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Compound Interest Calculator
See how your money grows over time with compound interest and regular contributions.
The amount you are starting with today.
How much you plan to add each month.
Historical S&P 500 average is about 10% (7% after inflation).
How long you plan to invest.
How often interest is calculated and added. Most investments compound monthly.
About This Tool
Compound interest is the single most powerful wealth-building force available to you. This compound interest calculator shows exactly how your money grows when you combine a starting investment, regular contributions, and the power of compounding. Even small monthly amounts turn into serious wealth given enough time. Building financial health is just as important as physical health — use our TDEE Calculator to invest in both.
How to Use
Enter your starting investment, monthly contribution, expected annual return rate (7% is a common inflation-adjusted benchmark for S&P 500 index funds), and investment period. Select compounding frequency and calculate. The results show your final balance, total contributions, compound interest earned, and effective annual rate.
FAQ
What annual return rate should I use?
The S&P 500 has historically returned about 10% annually before inflation, or roughly 7% after inflation. Use 7% for real purchasing power, 10% for nominal amounts, or 3-5% for bonds and savings accounts. The key is to start — time in the market matters more than timing the market.
How much difference does starting early really make?
Massive. Someone investing $500/month from age 25 will have roughly $1.2 million by 60 at 7% annual return. Starting at 35 instead yields about $567,000 — less than half, despite only missing 10 years. The early investor contributed just $60,000 more but ended up over $600,000 ahead. That is the power of compound interest and time.
Does compounding frequency make a big difference?
The difference between monthly and annual compounding is relatively small — typically a fraction of a percent in effective annual return. For example, 7% compounded monthly gives an effective rate of about 7.23%, versus 7% flat annually. Over long periods this adds up, but contribution amount, rate of return, and time horizon matter far more.
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