Compound Interest Calculator
Estimate investment growth with monthly contributions and your choice of compounding frequency.
Compound Interest Calculator
Estimate investment growth with monthly contributions and your choice of compounding frequency.
ⓘ Illustrative estimate for educational use. Actual results may vary.
How to Use This Calculator
- Enter your investment details
Enter your starting balance, annual interest rate, and monthly contribution.
- Choose a period and frequency
Set the number of years and how often interest compounds.
- Compare scenarios
See the ending balance, money you contributed, and interest earned.
Compound Interest Formula
FV = P(1 + i)m + C[((1 + i)m − 1) ÷ i]
i = (1 + r ÷ n)n/12 − 1
P is the initial investment, C is the contribution at the end of each month, m is the number of months, r is the nominal annual rate as a decimal, and n is the number of compounding periods per year. The equivalent monthly rate models deposits between compounding dates. At zero interest, FV equals P + Cm.
Worked Example
Start with $10,000, add $200 at each month-end, and earn a nominal 8% compounded monthly for 20 years.
Frequently Asked Questions
What is compound interest?
Compound interest is interest on the starting balance and on interest already earned.
How is this different from simple interest?
Simple interest on a lump sum grows in proportion to time. Compound interest also earns interest on prior interest.
When do monthly contributions earn interest?
This calculator assumes deposits occur at month-end. Each deposit starts earning from the following month.
What does this estimate exclude?
The model assumes a constant rate. It excludes taxes, fees, inflation, and market fluctuations.