ToolNestFree Compound Interest Calculator
Last updated: October 10, 2026
Quick answer: Compound interest is interest earned on both your original money and the interest it has already earned — the snowball that lets small, consistent investing beat large, late deposits. This free compound interest calculator shows exactly how that snowball grows: enter your starting amount, a monthly contribution, your expected annual return, and the number of years, and it computes the future value, your total contributions, and the interest earned — plus a year-by-year growth table so you can watch every step of the journey. Switch the compounding frequency (monthly, quarterly, annually, even daily) to see how it changes the outcome. Everything runs in your browser: your numbers never leave your device.
Estimates only — real returns vary, and this ignores taxes, fees, and inflation. Formula: A = P(1+r/n)nt plus the future value of your monthly contributions.
✓ Free forever · ✓ No signup · ✓ Tool inputs stay in your browser — never sent to our servers.
How to Use the Compound Interest Calculator
- Enter your starting amount — the lump sum you're investing today (enter 0 if you're starting from scratch).
- Add a monthly contribution — most compounding wealth comes from consistent deposits, not the starting amount.
- Set your expected annual return and the number of years. A common long-term planning figure is 7% for stock-market investing.
- Pick how often interest compounds — monthly is the standard default; try daily versus annually to see how much the schedule matters.
- Read your results: future value, total contributions, interest earned, the visual breakdown, and the full year-by-year table.
Why Use Our Compound Interest Calculator?
- Monthly contributions built in — most free calculators only grow a lump sum; this one adds your recurring deposits, which is how real saving works.
- Year-by-year growth table — see the balance, contributions, and interest earned at the end of every year, not just one final number.
- Five compounding frequencies — annually, semi-annually, quarterly, monthly, or daily, with the standard future-value math for each.
- Visual contribution-vs-interest split — an instant bar showing how much of your final balance you funded versus what compounding did.
- Rule-of-72 doubling callout — your money's doubling time at the chosen rate, the mental shortcut behind every compounding comparison.
- Private by design — all math runs in your browser; your financial numbers never leave your device.
Common Use Cases
- Retirement planning: Test whether your monthly 401(k) or IRA contributions put you on track for your target number by retirement age — adjust the rate to model conservative versus optimistic markets.
- Comparing savings accounts: See what a 4.5% high-yield account does over 5 years versus a 0.5% checking balance. The gap is bigger than it looks once compounding runs.
- College savings: Model a 529 plan from birth to age 18 and check whether the monthly deposit actually covers projected tuition.
- Understanding debt in reverse: The same math powers credit-card balances working against you — run it with a 24% rate to see why carrying a balance is so expensive.
- Teaching the savings habit: Show a teenager what $50/month from age 16 does by age 40 — the year-by-year table makes the lesson visual instead of abstract.
Compound Interest Calculator vs Alternatives
- Investor.gov's compound interest calculator: The US government's calculator is trustworthy — it takes monthly contributions and shows a year-by-year table. Ours matches that, adds a contributed-vs-interest split and compounding-frequency options, on a clean ad-free page with no signup.
- Bankrate / NerdWallet calculators: Polished and credible, with helpful explainers — on ad-supported pages that nudge you toward accounts. Ours is a single clean page for a quick honest answer with the year-by-year table, no ads or signup.
- Excel / Google Sheets FV function: The FV function does the exact same math and lives where your budget already is — great if you're modeling. This page is faster for a one-off 'what if' and shows the year-by-year story without building the spreadsheet.
- Thin single-input blog calculators: Many finance blogs embed a principal-rate-years box with no contributions and no breakdown. Fine for a glance, but they hide where the growth actually comes from — which is usually the monthly deposits.
If you're already modeling in a spreadsheet, keep using FV() — it's the same math, and your model is the source of truth. For a quick, honest answer with monthly contributions, the year-by-year table, and no ads or signup, this page is the fastest way to see your money's future.
Frequently Asked Questions
What is compound interest?
Compound interest is interest calculated on both your original principal and the interest that has already accumulated — so your money earns returns on its own returns. Over long periods this creates exponential growth: the balance doesn't just grow, it grows faster each year.
What is the compound interest formula?
A = P(1 + r/n)nt, where A is the future value, P the starting principal, r the annual rate as a decimal, n the number of compounding periods per year, and t the years. With monthly contributions, this calculator adds the future value of an annuity on top: monthly deposit × (12/n) × [((1 + r/n)nt − 1) ÷ (r/n)].
How much difference do monthly contributions make?
A lot — often more than the starting amount. Example: $10,000 invested once at 7% for 20 years grows to about $38,700. Add $200/month and the same 20 years ends near $144,600 — the contributions plus their compounding dwarf the original deposit. That's the snowball in action.
Does compounding frequency matter much?
Less than you'd think. At 7% over 20 years on $10,000, annual compounding gives $38,697 while daily gives $40,547 — a few percent. Time in the market and the size of your contributions move the needle far more than the compounding schedule.
What is the Rule of 72?
Divide 72 by your annual rate to estimate how many years it takes your money to double: at 7%, about 10.3 years; at 10%, about 7.2 years. The calculator shows your exact doubling time under the results, so you can sanity-check any return assumption instantly.
Is compound interest the same as simple interest?
No. Simple interest pays only on the original principal, so growth is a straight line. Compound interest reinvests each period's earnings, so growth curves upward: after 20 years at 7%, simple interest on $10,000 earns $14,000 while compounding earns $28,697.