Compound Interest Calculator
Enter your starting balance, interest rate, compounding frequency, and optional recurring deposits to see exactly how your savings grow. Get the future balance, total interest, effective annual yield (APY), the time needed to double your money, a year by year schedule, and a growth chart that compares compounding against simple interest.
Formula
Worked example
$10,000 at 7% compounded monthly for 10 years with $200 added monthly: the lump sum grows to about $20,097 and the deposits add roughly $34,600, for a future balance near $54,700. The APY is about 7.23%, and at that yield money doubles in about 9.9 years.
How the Calculation Works
The calculator applies the standard compound interest formula, where your balance is recalculated at each compounding period so that previously earned interest itself earns interest. Every recurring deposit you make is also compounded forward from the date it lands to the end of the term, which is why steady contributions add so much over time. Compounding frequency matters: daily compounding produces a slightly higher balance than monthly or annual compounding at the same stated rate, because interest is reinvested more often. The effective annual rate (APY) captures this, turning a nominal rate plus a compounding frequency into the real yearly growth your money actually sees.
How to Use This Calculator
Enter your starting balance (principal), the annual interest rate as a percentage, how often interest compounds (daily, monthly, quarterly, or annually), and the length of time in years and months. If you save regularly, enter a deposit amount, choose how often you add it, and pick whether it lands at the start or the end of each period. Deposits made at the start of a period earn one extra period of interest, so the timing toggle changes the result. You can also set an annual deposit increase to model raises or stepped-up saving. Pick your currency, the math is identical regardless of currency, and the results update instantly.
What the Results Tell You
The future balance is what your account is worth at the end of the term. Total deposited is everything you put in (principal plus every contribution), and total interest is the gap between the two, the money compounding earned for you. The effective annual rate (APY) is the true yearly growth after compounding, which is the right number to compare savings accounts on. Time to double estimates how long, at this rate and frequency, it takes your money to double if left to grow on its own. The year by year schedule shows deposits, interest, and the running balance for each year, and the chart contrasts compound growth with a simple interest line so you can see exactly how much compounding adds.
What Affects Your Result the Most
Time is the most powerful variable: doubling your investment horizon has a far greater effect than doubling your principal, because compounding is exponential rather than linear. The annual interest rate is the second largest driver, even a one percentage point difference produces meaningfully different outcomes over a decade or more. Regular deposits amplify growth substantially, since each one begins compounding from the moment it is added rather than sitting idle until the end. Compounding frequency and contribution timing make a smaller but real difference, especially over long horizons.
Limitations and Important Caveats
This calculator assumes a fixed, constant interest rate for the entire period, which is rarely the case for real savings accounts, money market funds, or investment portfolios. It does not account for taxes on interest income, account fees, or inflation, all of which reduce your real purchasing power over time. Results are projections for general planning purposes only and should not be treated as a guarantee of future returns. For decisions involving significant sums, consult a licensed financial adviser.
Nominal rate vs effective annual yield (APY)
| Nominal rate | Monthly APY | Daily APY | Doubling time (daily) |
|---|---|---|---|
| 2% | 2.02% | 2.02% | 34.3 yr |
| 5% | 5.12% | 5.13% | 13.9 yr |
| 7% | 7.23% | 7.25% | 9.9 yr |
| 10% | 10.47% | 10.52% | 6.9 yr |
How a stated rate turns into real yearly growth as compounding gets more frequent.
Frequently asked questions
How often should interest compound to maximize my savings?
The more frequently interest compounds, the higher your ending balance at any given nominal rate. Daily compounding yields slightly more than monthly, which yields more than annual. In practice the difference between daily and monthly compounding is small: a 5% rate compounded daily produces an effective annual rate of about 5.127%, versus 5.116% compounded monthly.
Does it matter whether I deposit at the start or end of each period?
Yes. A deposit made at the start of a period earns one extra period of interest compared with one made at the end, so start of period contributions (an annuity due) finish slightly ahead. The gap is small per period but compounds over a long horizon, which is why this calculator lets you choose the timing.
What is the difference between APR and APY in compound interest?
APR (Annual Percentage Rate) is the stated nominal rate before compounding is applied within the year. APY (Annual Percentage Yield) reflects the actual return after compounding, and it is always equal to or greater than the APR. When comparing savings accounts, APY is the more meaningful figure because it captures how much your balance truly grows in one year. This calculator reports the APY for your chosen rate and frequency.
How long will it take to double my money?
The rough Rule of 72 says divide 72 by the interest rate: at 8% your money doubles in about nine years. This calculator computes the exact doubling time for your effective annual rate using logarithms, which is more precise than the rule of thumb, especially at higher rates.