Compound Interest Calculator
See how savings or investments grow with compound interest: monthly and yearly contributions or withdrawals, nine compounding options, tax, inflation and a year-by-year schedule.
How it works
A = P × (1 + r/n)^(n×t) + contributions · contributions: PMT × ((1 + i)^N − 1) ÷ i, with i = (1 + r/n)^(n/12) − 1 per month · APY = (1 + r/n)^n − 1 · continuous: A = P × e^(r×t)
P is the starting amount, r the annual interest rate as a decimal, n the number of times interest is compounded per year and t the time in years. Monthly contributions (PMT) are added at the end of each month, or at the start if you choose, and then earn interest at the equivalent monthly rate i, so N = months. Under More options you can add a yearly lump sum, raise contributions each year, switch the regular amounts to withdrawals, take tax out of the interest as it is earned, and restate the result in today’s money with an inflation rate. The effective annual rate (APY) shows what the compounding is worth: 7% compounded monthly earns the same as 7.229% paid once a year. The rate comparison option re-runs the same plan at higher and lower rates so you can see how sensitive the result is.
Worked example
$10,000 at 7% compounded monthly for 20 years, with $200 added at the end of each month: the monthly rate is 0.07 ÷ 12 = 0.5833%. The starting amount grows to $10,000 × 1.005833^240 = $40,387.39 and the contributions to $200 × (1.005833^240 − 1) ÷ 0.005833 = $104,185.33, for a final balance of $144,572.72. You put in $58,000, so $86,572.72 is interest — about 60% of the balance. The APY is 7.229%, and the starting amount alone doubles in about 10 years. At 2.5% inflation the final balance is worth about $88,229 in today’s money.
Frequently asked questions
What rate should I assume?
Use the rate the account actually pays for savings and CDs. For stock investments, long-run index returns have been roughly 7% a year after inflation, with large swings from year to year. Use the rate comparison option to see a lower and a higher case next to your main one.
Does compounding frequency matter much?
Less than people think. $10,000 at 7% for 20 years grows to $38,696.84 compounded yearly, $40,387.39 monthly and $40,546.56 daily. The rate and the number of years matter far more.
Should contributions go at the start or the end of the period?
Money added at the start of each month earns one extra month of interest on every deposit, so the balance comes out slightly higher. Most savings plans and payroll deductions are closer to end-of-period deposits, which is the default.
Is the result adjusted for inflation or tax?
Only if you ask. Enter an inflation rate under More options to see the balance in today’s money, and a tax rate if the interest is taxed every year, as in an ordinary savings account. Leave tax at 0 for tax-free or tax-deferred accounts such as an IRA or 401(k).
Can I model withdrawals?
Yes. Set "Regular amounts are" to Withdrawals and the monthly and yearly amounts are taken out instead of added. The calculator shows the total withdrawn and, if the money runs out, when that happens.
What is the difference between APR and APY?
APR is the stated yearly rate before compounding. APY includes the effect of compounding within the year, so it is the rate you actually earn: 5% compounded daily is an APY of about 5.127%.
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This is an educational estimate, not financial advice. Lenders use their own rounding, fees and credit terms — confirm exact figures with your lender or a licensed advisor.