Compound Interest Calculator
$10,000 plus $500 a month at 7% grows to about $691,000 in 30 years. Enter your own amount, monthly contribution, rate and compounding to see the growth year by year.
Quick answer · Updated 2026
$10,000 invested with $500 added every month at a 7% annual return, compounded monthly, grows to about $691,150 in 30 years. You contribute $190,000 and compound interest adds about $501,150. The formula is FV = P(1 + r/n)^(nt) + PMT x [((1 + r/n)^(nt) - 1) / (r/n)].
| Years | Your contributions | Balance at 7% |
|---|---|---|
| 10 | $70,000 | $106,639 |
| 20 | $130,000 | $300,851 |
| 30 | $190,000 | $691,150 |
Sources: standard future value formulas as used by the SEC's Investor.gov compound interest calculator; figures computed with this calculator's defaults, before taxes and inflation. Researched general information, not financial, tax or legal advice.
Investment Details
Stock market average: 7-10%
Results Summary
Future Value
$0
In 30 years
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$0
Your money
Interest Earned
$0
Free money from compounding
Return on Investment
Growth Breakdown
Yearly Breakdown
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The Power of Compound Interest
Start Early
Time is your greatest asset. Starting at 25 vs 35 can double your retirement savings even with the same monthly contributions.
Consistency Matters
Regular monthly contributions matter more than timing the market. Dollar-cost averaging smooths out market volatility.
Let It Grow
Avoid withdrawing early. Each dollar withdrawn costs you its future compound growth. A $1,000 withdrawal at 30 could cost you $10,000 at 60.
Common questions
How do you calculate compound interest with monthly contributions?
Use FV = P(1 + r/n)^(nt) + PMT x [((1 + r/n)^(nt) - 1) / (r/n)], where P is the starting amount, PMT the monthly contribution, r the annual rate, n the compounding periods per year and t the years. With P = $10,000, PMT = $500, r = 7%, n = 12 and t = 30, the result is about $691,150.
What is the rule of 72?
Divide 72 by the annual return to estimate how many years it takes money to double. At 7% that is about 10.3 years, which is why a lump sum roughly doubles every decade at a stock-like return.
Does compounding frequency matter much?
Less than people expect. At 7%, monthly compounding gives an effective annual rate of about 7.23% against 7% for annual compounding. The contribution amount and the number of years matter far more.
Investing for retirement? Run the same growth through the 401(k) calculator, which adds your employer match and the 2026 contribution limits.
