Quick example
$10,000 at 5% with $100 monthly contributions
After 10 years of monthly compounding and end-of-month contributions, the projected balance is $31,998.32. The total amount contributed is $22,000.00, and estimated interest is $9,998.32.
Growth math
How the future balance is calculated
With a positive interest rate and equal end-of-period contributions:
FV = P × (1 + r)^N
+ C × ((1 + r)^N − 1) ÷ r
Pis the starting principal.Cis the contribution added at the end of each period.mis the number of compounding periods per year, such as 12 for monthly.ris the decimal rate per period: annual interest rate (%) divided by 100, then bym.Nis years multiplied bym, andFVis the future balance.
The first term grows the starting principal. The second grows each contribution for the periods remaining after it is deposited; the final contribution earns no interest within the selected term. Set the contribution to zero to calculate growth on the principal alone.
At zero interest
Use FV = P + C × N. Starting with $10,000 and adding $100 monthly for 10 years gives $22,000.00: $10,000 plus 120 contributions of $100.
For either formula, total contributions are P + C × N. Subtract that amount from the future balance to find interest earned. Display values are rounded to cents; intermediate calculations are not.
Compounding choices
Frequency changes growth and contribution timing
Annual
Interest compounds once and the contribution is added once each year.
Quarterly
Four compounding periods and four end-of-period contributions per year.
Monthly
Twelve periods per year, useful for regular monthly deposits.
Daily
Uses 365 periods and treats the entered contribution as a daily amount.
Projection limits
Actual returns are rarely fixed
This calculator assumes a constant rate, no taxes, no fees, and perfectly regular contributions. Investment returns can vary and losses are possible. Treat the output as a mathematical illustration, not financial advice.
Common questions
Compound interest FAQ
What is compound interest?
It is growth on both the starting principal and interest accumulated in earlier periods.
When are regular contributions added?
This calculator adds the entered contribution at the end of each compounding period. A monthly setting means monthly contributions; a daily setting means daily contributions.
What is the effective annual rate?
It is the one-year growth rate after accounting for the selected number of compounding periods, without contributions. It is calculated as ((1 + r)^m - 1) times 100, where r is the decimal rate per period and m is the number of periods per year.
What happens at zero interest?
The final balance is the starting principal plus all contributions. No interest is earned, regardless of the number of periods.