Compound Interest Calculator
See how your investments grow over time with the power of compound interest. Enter your initial investment, monthly contributions, and expected return rate.
Investment Details
The amount you're starting with
How much you'll add each month
Expected annual return (7% is a common long-term average)
Enter values and click Calculate to see results
How the Compound Interest Calculator Works
Compound interest earns interest on your interest, creating exponential growth over time. Unlike simple interest (which only earns on your original deposit), compounding accelerates as your balance grows. The more frequently interest compounds, the faster your money grows.
A = P(1 + r/n)^(nt) + PMT × [((1 + r/n)^(nt) - 1) / (r/n)]A= Future value of the investmentP= Initial principal (starting amount)r= Annual interest rate (decimal)n= Number of times interest compounds per yeart= Number of yearsPMT= Regular contribution amountExample Scenarios
Lump Sum Only
Even without adding a single dollar, your $10K grows to over $76K in 30 years at 7% — a 7.6x return.
Monthly Contributions
$500/month for 25 years means $150K contributed — but compound growth turns it into over $430K.
Compounding Frequency
Daily compounding earns about $2,900 more than annual compounding on $50K over 20 years at 8%.
The Rule of 72
To quickly estimate how long it takes to double your money, divide 72 by your interest rate. At 7% annual return, your money doubles approximately every 10 years (72 ÷ 7 ≈ 10).
Why Starting Early Matters
Time is the most powerful factor in compound growth. Starting to invest 10 years earlier can result in significantly more wealth than investing larger amounts later, thanks to the extra years of compounding.