account for inflation?
No. The future value is in nominal dollars. A balance of $300,851 in twenty years will buy less than $300,851 buys today. Run the result through an inflation adjustment separately if you want the figure in today’s purchasing power.

How does this relate to a mortgage?
It is the same mathematics viewed from the other side of the table. The compounding that builds your balance also builds a lender’s interest charges — see it from the borrower’s side in the mortgage calculator.

For education only — not financial or tax advice. Projections are hypothetical and do not predict actual investment performance. Consider speaking with a qualified financial professional before making decisions with your money.