Compound Interest Calculator
See how savings grow with compound interest and regular contributions.
100% freeNo sign-up Your inputs stay in your browserPrivacy
Enter a starting amount, an annual rate, and a time horizon to see how compound interest grows your money — with an optional monthly contribution. You'll get the final balance, the total interest earned, and a year-by-year breakdown with a growth chart.
Compounding is the engine of long-term saving: you earn interest on your interest, so balances grow faster the longer you leave them and the more often interest compounds.
📖 Read the guide: Compound Interest Explained: Formula, Examples, and the Rule of 72
Formula
Compound interest
A = P(1 + r/n)^(nt)
P = principal, r = annual rate (decimal), n = compounds per year, t = years. Example: $1,000 at 10% for 1 year, compounded monthly ≈ $1,104.71.
With regular contributions
+ PMT added at the end of each month
Contributions are monthly no matter how often interest compounds — choosing annual compounding does not turn twelve deposits into one. Each deposit is added at the end of its month and earns interest for the rest of the term.
The figures shown
every amount in whole cents
Money is entered and reported to the cent: a starting balance of $99,999,999,999,999.99 is the amount used, not a value a fraction of a cent away from it, and the balance, contributions and interest shown are exact two-decimal amounts that always add up to each other. The cent itself is never estimated. Some calculations can be settled directly with exact arithmetic; others are settled with upper and lower bounds that are guaranteed to contain the true answer, and there a figure appears only once both bounds round to the same cent. Whichever way a calculation is settled, the cent you see is the correctly rounded cent of the formula above — and where it cannot be settled at all, the calculator tells you so instead of picking one.
Examples
| Example | Input | Result |
|---|---|---|
| One year | $1,000 · 10% · annually | $1,100 |
| Monthly compounding | $1,000 · 10% · monthly | $1,104.71 |
| Long term | $1,000 · 7% · 30 yrs | ≈ $7,612 |
How to use the compound interest calculator
- 1Enter your starting amount (principal), the annual interest rate, and the number of years.
- 2Pick how often interest compounds, and add an optional monthly contribution.
- 3Read the final balance, total interest, and contributions; explore the year-by-year table and chart.
What is compound interest?
Compound interest is interest calculated on both your original principal and the interest already added. Unlike simple interest (which is only ever a percentage of the principal), compounding makes the balance grow by larger and larger amounts each period — an effect that becomes dramatic over long horizons.
Regular contributions amplify this further. Adding a fixed amount each month means more principal earning interest over time, which is why steady investing tends to outperform a single large deposit left untouched.
Compounding frequency
The more often interest compounds, the more you earn, because interest starts earning its own interest sooner. Daily compounding edges out monthly, which beats annual — though the differences are modest at typical rates. This calculator lets you compare annual, semi-annual, quarterly, monthly, and daily compounding.
The Rule of 72
A handy mental shortcut: divide 72 by the annual interest rate to estimate how many years it takes for money to double. At 8%, that's about 9 years; at 6%, about 12. It's an approximation, but a remarkably good one for everyday rates.
This tool provides estimates for educational purposes only and is not financial advice. Investment returns are not guaranteed — consult a qualified financial professional before making decisions.
Frequently asked questions
What is the compound interest formula?
A = P(1 + r/n)^(nt), where P is the principal, r is the annual rate as a decimal, n is the number of compounds per year, and t is the number of years.
How often should interest compound?
More frequent compounding earns slightly more — daily beats monthly beats annual — but the difference is small at normal rates. The rate and time horizon matter far more.
What's the difference between compound and simple interest?
Simple interest is calculated only on the principal. Compound interest is calculated on the principal plus accumulated interest, so it grows faster over time.
What return should I assume for long-term projections?
There's no guaranteed figure. Historically, broad stock-market returns have averaged roughly 7% after inflation, but actual results vary widely — treat any projection as an estimate, not a promise.
What amounts, rates and terms does this calculator accept?
Starting amounts and monthly contributions up to $1,000,000,000,000,000, entered to the cent — an amount finer than a cent is refused rather than quietly rounded, because rounding it away would turn two different plans into the same one. Rates from 0% to 100%, and terms from just above zero up to 100 years. Anything outside that gets a message explaining why, instead of a figure.
How exact are the results?
The money you enter and the amounts shown are kept to the cent, and the balance, total contributions and total interest always reconcile with each other exactly. Every cent shown is the correctly rounded cent of the compound interest formula, not an estimate of it. Some cases can be settled with exact arithmetic; others need numerical bounds, and there a figure is shown only when the upper and lower bounds round to the same cent. In the rare case they cannot be narrowed that far, you get a short message explaining it rather than a number the calculator cannot justify. Results are still projections about the future: the figure is proven to match the formula, but no formula can promise a return.
Embed this calculator
Add the Compound Interest Calculator to your own website — free. Copy and paste this snippet:
<iframe src="https://numvella.com/embed/compound-interest-calculator" width="100%" height="460" style="border:1px solid #e2e8f0;border-radius:12px" title="Compound Interest Calculator — Numvella" loading="lazy"></iframe>Related calculators
Savings Goal Calculator
Find the monthly amount to save to hit your target on time.
Mortgage Calculator
Estimate your full monthly mortgage payment — PITI, PMI, and HOA.
Loan Calculator
Monthly payment, total interest, and payoff schedule for any loan.
Amortization Schedule
A complete payment-by-payment breakdown for any loan.