Compound Interest Calculator
See how your savings or investments could grow over time with regular monthly contributions. Then compare how the rate, the time horizon and the compounding frequency change the result.
Projections assume a constant rate and contributions made at the end of each month. Real investment returns vary from year to year and aren't guaranteed, and taxes and fees aren't included. See our terms of use.
How to use this compound interest calculator
Enter what you're starting with, how much you'll add each month, the yearly rate you expect to earn and how many years the money will grow. For a savings account or CD, use its stated rate. For investments, use an assumed average annual return, and try a few different rates to see a range of outcomes.
Add an expected inflation rate to see what the future balance would be worth in today's money. That's the more honest number when you're planning for something decades away.
What is compound interest?
With simple interest, you earn interest only on the money you put in. With compound interest, you also earn interest on the interest you've already earned. Each year's growth is added to your balance, so the next year's growth is calculated on a bigger number.
The difference looks small at first and becomes huge over time. Take $10,000 at 7% for 30 years:
- With simple interest, you'd end up with $31,000.
- With interest compounded annually, you'd end up with $76,123, about two and a half times as much, without adding a single dollar.
The compound interest formula
For a one-time deposit, the classic formula is:
A = P × (1 + r/n)n×t
Here A is the final amount, P is the starting principal, r is the annual rate as a decimal, n is how many times per year interest compounds and t is the number of years. When you add monthly contributions, each deposit compounds for the time it's invested. This calculator works through that month by month, so you don't have to.
Example: start with $10,000, add $300 a month and earn 7% compounded monthly for 30 years. You'd contribute $118,000 in total, and the balance would reach about $447,156. More than $329,000 of that is interest.
Why starting early matters most
Time is the most powerful input in the calculator. Compare two savers who each invest $300 a month at 7%:
| Starts at age | Years invested (to 65) | Total contributed | Balance at 65 |
|---|---|---|---|
| 25 | 40 | $144,000 | $787,444 |
| 35 | 30 | $108,000 | $365,991 |
The early starter puts in only $36,000 more but ends up with about $421,000 more. Those extra ten years at the beginning have the longest time to compound.
Does compounding frequency matter?
More frequent compounding helps, but much less than the rate or the time horizon. Here's $10,000 at 5% for 10 years:
| Compounding | Balance after 10 years |
|---|---|
| Annually | $16,288.95 |
| Quarterly | $16,436.19 |
| Monthly | $16,470.09 |
| Daily | $16,486.65 |
This is why banks advertise an APY (annual percentage yield), which already includes the effect of compounding. A 5% rate compounded monthly is a 5.12% APY, and compounded daily it's about 5.13%. When comparing savings accounts, compare APYs.
The Rule of 72
For a quick mental estimate, divide 72 by the annual rate to find roughly how many years it takes your money to double. At 7%, that's 72 ÷ 7 ≈ 10.3 years. At 4%, about 18 years. The rule works best for rates between about 4% and 12%.
Don't forget inflation
A dollar in 30 years will buy less than a dollar today. If inflation averages 2.5% a year, the $447,156 from the example above would have the buying power of about $213,000 in today's money. Enter an inflation rate in the calculator to see this for your own numbers. The real return, your rate minus inflation, is what actually grows your purchasing power.
Where compound growth happens
- High-yield savings accounts and CDs: low risk, with stated rates. Deposits at FDIC-insured banks are protected up to $250,000 per depositor, per bank, for each account ownership category.
- Bonds and bond funds: interest payments that can be reinvested.
- Stock index funds: no fixed rate. Over long periods, stocks have historically grown faster than savings accounts, but with real ups and downs, including years of losses.
- Retirement accounts such as a 401(k) or IRA: their tax advantages let your money compound without being reduced by taxes on gains every year.
Compounding works against you just as powerfully when you owe money. Credit card balances compound at rates far higher than most investments earn. That's why paying off high-interest debt is often the best "return" available. See our credit card payoff calculator and debt payoff calculator.
Frequently asked questions
Is the result guaranteed?
Only if your rate is fixed and guaranteed, as with a CD held to maturity. For investments, the result is a projection based on the average return you enter. Actual returns will vary from year to year.
What rate should I use for investments?
There's no single right answer. Many people run a conservative scenario and an optimistic one to see a range. Remember to subtract fund fees, such as expense ratios, from the return you assume.
Does the calculator include taxes?
No. Interest in a regular savings account is usually taxed each year, which reduces your effective growth. Money in tax-advantaged retirement accounts follows different rules.
When are contributions added?
At the end of each month. Contributing at the start of each month instead would give a slightly higher balance.
Can I enter a negative rate?
Yes. A negative rate shows how a balance shrinks, which is useful for modeling a bad stretch or the effect of inflation on cash.
Last reviewed: October 2026