Compound Interest Calculator

What is a compound interest calculator?

A compound interest calculator shows what your money becomes when its returns are reinvested and start earning returns of their own. That's the difference between simple interest, which pays only on your original amount, and compound interest, which pays on your amount plus all the interest already added. Over a few years the gap is modest; over a few decades it's the difference between a tidy sum and a small fortune.

The tool above models the full picture. Enter a starting amount, an annual rate and a number of years, choose how often interest compounds (daily, monthly, quarterly or yearly), and — the part most calculators skip — add a regular contribution if you plan to keep paying in. It returns your future value, the total you actually contributed, the interest earned, a year-by-year table, and a simple stacked chart showing contributions versus interest growing apart over time.

It all runs in your browser. Your savings figures are personal, so nothing you enter is uploaded — the projection is calculated on your own device, live as you adjust the inputs.

How to use it

1. Enter your starting amount. This is the principal you begin with. It can be zero if you're starting from scratch with contributions only.
2. Set the annual rate. Use the expected return as a percentage — 7 for 7%. For cash savings this is the quoted interest rate; for investments it's an assumed average.
3. Choose the number of years you'll stay invested.
4. Pick the compounding frequency. Savings accounts often compound daily or monthly; many models use yearly.
5. Add a regular contribution (optional). Enter the amount, choose monthly or yearly, and set whether you pay at the start or end of each period.
6. Read the results. The headline is your future value; the cards show what you contributed and what compounding added. Scroll the chart and table to watch it build.

The compound interest formula

For a single lump sum with no further contributions, the classic formula is:

A = P · (1 + r/n)^(n·t)

• A is the final amount (future value).
• P is the principal (starting amount).
• r is the annual interest rate as a decimal (7% → 0.07).
• n is how many times interest compounds per year (yearly = 1, monthly = 12, daily = 365).
• t is the time in years.

The exponent n·t is the engine: it's the total number of compounding periods, and growth rises exponentially with it. That single exponent is why doubling your time horizon does far more than doubling your final balance.

The contributions (annuity) formula

Most people don't invest once and walk away — they keep paying in. Adding a fixed contribution each period turns the calculation into a lump sum plus an annuity. The future value of those contributions is:

FV_contributions = PMT · ((1 + i)^N − 1) ÷ i

where PMT is the contribution per period, i is the periodic rate (r ÷ n), and N is the total number of periods (n × t). If you contribute at the start of each period rather than the end, multiply that result by (1 + i) — each payment gets one extra period to grow. Your total future value is the lump-sum result plus this annuity result. The calculator computes it period by period, which handles every compounding frequency and contribution timing exactly.

A worked example

Say you start with $10,000, expect a 7% annual return compounded monthly, contribute $200 a month, and stay invested for 20 years.

• The original $10,000, left alone, grows to about $40,387 — it roughly quadruples.
• Your contributions total $200 × 12 × 20 = $48,000, and with monthly compounding they grow to roughly $104,200.
• Together the account reaches about $144,600.

Now total what you actually put in: $10,000 + $48,000 = $58,000. The remaining ~$86,600 is interest — more than you contributed. The year-by-year table makes the turning point visible: in the early years contributions dominate the balance, but somewhere around the midpoint the interest line overtakes them and never looks back.

The power of compounding — and starting early

Compounding rewards time more than any other input, and it does so unfairly: the first dollars you invest have the most years to multiply, so they end up worth the most. This is why a saver who invests modestly in their twenties often ends up ahead of one who invests much more in their forties.

A simple illustration: at 7%, money roughly doubles every ten years (the "Rule of 72" says years-to-double ≈ 72 ÷ rate). A dollar invested at age 25 doubles about four times by age 65 — becoming sixteen dollars. The same dollar invested at age 45 doubles only twice — becoming four. Same dollar, same rate; four times the result, purely from starting twenty years sooner. The practical takeaway is blunt: the best time to start was years ago, and the second best time is now.

How compounding frequency changes things

More frequent compounding helps, but less than people expect. Compounding daily instead of yearly lets interest start earning interest a little sooner, nudging the final balance up — but at ordinary rates the difference is small. On a 7% lump sum over 20 years, switching from yearly to daily compounding adds well under one percent to the final figure. It's a genuine effect, just a minor one.

What this means in practice: don't choose an account purely for its compounding frequency. The headline rate, how much you contribute, and how long you leave it invested swamp the frequency every time. Use the toggle in the calculator to see for yourself — flip between daily and yearly and watch how little the future value moves compared with, say, adding two years to the term.

Tips for making compounding work for you

• Start now, even small. A modest amount with a long runway beats a large amount with a short one. Time is the input you can't buy back.
• Automate contributions. Regular, automatic paying-in removes the temptation to skip months and keeps the annuity engine running.
• Reinvest everything. Compounding only works if returns stay in the account. Withdrawing interest turns compound growth back into simple growth.
• Mind real returns. This model ignores tax and inflation. A 7% nominal return with 3% inflation is closer to 4% in real buying power — useful to remember when reading the headline, and our Percentage Calculator makes quick work of adjustments like that.
• Run scenarios. Try a higher rate, a longer term and a bigger contribution one at a time to see which lever moves your future value most for your situation.

Private, instant and always free

This compound interest calculator runs entirely in your browser. There are no accounts, no limits, and your savings figures never leave your device. Bookmark it for whenever you're weighing a savings or investment plan — or our Loan & Mortgage Calculator when you're on the borrowing side instead. It turns an amount, a rate and a time horizon into a clear future value and a year-by-year story of how compounding builds it, while your numbers stay yours.