Financial Calculators
Compound Interest Calculator
See how a principal plus optional monthly contributions grows with compound interest, with a year-by-year growth chart.
Quick guide
Enter a starting amount, an optional monthly contribution, the annual rate, the number of years, and how often interest compounds. The result shows the ending balance and how much of it is interest, with a year-by-year growth chart.
Read the full explanationHow It Works
Compound interest earns interest on interest, so growth accelerates over time. The selected frequency applies to the initial principal; monthly contributions use month-end deposits and monthly compounding in this model. Compare only scenarios with the same deposit assumptions.
Formula Used
A = P(1 + r/n)^(nt) for the lump sum, plus c · [((1 + r/12)^(12t) − 1) / (r/12)] for level monthly contributions.
Example Calculation
$10,000 at 7% compounded monthly for 20 years grows to about $40,387; adding $100 per month lifts it to roughly $92,500.
Practical Use Cases
- Project long-term savings growth.
- Show why starting early beats contributing more later.
- Compare compounding frequencies.
Use This Calculator Step by Step
- Enter the principal, annual rate, time period, and the available compounding frequency.
- Read the frequency before comparing with an account quote; a nominal rate is different from APY.
- Compare the ending value with starting principal and inspect the interest portion separately.
Check Your Result
At a nominal annual rate of 10%, $1,000 compounded annually becomes $1,100 after one year. With monthly compounding it becomes about $1,104.71. These are different assumptions, not conflicting answers to the same model.
Assumptions and Limits
- The rate is held constant; taxes, fees, inflation, and uncertain investment returns are outside this mathematical illustration.
- The selected frequency applies to the starting principal. Contributions follow the monthly model in the stated formula, even when another frequency is selected.
A Common Question
Which tool should I use for a savings account quoted in APY?
Use the Savings Calculator for an effective APY input. Use this compound-interest model when you have a nominal rate and the compounding frequency required by its formula.