Compound Interest Calculator

What compounding does to a balance

The Compound Interest Calculator projects how an initial sum grows when interest is reinvested and compounded over time, with optional monthly contributions added on top. Savers and investors use it to visualize the gap between what they deposit and what their money actually earns.

It is a fast way to plan retirement, a college fund, or an emergency fund with realistic numbers, all computed instantly in your browser.

What the compounding frequency really changes

The frequency is the input people overthink. Compounding more often does help, but the effect is far smaller than the effect of adding money regularly, and the table below shows both at once: $10,000 to start, $500 added every month, 7% a year, held for ten years.

Read the range rather than the winner. Moving from annual to daily compounding - a change of 364 periods a year - is worth about $4,400 over the decade, while the $500 monthly contributions put $60,000 of that balance there in the first place. If you are choosing between two accounts, the contribution schedule and the rate matter more than whether interest is posted monthly or daily.

Choosing the inputs that matter

1. Enter your starting amount in the "Initial principal ($)" field.
2. Set the expected "Annual interest rate (%)" and the "Time (years)" you plan to stay invested.
3. Add any regular deposits in the "Monthly contribution ($)" field.
4. Choose how often interest compounds from the "Compounding frequency" dropdown — Monthly, Quarterly, Semi-annually, or Annually.
5. Click "Calculate" to see the "Future value", "Total invested", and "Interest earned" cards, plus a year-by-year chart and table. Use "Copy result" to save a summary to your clipboard.

Where the arithmetic stops and reality starts

• Compounding monthly yields a slightly higher result than annually at the same rate, so compare the dropdown options side by side.
• Keep the annual rate realistic — a small change produces a large gap over many years.
• Watch the blue-versus-green chart to see how much of the final value is your own deposits versus earned interest.
• Re-run after raising your monthly contribution to see how much sooner you could hit a savings goal.

Three assumptions are baked into the number you see, and knowing them is the difference between a useful estimate and a misleading one. Contributions are assumed to arrive at the end of each period, which is the conservative choice: depositing on the first of the month instead would leave the result slightly higher. The rate is treated as constant for the whole term, while real returns arrive unevenly and a single bad year early on changes everything after it. And the figure is nominal, so it ignores tax on the growth and the fact that prices rise too - $106,639 in ten years will not buy what $106,639 buys today.

The arithmetic itself is the easy part, and it is the part this page does for you: the balance is the starting amount grown at the periodic rate, plus the contributions grown as an ordinary annuity. What the page will not do is tell you which rate to assume, because that is a judgement about the future rather than a calculation. For the percentage work behind the rate itself, the percentage calculator handles the growth, the difference and the share of a total; and for how this page is built and checked, see how we verify a tool.

One sanity check is worth memorising, because it lets you judge a rate without any calculator. Divide 72 by the annual percentage rate and you get the rough number of years a lump sum needs to double: at 7% that is 72 / 7 = 10.3 years, and the tool agrees closely, turning $10,000 into $20,096.61 after ten years. At 3% the same money needs 24 years, which is the whole argument for starting early rather than saving harder later.

FAQ

How is the future value calculated?

The tool compounds the principal and each monthly contribution at your chosen frequency, treating contributions as periodic deposits rather than a single lump sum.

Does it account for taxes or fees?

No. It models pure growth and ignores taxes, inflation, and account fees, so treat the result as an estimate rather than a guarantee.

Why does the interest earned stay at zero?

If the annual rate is zero, the future value simply equals your principal plus contributions, so there is nothing extra to earn.