Compound Interest Calculator
Project how your savings grow over time with compound interest and optional regular contributions.
Example
Start with $10,000 at a 7% annual rate, compounded monthly, for 10 years, adding $200 every month.
The $10,000 grows to about $20,096.61, while the $24,000 of contributions grows to about $34,616.96. Final balance is roughly $54,713.58, of which $20,713.58 is interest earned.
How it works
Enter your starting principal, annual rate, years, how often it compounds, and any recurring contribution. The tool applies A = P(1+r/n)^(nt) for the principal and the future-value-of-a-series formula for contributions, then shows your final balance and interest earned.
Good to know
This Compound Interest Calculator projects how a lump sum and a stream of regular deposits grow when interest is reinvested over time. You enter a starting principal, an annual rate, a number of years, how often interest compounds, and an optional recurring contribution, and it returns a single final balance broken into three parts: your original principal, everything you contributed, and the interest those amounts earned. It is built for savers, investors, and anyone weighing a long-term goal like retirement, a house deposit, or an emergency fund who wants a quick sense of how money snowballs.
It is most useful when you want to compare scenarios rather than predict an exact future number. Try nudging one input at a time, such as adding a few years, raising the monthly deposit by $50, or switching the rate down, and watch how the final balance and interest-earned figures shift. Because the math runs instantly in your browser, it is a fast way to see whether time or contribution size moves the needle more for your situation.
Read the result as three stacked bars: principal is what you put in once, contributions is the running total you added over the years, and interest is the growth on top. When interest starts to rival or exceed your contributions, that is compounding doing the heavy lifting, which typically happens only over long horizons. Keep these points in mind:
- The rate is a single fixed annual percentage, so it cannot model real-world ups and downs or a variable savings APY.
- The figures are nominal and do not subtract inflation, taxes, or fees, which all reduce what the balance is actually worth.
A practical tip: enter the rate as a flat number rather than a percentage of itself, and match your contribution frequency to how you really save. If you invest monthly, leave contributions on monthly so the future-value math lines up with your habit. Treat the output as a directional estimate to inform planning, not a guaranteed forecast.
Frequently asked questions
Does compounding frequency really change my returns?
Yes, slightly. For a given annual rate, compounding more often (e.g. daily vs annually) produces a higher final balance because interest is added to the balance sooner and then earns interest itself. The effect grows with higher rates and longer time horizons.
Are contributions added at the start or end of each period?
This calculator assumes contributions are made at the end of each period (an ordinary annuity). Depositing at the start of each period would earn a bit more interest, so treat the result as a slightly conservative estimate.
Is my data uploaded anywhere?
No — this calculator runs entirely in your browser; nothing is uploaded.
Is this financial advice?
No. These are educational estimates — consult a qualified financial professional before making decisions.
People also ask
What is the formula for compound interest with regular contributions?
The principal grows by A = P(1 + r/n)^(nt), where P is the starting amount, r the annual rate, n the compounds per year, and t the years. Recurring deposits are added using the future value of a series formula, which sums each contribution plus the interest it earns until the end of the term.
How is compound interest different from 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 previously earned interest, so the balance grows faster over time, with the gap widening the longer the money is invested.
Does adding money monthly beat one large lump sum?
It depends on the amounts and timing. A lump sum invested earlier has more time to compound, while regular contributions spread deposits out and add new principal over the years. This calculator lets you model both at once by setting a starting principal and a recurring contribution together.
How long does it take to double money with compound interest?
A rough shortcut is the Rule of 72: divide 72 by the annual interest rate to estimate the years to double. For example, at a 7% rate money roughly doubles in about 10 years, though the exact figure varies with compounding frequency.
Does this calculator account for taxes and inflation?
No. The results are nominal figures based only on your rate, time, and contributions. Real purchasing power would be lower after accounting for inflation, and investment or interest gains may be subject to taxes depending on the account type and jurisdiction.
What annual rate should I use for a long-term estimate?
That is a personal choice tied to where the money sits. Savings accounts and CDs reflect their stated APY, while long-term stock-market projections often use a single assumed average rate. Because the tool uses one fixed rate, lower it if you want a more conservative estimate.
Why does compounding monthly give more than compounding annually?
More frequent compounding adds interest to the balance sooner, so that interest begins earning its own interest earlier in the year. For the same stated annual rate, switching from annual to monthly or daily compounding produces a modestly higher final balance, and the difference grows with higher rates and longer terms.
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