Compound Interest Calculator
This compound interest calculator shows exactly how your money grows when you reinvest the interest you earn — the effect often called "interest on interest." Enter a starting amount, an optional monthly contribution, a rate, and a time horizon to project your future balance instantly.
It is built for savers, investors, and anyone planning ahead, from a simple savings account to a long-term index-fund portfolio. You can fine-tune the compounding frequency and contribution timing to match a real account, then read the results as summary cards, growth charts, and a full year-by-year table.
The Compound Interest Formula
The future value of an investment with compound interest and regular contributions is:
FV = P(1 + r/n)nt + PMT × [((1 + r/n)nt − 1) / (r/n)]
Where P is the initial principal, r is the annual interest rate, n is the compounding frequency per year, t is the number of years, and PMT is your regular contribution.
How to Use the Calculator
Enter your initial investment
Set the lump sum you are starting with under Initial Investment. Drag the slider for a quick estimate or type the exact figure in the input field.
Add a monthly contribution
Enter the amount you plan to add each month under Monthly Contribution, or leave it at zero to model a one-time deposit.
Set the rate and period
Adjust the expected annual Interest Rate (0.1% to 30%) and the Investment Period in years (up to 50), or tap a quick preset: 5, 10, 20, 30, or 40 years.
Fine-tune advanced options
Choose a Compound Frequency (daily, monthly, quarterly, semi-annually, or annually) and set Contribution Timing to the beginning or end of each period.
Read the results
Everything updates in real time. Compare Future Value, Total Contributions, and Total Interest, study the charts, and expand the year-by-year table for the full schedule.
Features
Slider and Input Controls
Every amount has both a slider for quick changes and a text field for precise values, and results recalculate the instant you adjust them.
Adjustable Rate and Term
Dial in an annual interest rate from 0.1% to 30% and an investment period of up to 50 years, with one-tap presets for 5, 10, 20, 30, and 40 years.
Five Compounding Frequencies
Switch between daily, monthly, quarterly, semi-annually, and annually to match savings accounts, bonds, CDs, or any investment.
Contribution Timing
Choose whether monthly contributions land at the beginning or end of each period — beginning earns slightly more thanks to an extra period of growth.
Clear Summary Cards
See future value, total contributions, and total interest earned at a glance, so you know how much growth came from compounding alone.
Growth Over Time Chart
An area chart plots your total balance against contributions, with a toggle to show the contributions line for an easy with-and-without-interest comparison.
Balance Breakdown Chart
A donut chart splits your final balance into initial investment, total contributions, and interest, with percentages on hover.
Year-by-Year Table
Expand a collapsible schedule to see cumulative contributions, interest, and balance for every single year of your plan.
Auto-Detected Currency
The calculator detects your locale to show the right currency symbol and number formatting, and scales the default amounts to suit it.
Runs Entirely Offline
All math happens locally in your browser — nothing you type is sent to a server, so your financial figures stay private.
Frequently Asked Questions
How is compound interest calculated?
Compound interest is earned on your principal plus all the interest already added, so each period grows on a slightly larger balance. The standard formula is FV = P(1 + r/n)nt, extended with an annuity term when you make regular contributions. This calculator applies that formula across your chosen period and shows the running totals year by year.
What is the difference between compound and simple interest?
Simple interest is calculated only on the original principal, so it grows in a straight line. Compound interest is calculated on the principal plus accumulated interest, so it grows faster over time because you keep earning "interest on interest." The longer the horizon, the wider the gap between the two.
Daily vs monthly vs annual compounding — which grows faster?
More frequent compounding produces a slightly higher final balance, so daily compounding edges out monthly, which edges out quarterly, semi-annual, and annual. In practice the difference between daily and monthly is usually small. Use the Compound Frequency selector to try each one and see the impact on your numbers.
How do regular monthly contributions affect compound interest?
Each contribution adds to your balance and then earns interest for the rest of the term, so steady deposits can dwarf your starting amount over the years. The balance breakdown chart makes this clear by splitting the final total into initial investment, contributions, and interest earned.
Should I set contribution timing to "Beginning" or "End"?
If you invest at the beginning of each month, that money has one extra month to earn interest, so the final balance is a little higher than investing at the end. Pick the option that matches when you actually move money into the account for the most accurate projection.
What interest rate should I use?
Common benchmarks are roughly 1%–5% for savings accounts, 3%–6% for bonds, and a 7%–10% long-term historical average for the broad stock market, with index funds often cited near 7%–9%. Use a conservative estimate for long-term planning, since real returns vary with markets, fees, taxes, and inflation, none of which this tool factors in.
How accurate are the results, and is my data private?
The projections use standard compound interest math and are accurate for the inputs you enter, though actual returns differ due to market swings, fees, and taxes. Every calculation runs entirely in your browser, so none of your financial figures are uploaded or stored anywhere.
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