See how a savings account, index fund, or retirement account grows when interest earns interest. Enter a starting balance, a rate, how often it compounds, and what you add each month or year, and the calculator shows the final balance, how much of it you put in, and how much came from interest — with a year-by-year table and a chart that makes the snowball visible. Everything runs in your browser; nothing is sent anywhere.
Compounding works against you on debt the same way it works for you on savings. To see the other side of the ledger, the mortgage calculator and loan calculator show how interest accumulates on a balance you owe, and the percentage calculator handles quick growth-rate and percent-change questions. For the debt side worked out in full, see why your first mortgage payment is mostly interest.
Comparing accounts or loans? APR vs APY, and why compounding frequency matters less than you think shows the conversion, which number US rules require each product to quote, and why rate and time swamp daily-vs-monthly compounding.
Frequently asked questions
How is compound interest calculated?
The balance grows by FV = P(1 + r/n)^(nt), where P is the starting amount, r the annual rate, n the number of compounding periods per year, and t the years. Regular deposits add an annuity term, PMT × ((1 + r/n)^(nt) − 1) ÷ (r/n). Each period, interest is earned on the previous interest as well as the original money, which is why the curve bends upward the longer you leave it. At 7% a year, $10,000 becomes about $19,672 in 10 years and $76,123 in 30 — the last decade adds more than the first two combined.
Does compounding frequency matter much?
Less than most people expect. At 5%, annual compounding gives an effective annual rate of 5.000%, monthly 5.116%, and daily 5.127% — continuous compounding tops out at 5.127%. Going from monthly to daily is worth about a dollar a year per $10,000. The rate itself and how long you stay invested matter far more. When comparing savings accounts, look at the APY, which already includes compounding; this calculator shows that effective rate so you can compare like with like. If a bank quotes an APY, enter it with annual compounding to reproduce it exactly.
What is the difference between start and end of period contributions?
A deposit made at the start of a month earns interest for that whole month; one made at the end earns nothing until the following month. Over long periods the gap adds up to roughly one extra period of interest on every deposit. Finance texts call these an annuity due (start) and an ordinary annuity (end). Most automatic payroll contributions behave like end-of-period deposits, while funding an IRA on January 1 each year is a start-of-period deposit. Pick the timing that matches how the money actually arrives.
How do monthly deposits work with daily or quarterly compounding?
When deposits and compounding happen on different schedules, the calculator converts the nominal rate to its effective annual rate and grows the balance by the equivalent monthly factor. That is exact at every month boundary no matter how often interest compounds, and it matches the textbook formula when the two schedules line up. Banks sometimes credit interest on a different day than you deposit, so real statements can differ by a few cents. For planning purposes that difference is far smaller than the uncertainty in any future rate of return.
What does the inflation adjustment show?
It divides each balance by (1 + inflation)^years to express it in today's purchasing power. A $500,000 balance in 30 years sounds large, but at 3% inflation it buys what about $206,000 buys today. Use it to sanity-check retirement or savings goals. The adjustment does not account for taxes on the interest or investment fees, both of which also reduce real growth, and actual inflation varies year to year. Try a few inflation rates, such as 2%, 3%, and 4%, to see how sensitive a long-range goal is to that assumption.