Calculator
Compound Interest Calculator
Project how a starting balance plus monthly contributions grow over time with the power of compounding.
1. Your inputs
Results update instantly. Everything runs in your browser.
2. Your results
Projected future value
$691,150
in 30 years
Total contributed
$190,000
Interest earned
$501,150
Ending balance
$691,150
Starting amount
$10,000
Monthly deposit
$500
Growth multiple
69.1x
Growth over time
Final balance breakdown
$691,150
Total
- Contributions27%
$190,000
- Interest earned73%
$501,150
What does this mean?
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In plain English
Starting with $10,000 and adding $500 every month for 30 years at 7% annual return, you end up with roughly $691,150.
Of that, $501,150 — about 73% — comes from compound growth, not your contributions.
Try shortening the timeline by 5 years or dropping the return by 1% to see how sensitive the final number is to each variable.
Assumptions used
The math relies on these assumptions. Real-world numbers can vary.
- Constant annual return each year.
- Monthly contributions made at the end of every month.
- No taxes, fees, or account minimums applied.
- Results are nominal (not adjusted for inflation).
- Compounding calculated monthly.
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Frequently asked
What is compound interest?
Compound interest is interest paid on both your original balance and any interest already earned. Over time it grows exponentially — the sooner you start, the more it works for you.
Why does starting early matter so much?
Because the earliest dollars have the most time to compound. A dollar invested at 25 is worth roughly 4x more at 65 than a dollar invested at 35 (at 7% annual return).
Is 7% a realistic return?
For long-term diversified stock-market portfolios, 6–8% real return is a common historical range. Actual returns vary — never guaranteed.
Should I include inflation?
The default calculation is nominal. If you want real (inflation-adjusted) growth, subtract your expected inflation rate (e.g. 3%) from the return to model it.
What if I stop contributing?
Set the monthly contribution to $0 to see how the balance grows on its own — it's a good exercise to see the power of your existing balance.
About the Compound Interest Calculator
Compound interest is the single most powerful concept in personal finance. Money you invest earns returns, and those returns then earn their own returns — an exponential curve that quietly transforms modest savings into serious wealth.
The two variables that matter most are time and rate of return. Doubling the monthly contribution helps, but starting five years earlier often helps more.
Use the projection to model any long-term goal — retirement, a child's education, or just building a solid long-term portfolio.
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These calculators are for education only and are not a substitute for personalized advice from a licensed professional. Read our full disclaimer.