Free online compound interest calculator

How to use

1. Enter your starting balance (can be $0), the annual interest rate, and the time horizon in years.
2. Choose how often interest compounds — monthly is the most common for savings accounts.
3. Add a monthly contribution if you plan to deposit regularly.
4. The calculator shows the final balance, total interest earned, and total amount contributed.
5. Press “Show year-by-year” for a detailed breakdown of growth each year.

The formula

For a lump sum with no contributions:

A = P × (1 + r/n)^(n × t)

Where P is the starting balance, r is the annual rate (as a decimal), n is the compounding periods per year, and t is the number of years.

When you add monthly contributions, the calculator converts to an effective monthly rate:

r_monthly = (1 + r/n)^(n/12) − 1

Then simulates month-by-month: each month, the balance earns balance × r_monthly, then the contribution is added.

Worked example

Starting with $10,000 at 7% annually, compounded monthly, with $500/month added over 10 years:

• Effective monthly rate: (1 + 0.07/12)^1 − 1 ≈ 0.5833%
• After 10 years (120 months): final balance ≈ $96,762
• Total contributions: $10,000 + (120 × $500) = $70,000
• Total interest: ≈ $26,762

The power of compounding is that more than a quarter of the final balance is pure interest — money that comes from growth, not your deposits.

Notes

• This calculator uses end-of-period contributions (deposits happen at the end of each month). Some plans use beginning-of-period, which yields slightly more.
• Taxes on interest or dividends reduce real returns. For tax-advantaged accounts (401k, IRA, Roth), the numbers here are more accurate.
• Past stock market returns don’t guarantee future results. The 7% figure is often cited as a historical average; actual returns vary year to year.