Compound Interest Calculator
See how your savings grow with compound interest. Adjust starting amount, rate, contributions, and frequency. No signup, no ads.
Investment details
Balance after 20 years
$40,387
Total contributed
$10,000
Interest earned
$30,387
How does compound interest work?
Compound Interest Formula
Variables
Final amount-Balance after all compounding periods
Principal-Starting amount
Annual rate-Yearly rate as a decimal (8% = 0.08)
Periods per year-1 for annual, 12 for monthly, 365 for daily
Years-Investment duration
Interest Rate Comparison
Growth of $10,000 over 20 years
| Rate | Total Interest | Ending Balance |
|---|---|---|
| 4% | $11,911.23 | $21,911.23 |
| 6% | $22,071.35 | $32,071.35 |
| 8% | $36,609.57 | $46,609.57 |
| 10% | $57,275.00 | $67,275.00 |
Frequently Asked Questions about the Compound Interest Calculator
What is compound interest?
Compound interest is interest you earn on both your initial deposit and on the interest that builds up over time. The more often it compounds (monthly vs annually) and the longer it runs, the more dramatic the growth.
How is compound interest different from simple interest?
Simple interest only pays you on the original amount you deposited. Compound interest pays you on the original amount plus all previously earned interest, which is why it accelerates over long periods.
What compound frequency should I use?
Most savings accounts compound daily and most CDs and bonds compound semi-annually. For a 401(k), monthly is a reasonable assumption. Use the frequency your account actually uses if you know it.
Does this account for taxes or inflation?
No. The result is a pre-tax, nominal return. To get a real return, subtract your expected inflation rate from the interest rate. To estimate after-tax returns, subtract your marginal tax rate from the gross interest.
Is 7 percent a realistic stock-market return?
Historically, the S&P 500 has returned about 10 percent per year nominally, or roughly 7 percent after inflation, over multi-decade periods. Short windows can vary widely. Adjust the rate to match your own assumptions.
Can I model monthly contributions?
Yes. Enter the amount you plan to add each month under monthly contribution. The calculator applies it at the start of every compounding period and includes it in both the final balance and the contributions total.
What is the rule of 72?
Divide 72 by your interest rate to estimate how many years it takes to double your money. At 6 percent, money doubles in roughly 12 years; at 9 percent, in roughly 8 years.
How accurate is this calculator?
The math is exact. The accuracy of your result depends on whether your input assumptions match reality. Returns are not guaranteed and past performance does not predict future results.