Investing · compound interest

Compound interest calculator

See what a deposit grows into when interest earns interest. Add a monthly contribution, choose how often it compounds, and watch how much of the final balance is pure growth.

Final balance —

Your money vs. interest earned

Total contributed: —
Total interest earned: —
Interest as % of balance: —
Effective annual yield: —

Balance over time — your money vs. total balance

What you put in Total balance

Year-by-year growth

Balance, what you put in, and interest earned each year

Year	Contributed	Interest	Balance

How compound interest works

Compound interest is the engine behind almost every long-term savings and investing plan. The idea is simple: you earn interest not just on the money you put in, but on the interest that money has already earned. Once that interest is added to your balance, it starts earning interest too — so the balance grows on itself, period after period. Early on the effect is barely noticeable. Given enough time it becomes the dominant force in your account, and the growth quietly overtakes everything you contributed.

The standard formula is A = P(1 + r/n)^nt. Read it plainly: P is your starting principal, r is the annual interest rate written as a decimal (7% becomes 0.07), n is how many times a year interest is added (12 for monthly), and t is the number of years. The r/n piece is the interest rate for a single period, and the nt exponent is the total number of periods. Raising one plus the periodic rate to that power is what produces the curve. This calculator applies that formula to your initial deposit, then adds the future value of your monthly contributions — each one compounding from the month you make it — and reports the combined final balance.

The effect of compounding frequency

Compounding frequency is how often interest is calculated and added back to your balance — annually, semiannually, quarterly, monthly or daily. The more often it compounds, the sooner each chunk of interest starts earning its own interest, so a higher frequency produces a slightly higher final balance at the same stated rate. That is exactly why banks advertise an annual percentage yield (APY) alongside the nominal rate: the APY bakes in the compounding so you can compare accounts fairly.

The catch is that the gains shrink fast as you compound more often. Going from annual to monthly compounding gives a real, visible bump. Going from monthly to daily adds only a sliver. At 7% on a $10,000 deposit over 20 years, the difference between annual and daily compounding is a few hundred dollars — meaningful, but dwarfed by what an extra few years or a higher contribution would add. Use the dropdown to see it for yourself: the bar barely moves between monthly and daily, but stretches dramatically when you add years.

Why time beats rate

If you can only optimize one thing, optimize time. Because compounding is exponential, each additional year works on a balance that is already bigger than the year before, so the last decade of a long investment adds far more in raw dollars than the first. That front-loaded patience is why a saver who starts at 25 with modest amounts routinely ends up ahead of someone who starts at 35 contributing more. A couple of extra percentage points help, but they can’t easily make up for a decade of lost compounding. The honest headline of this calculator is the same one every advisor repeats: the best time to start was years ago, and the second-best time is today.

A worked example

Suppose you deposit $10,000, add $100 a month, expect a 7% annual return compounding monthly, and leave it for 20 years. Over that period you personally put in $34,000 — the original $10,000 plus $24,000 of contributions. But the projected balance lands well above $60,000, because roughly two decades of compounding more than doubles the deposit and grows every contribution along the way. More than half of the ending balance is interest you never deposited. Shorten the term to 10 years and the interest slice collapses; extend it to 30 and it balloons. That sensitivity to time is the whole point of the bar chart.

Common mistakes

Assuming the rate is guaranteed. A savings rate is fixed for a while; an investment return is a long-run average that swings year to year. Model a range, not a single promised number.
Obsessing over compounding frequency. Daily vs. monthly is a rounding error next to your time horizon and contribution. Don’t chase it.
Ignoring inflation. A balance decades away buys less than the same number today. For a real-terms view, run the inflation calculator alongside this one.
Forgetting to keep contributing. The deposit alone is only part of the story — the monthly contributions are what keep feeding the snowball. Stopping them early flattens the curve.
Waiting for the “right” amount. Starting small now beats starting big later, because the early years are the ones that get the most time to compound.

Related tools & guides

Compound interest is the math under every long-term plan. To project a portfolio with a chart over time, use the investment growth calculator. To see whether you’re on track for retirement specifically, switch to the retirement calculator. And you can browse the full set on the calculators page. This calculator is an educational projection, not financial advice — returns aren’t guaranteed, so treat the result as a scenario rather than a promise.

Compound interest calculator FAQ

What is compound interest?

Compound interest is interest earned on both your original money and the interest it has already earned. Instead of paying out a flat amount each year, the balance grows on itself, so each period’s interest is calculated on a slightly larger number than the last. Over a long horizon that snowball effect is what turns steady saving into real wealth — the growth eventually dwarfs the amount you put in.

How is compound interest calculated?

The core formula is A = P(1 + r/n)^(nt), where P is your starting principal, r is the annual interest rate as a decimal, n is how many times a year it compounds, and t is the number of years. This calculator runs that formula on your initial deposit, then adds the future value of your monthly contributions compounded at the monthly rate, and reports the combined final balance.

Does compounding frequency really matter?

Yes, but less than people expect. Compounding more often — daily instead of annually — raises your return because interest starts earning interest sooner. The jump from annual to monthly is meaningful; from monthly to daily it is tiny. At 7% on $10,000 for 20 years, switching from annual to daily compounding adds a few hundred dollars, while doubling your time horizon roughly doubles your balance. Time and rate move the needle far more than frequency.

Why does time matter more than the interest rate?

Because compounding is exponential, extra years multiply your money in a way extra percentage points can’t easily match. Each additional year compounds on an already-larger balance, so the final stretch of a long investment grows the most in absolute dollars. Starting ten years earlier often beats earning a couple of points more, which is why the single best move is usually to start now rather than wait for a higher rate.

What interest rate should I use?

Use a rate that matches the kind of account. A high-yield savings account or CD might pay 4–5%; the long-run historical return of a broad stock-market index fund is often modeled around 7% after inflation. Investment returns are not guaranteed and vary year to year, so treat any single rate as a scenario, not a promise. Run an optimistic and a conservative number and plan around the range.

Is compound interest the same for savings and investments?

The math is the same, but the certainty isn’t. A savings account or bond pays a stated rate that compounds predictably. Stock investments don’t pay a fixed rate — their value compounds through reinvested dividends and price growth that swing up and down, so the “rate” is a long-run average rather than a guaranteed annual figure. This tool models steady compounding, which is a clean way to project either, as long as you remember real markets are bumpier.

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