Finance & business
Compound Interest Calculator
Compound interest is the interest you earn on both your original money and the interest it has already earned — so a balance grows faster every year. Enter a starting amount, rate, term and optional monthly contribution to see your future value, a year-by-year growth chart, and how long your money takes to double. Example: $10,000 at 7% for 10 years grows to about $20,097.
See how an investment grows over time. Enter your starting principal, annual interest rate, number of years and how often interest compounds — plus optional monthly contributions — to get the future value, the total you put in, and the interest earned. This is the engine behind long-term savings vehicles Americans use every day, from a high-yield savings account or certificate of deposit to a 401(k), IRA or brokerage account. Because compounding rewards time more than any other factor, the calculator makes it easy to see why starting to invest in your twenties can dwarf a larger but later effort. It updates as you type, runs entirely in your browser, and shows the exact formula below.
Investment details
balance at the end of the term
Assumes a constant rate, contributions made at the start of each month, and no taxes or fees.
How your balance grows
Each bar is your balance at the end of that year, split into the money you put in and the interest compounding on top. Watch the interest portion overtake your deposits — that crossover is the entire case for investing early.
How long until your money doubles?
At 7% a year, the Rule of 72 estimates your money doubles about every 10.3 years. Compounded at your selected frequency, the precise doubling time is 10.2 years.
The Rule of 72 is the fast mental shortcut: divide 72 by your rate. It's accurate for rates between roughly 5% and 12% and is the quickest way to sanity-check any investment claim.
Year-by-year breakdown
The full schedule of what you've deposited, the interest accumulated, and your balance at the end of each year.
| Year | Deposited | Interest earned | Balance |
|---|---|---|---|
| Enter your details to see the schedule. | |||
How the compound interest calculator works
Compound interest means you earn interest on your interest, not just on your original deposit. The more often interest compounds (monthly or daily versus annually), the faster the balance grows. This tool grows your starting principal with the compound interest formula, then adds the future value of any regular monthly contributions on top.
Compound interest formula
A = P × (1 + r ÷ n)^(n × t)where: P = starting principal r = annual interest rate (as a decimal, e.g. 7% = 0.07) n = compounding periods per year (1, 2, 4, 12 or 365) t = time in years A = future value of the principal
Future value of monthly contributions
FV = PMT × [ ((1 + i)^m − 1) ÷ i ] × (1 + i)where: PMT = monthly contribution i = monthly rate = r ÷ 12 m = total months = t × 12 (the trailing × (1 + i) reflects start-of-month deposits)
Total interest earned is the future value minus everything you deposited (principal + all contributions).
Notes & assumptions
- Contributions are assumed to be made at the start of each month and to grow at the same annual rate.
- Does not account for taxes, inflation, fees or variable returns.
- Calculations are for general information only — verify before relying on them.
Worked example
Start with $10,000, earn 7% a year compounded monthly, and leave it untouched for 10 years. The principal alone grows to about $20,097 — it roughly doubles. Now add a $200 monthly contribution: you deposit an extra $24,000 over the decade, but it grows to around $34,800, pushing your total balance to roughly $54,900. Of that, nearly $21,000 is interest you never deposited. Run it for 30 years instead of 10 and the same habit can grow past $300,000 — the clearest illustration of why time in the market matters so much.
Frequently asked questions
What's the difference between compound and simple interest?
Simple interest is calculated only on your original principal, so it grows in a straight line. Compound interest is calculated on your principal plus all the interest already earned, so your balance grows faster and faster over time. Nearly every U.S. savings account, CD and investment uses compounding, which is why this calculator models it rather than simple interest.
How does compounding frequency affect growth?
The more often interest is added to your balance, the sooner it starts earning interest of its own. Daily compounding edges out monthly, which edges out quarterly and annual — but at typical rates the difference is modest. For example, 7% compounded daily versus annually adds only a fraction of a percent to your effective return. Frequency matters far less than the rate and the number of years.
What's the difference between APY and APR?
APR (annual percentage rate) is the simple yearly rate before compounding is taken into account, while APY (annual percentage yield) reflects what you actually earn once compounding is included. On savings accounts and CDs, banks advertise APY because it's the higher, more accurate figure for a depositor. The more frequently interest compounds, the more APY exceeds the stated APR.
What is the Rule of 72?
The Rule of 72 is a quick mental shortcut for estimating how long it takes money to double: divide 72 by your annual rate of return. At 7% a year, 72 ÷ 7 is roughly 10, so your money doubles about every 10 years. It's an approximation that works best for rates between about 5% and 12%, but it's handy for sanity-checking investment expectations.
Does this calculator account for taxes and inflation?
No. The future value shown is a pre-tax, nominal figure that assumes a steady rate of return. In reality, interest in a taxable account may be reduced by federal and state taxes, and inflation lowers the purchasing power of your future balance. Tax-advantaged accounts like a Roth IRA or 401(k) change the picture, so treat the result as an illustration rather than a guarantee.
How do monthly contributions change the result?
Adding a regular monthly contribution is the single biggest lever most people control. Each deposit starts compounding the moment it lands, so a steady habit usually ends up dwarfing the starting principal over a few decades. Enter a monthly contribution above and the calculator adds its future value (as a start-of-month annuity) on top of your principal's growth, and the year-by-year table shows exactly how the contributions and interest stack up.
How long does it take to double your money?
Divide 72 by your annual return — the Rule of 72. At 6% your money doubles in about 12 years, at 8% about 9 years, and at 10% roughly every 7 years. The "time to double" panel above shows both the Rule-of-72 estimate and the precise figure for the rate and compounding frequency you entered. Higher rates and more frequent compounding both shorten the doubling time, but the rate matters far more.
Is daily or monthly compounding better?
Daily compounding earns slightly more than monthly, which earns slightly more than quarterly or annual, because interest is added to the balance sooner and starts earning its own interest. But the gap is small: at 7%, daily versus annual compounding adds only about a quarter of a percent to your effective yearly yield. Choose the frequency your account actually uses, but don't expect frequency to outweigh the rate or the number of years.