Compound Interest Calculator
Estimate how an initial balance and regular contributions could grow over time. Adjust the assumptions and compare outcomes instantly.
Model a scenario
Starting amount, up to $1 trillion.
Added monthly, up to $1 billion per month.
Hypothetical annual rate from -100% through 100%.
Use a whole number from 0 through 100.
In 10 years, about 34% of the ending balance is modeled interest.
- Total contributed
- $70,000.00
- Modeled interest
- $36,639.02
- Effective annual yield
- 7.229%
What if the rate changes?
How compound interest is calculated
Compound interest adds earned interest back to the balance, so future interest is calculated on both the original principal and earlier interest. Regular contributions are modeled as an annuity and added at the end of each contribution period.
FV = P(1 + r/n)nt + PMT × (((1 + i)kt − 1) / i)- FV
- Future value
- P
- Initial principal
- r
- Nominal annual rate
- n
- Compounding periods per year
- PMT
- Contribution per period
- i
- Effective rate per contribution period
$10,000 plus $500 per month for 10 years
At a hypothetical 7% nominal annual rate compounded monthly, $10,000 initially plus 120 monthly contributions of $500 produces an estimated ending balance of $106,639.02. The person contributed $70,000 in total; the remaining $36,639.02 is modeled interest.
This is an educational projection. Actual savings or investment returns can change, and taxes, fees, inflation, and market volatility are not included unless explicitly modeled.
What this calculator assumes
- The entered annual rate remains constant for the full period.
- Contributions are made at the end of each selected period.
- All interest remains in the account and compounds.
- Taxes, fees, withdrawals, and inflation are excluded.
- Money values are rounded to the nearest cent for display.
Understanding the Compound Interest Calculator
Compound interest is the engine behind almost every long-term savings and investing plan. It is the interest you earn not only on the money you put in, but also on the interest that money has already earned. Given enough time, that reinvested interest can grow to dwarf your original contributions — which is exactly what this calculator is built to show you.
Use it to turn an abstract idea — “my money grows over time” — into concrete numbers: how much a starting balance plus a monthly contribution could become, and how much of that final figure is your own money versus growth.
Who this calculator is for
- New savers and investorswho want to see why starting early matters more than starting big.
- Parents and long-term plannersmodeling a college fund, a house deposit, or a retirement pot decades away.
- Anyone comparing accountschecking how a higher rate or more frequent compounding changes the outcome.
- Teachers and studentsillustrating the mathematics of growth with real, adjustable figures.
Why it matters
- It makes the cost of waiting visible — delaying contributions by even a few years can noticeably shrink the final balance because that money has less time to compound.
- It separates your contributions from growth, so you can see how much of your goal your own deposits cover and how much the market or interest is expected to do.
- It lets you stress-test assumptions — drop the rate, shorten the horizon, or cut the monthly amount and watch the result respond, which is far more useful than a single headline number.
How to use this calculator
- Enter your initial investment — the amount you already have set aside today. Use 0 if you are starting from scratch.
- Add your regular contribution — the amount you plan to add each month. Consistency here usually matters more than the starting balance.
- Set an expected annual rate of return. For cash savings this is the account’s APY; for long-term investing, many people model a conservative long-run figure and treat it as hypothetical, not guaranteed.
- Choose your time horizon in years, and — under Advanced — the compounding frequency if it differs from monthly.
- Read the projected balance, then use the chart or the year-by-year table to see how the growth accelerates over time.
How to read your result
The headline number is the estimated future value — what your account could be worth at the end of the period under the assumptions you entered. Beneath it, the breakdown splits that value into what you contributed and the interest earned on top.
Pay attention to the shape of the curve, not just the endpoint. Compound growth is slow at first and steep later, because the balance doing the compounding keeps getting larger. This is why the final decade of a long plan often adds more than the first two combined.
- The rate is an assumption, not a promise. Real investment returns vary year to year and can be negative; a constant rate smooths over that volatility.
- This projection ignores taxes, account fees, and inflation unless you account for them yourself. To see growth in today’s purchasing power, use a rate net of expected inflation.
- Compounding frequency helps, but far less than the rate, the time, or the amount you contribute — do not chase daily compounding at the expense of a better rate or a longer horizon.
- Withdrawals break compounding. The model assumes every dollar of interest stays invested.
- Increase the monthly contribution by even a small amount and re-run it — regular contributions are usually the biggest lever you actually control.
- Model a pessimistic and an optimistic rate to get a realistic range rather than a single false-precision figure.
- Automate your contributions so the plan actually happens; the calculator’s math only works if the deposits are made.
Frequently asked questions
What is a realistic rate of return to use?
For a savings account, use the account’s stated APY. For long-term diversified investing, people often model a conservative long-run average and clearly treat it as hypothetical. Because markets are volatile, it is wise to test a range rather than rely on one figure.
How is compound interest different from simple interest?
Simple interest is calculated only on your original principal, so it grows in a straight line. Compound interest is calculated on the principal plus all previously earned interest, so it grows faster and faster over time.
Does compounding frequency (daily vs monthly) make a big difference?
For the same nominal rate, more frequent compounding produces a slightly higher balance, but the effect is usually small compared with changing the rate, the time horizon, or your contribution amount.
Should I use a rate before or after inflation?
If you want the result in today’s purchasing power, subtract your expected inflation rate from the return before entering it (a “real” rate). If you want the raw future dollar amount, use the nominal rate.
Why does starting a few years earlier matter so much?
Because the earliest dollars have the longest time to compound, they end up generating the most interest. Starting earlier — even with smaller amounts — often beats starting later with larger ones.
Are my inputs saved or shared?
No. Every calculation runs in your browser; nothing is stored on our servers or sent to analytics. A shareable link only encodes the numbers you choose to share.
Sources and review notes
- U.S. Securities and Exchange Commission, Investor.gov — Compound Interest Calculator
- Financial Consumer Agency of Canada — Savings accounts and compound interest
Methodology last checked Jul 14, 2026. Formula implementation is covered by deterministic unit tests. No financial professional review is claimed yet.