How to use the compound interest calculator
Enter what you have today, how much you plan to add each month or year, the annual interest rate and how often interest compounds, then the number of years. The calculator shows your future balance, how much of it is your own money and how much is interest, the effective annual rate (APY), and a year-by-year table.
Choose whether deposits go in at the start or the end of each period. Money added at the start earns interest for one extra period, so it grows a little more. Nothing you enter leaves your device; the maths runs in your browser.
What compound interest is
Compound interest is interest paid on the principal and on the interest already earned (Investor.gov (SEC): compound interest). The CFPB's example (CFPB: How does compound interest work?): $1,000 at 5% compounded once a year grows to $1,050.00 after one year and $1,102.50 after two. The second year earns $52.50 instead of $50.00 because the first year's interest is now earning interest too.
Over short periods the effect is small; over long periods it is large. The Department of Defense's financial readiness site (FINRED (U.S. Department of Defense): Understanding Interest) shows $10,000 at 4% compounded yearly earning $1,248.64 in 3 years but $38,010.21 in 40 years, more than twice the $16,000 that simple interest would pay. To compare the two methods for your own numbers, use the simple interest calculator.
The compound interest formula
For a single deposit, the balance after t years is A = P × (1 + r/n)n×t, where P is the principal, r the annual rate as a decimal (7% = 0.07) and n the number of times interest compounds per year: 1 for yearly, 4 for quarterly, 12 for monthly, 365 for daily.
Regular deposits add a second part. With a deposit D at the end of every period and a rate i per period, the deposits grow to D × ((1 + i)periods − 1) ÷ i. This calculator works month by month, converting your rate and compounding frequency into an equivalent monthly rate, so any mix of deposit and compounding schedule adds up correctly.
Worked example (the default above): $10,000 plus $200 a month at 7% compounded monthly for 10 years grows to $54,713.58. You put in $34,000.00; the other $20,713.58 is interest. Making the same deposits at the start of each month gives $54,915.51.
Daily vs. monthly vs. yearly compounding
The more often interest compounds, the more you earn at the same stated rate, but the gain shrinks quickly. $10,000 at 6% for 10 years:
Yearly: $17,908.48 (APY 6.000%)
Quarterly: $18,140.18 (APY 6.136%)
Monthly: $18,193.97 (APY 6.168%)
Daily: $18,220.29 (APY 6.183%)
Going from yearly to monthly compounding adds $285.49; going from monthly to daily adds only $26.32. The rate and the time matter far more than the compounding schedule. That is why US banks must quote savings and CD yields as an annual percentage yield, which already includes compounding (CFPB: Regulation DD, Appendix A (annual percentage yield)). If your bank gives you an APY, enter it here with yearly compounding, or use the high-yield savings calculator, which takes APY directly.
The Rule of 72
To estimate how long money takes to double, divide 72 by the annual rate. The SEC's Investor.gov (Investor.gov (SEC): What is compound interest? (Rule of 72)) gives the example of a 9% return doubling in about 8 years (72 ÷ 9). The exact answer with yearly compounding is 8 years, so the shortcut is close. The calculator shows both, for your rate and compounding choice.
Why starting early matters
Time does most of the work. Saving $200 a month at 7% compounded monthly grows to $524,962.68 after 40 years, but only $243,994.20 after 30. The extra 10 years add just $24,000 of deposits but more than double the result.
For retirement savings, the 401(k) calculator and Roth IRA calculator apply the same idea with contribution limits and employer matches. For a fixed-term deposit, try the CD calculator.
Tips
Use a realistic rate. Savings accounts and CDs pay a known rate. Stock and bond returns vary from year to year and can be negative, so a single long-run rate is only an estimate.
Remember taxes and fees. Interest in a taxable account is usually taxed each year, and fund fees reduce returns. Both slow compounding.
Think in today's dollars. A balance decades from now buys less than the same amount today. The US inflation calculator shows how much prices have risen in the past.
Debt compounds too. Unpaid credit card interest is added to your balance and then charged interest, so the same maths works against you when you borrow.
Updated October 27, 2026 · Sources are linked in the text above · How we build and check our calculators