Compound Interest Calculator

How it works

Compound interest is interest that earns interest. Each period, the balance grows by the rate, and the next period earns on that larger balance. The future value follows this formula:

• P — the starting amount
• r — the annual interest rate
• n — how many times a year it compounds
• t — the number of years

With the defaults, $10,000 at 5% compounded monthly grows to about $16,470 in 10 years, of which roughly $6,470 is interest. Compounding more often helps a little: daily beats monthly beats annual, but the rate and the time matter far more than the frequency.

✓ Every result is checked against independent reference math. See how we test the calculators →

A worked example: $10,000 at 5% for 10 years

Drop $10,000 into an account paying 5% a year and leave it completely alone for ten years, with interest compounding monthly. By the end it has quietly grown to $16,470 without you adding a single dollar of your own.

Of that total, $10,000 is your original deposit and $6,470 is pure interest, earnings stacking on top of earlier earnings month after month. That snowball is the whole point of compounding: the account pays you on the interest it already paid you.

Compounding frequency nudges the result too. The same money compounded annually reaches $16,289, while daily compounding gets to $16,487, a gap of $198 between the slowest and fastest schedules. Try your own balance, rate, and timeline to see where you land.

Time is the real engine

The longer money compounds, the more the growth curve bends upward, because each year’s interest joins the principal that earns the next year’s interest. That is why starting early matters more than starting big: a modest sum with decades to grow can overtake a larger sum that starts late. To add regular deposits on top of a lump sum, use the Future Value calculator instead.

The Rule of 72, a shortcut for doubling

You do not need a calculator to estimate how fast money doubles. Divide 72 by the interest rate, and the answer is roughly the number of years the balance takes to double at that rate.

• At 5%, 72 divided by 5 is about 14 years to double.
• At 8%, 72 divided by 8 is about 9 years.
• At 10%, 72 divided by 10 is about 7 years.

The rule is an approximation, most accurate for rates between about 4% and 12%, but it is close enough to be genuinely useful for a quick gut check. It also runs in reverse: if you want your money to double in ten years, you need roughly a 7% return. Framing growth as doubling time makes the value of a slightly higher rate obvious, because shaving even a year or two off each doubling compounds across a lifetime into a large gap.

The cost of waiting

Because time is the biggest lever, delay is expensive in a way that is easy to underestimate. Picture two people who each invest $10,000 once and then leave it untouched at 7% a year.

• Ana invests at 25 and leaves it for 40 years. Her $10,000 grows to roughly $150,000.
• Ben invests the same $10,000 at 35 and leaves it for 30 years. His grows to roughly $76,000.

The only difference is a ten-year head start, yet Ana ends with nearly double the balance. Those first years look unremarkable while the numbers are still small, but they are doing the heaviest lifting, because they are the years all the later interest gets to compound on top of. The lesson is not that Ben did badly.

It is that the cheapest way to build wealth is simply to start sooner, even with a smaller amount.

Compounding cuts both ways

The same math that grows savings also grows debt, and on the borrowing side it works against you just as relentlessly.

• On your side: savings accounts, index funds, and retirement accounts all compound in your favor. Leaving the returns to reinvest, rather than spending them as they arrive, is what keeps the curve bending upward.
• Against you: credit cards compound too, often at 20% or more, so an unpaid balance snowballs the same way a good investment does, only pointed at your wallet. Clearing high-interest debt is effectively a guaranteed return equal to that rate.

The practical takeaway is to get compounding working for you and shut it down where it works against you: pay off high-rate debt first, then leave your investments to compound undisturbed for as long as you possibly can.

What moves the final number

Four inputs decide how much a lump sum becomes, and they do not pull with equal force. Knowing which lever does the most work tells you where to put your attention.

• Time does the most. Because growth compounds, an extra decade usually beats an extra chunk of principal. It is also the one lever you cannot buy back once it is gone.
• Rate is next. A point or two sounds small, but it changes the doubling time and stacks up across many years. Higher rates usually carry more risk, so reach for them with care.
• Principal is linear. Doubling the starting amount doubles the result, no more and no less, which is why a big head start still loses to a long one.
• Frequency barely matters. Daily versus monthly compounding changes the total by little more than a rounding error next to the other three.