What is Compound Interest Calculator?

The Compound Interest Calculator shows how your money grows over time through compounding. Enter your principal, interest rate, and time period to see the future value of your investment or savings.

Compounding monthly versus annually changes the answer more than most people expect. At 7% over 30 years, the same $10,000 principal grows to about $76,123 when compounded annually but $81,165 when compounded monthly. The calculator uses the standard formula A = P(1 + r/n)^(nt), so you can verify the math yourself.

How to use

1. Enter your initial investment amount (principal)
2. Set the annual interest rate and pick how often it compounds, anywhere from daily to yearly
3. Choose the time period in years and see your projected future value and total interest earned

When to use

• Comparing how a savings account, CD, or index fund stacks up over 10 to 30 years.
• Working out whether to take a lower-rate loan with monthly compounding or a higher-rate one with annual.
• Showing kids or family members how a small amount left alone for decades actually grows.

Result

$10,000 at 7% annual interest compounded monthly for 20 years grows to $40,387 — earning $30,387 in interest.

FAQ

Can I add regular contributions, or only a one-time deposit?

Both. Leave the contribution amount blank for a single lump sum, or enter an amount and pick a cadence (monthly, quarterly, or yearly) to add money on a schedule. Each deposit lands at the end of its period and starts earning from there, and the results split your final balance into what you put in versus what the interest added.

Why does monthly compounding give more than annual at the same rate?

Because each compounding event puts the new interest to work right away. Annual compounding waits 12 months before earning anything on the new interest, while monthly does it 12 times faster. Over 20 years the gap is usually 1 to 2% of the final balance.

Are taxes or inflation included?

No. The result is a nominal future value before tax. To see real growth, subtract roughly 2 to 3% from the annual rate to approximate inflation, then run the numbers again. For taxes, subtract your marginal rate from any interest portion.

What rate should I use for stocks or an index fund?

Historic S&P 500 returns average around 7% real (about 10% nominal) but past performance is not a guarantee. For conservative planning use 5 to 6%. For a savings account use the current APY from your bank, which is already an effective annual rate.

Can compound interest go negative if rates are negative?

The math works fine with a negative rate (you would type something like -0.5) and the future value will be lower than the principal. In reality consumer accounts rarely go below zero, but bond yields and some European deposit rates have, so the formula stays useful.

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