Compound interest is the interest calculated on both the initial principal and the accumulated interest from previous periods. It's one of the most powerful concepts in finance — often called the "eighth wonder of the world." Whether you're saving for retirement, evaluating an investment, or planning a college fund, this calculator helps you visualize exactly how your money grows over time with the power of compounding.
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Growth Over Time
Compound Interest Formula
A = P(1 + r/n)nt
A= Final amount (principal + interest)
P= Initial principal (starting amount)
r= Annual interest rate (as a decimal)
n= Number of times interest compounds per year
t= Number of years
Compounding works by earning interest on your interest. Instead of only earning returns on your original deposit, each compounding period adds earned interest to your balance, which then earns more interest in the following period. The more frequently interest compounds, the faster your money grows — daily compounding will yield slightly more than annual compounding over the same period and rate.
Compound Interest Examples
Example 1: Simple Growth
$1,000 invested at 5% annual interest, compounded annually for 10 years, no contributions.
Initial Principal
$1,000.00
Final Balance
$1,628.89
Total Interest Earned
$628.89
Your $1,000 grew by 62.9% over 10 years without any additional contributions — that's the power of compound interest.
Example 2: With Monthly Contributions
$5,000 initial deposit at 7% annual interest, compounded monthly, with $200/month contributions for 20 years.
Total Contributions
$53,000.00
Final Balance
$124,429.52
Total Interest Earned
$71,429.52
Regular contributions combined with compounding turned $53,000 in deposits into over $124,000 — more than doubling your money.
Why Compound Interest Matters
Investing
Stock market investments, index funds, and dividend reinvestment plans all benefit from compound growth. Starting early — even with small amounts — can lead to significantly larger portfolios over decades.
Savings Accounts
High-yield savings accounts and certificates of deposit (CDs) use compound interest to grow your emergency fund or short-term savings. The compounding frequency and APY directly impact how much you earn.
Retirement Planning
401(k)s, IRAs, and other retirement accounts rely heavily on compound interest. A 25-year-old investing $300/month at 8% will have far more at 65 than a 35-year-old investing the same — the extra decade of compounding makes an enormous difference.
Long-Term Wealth Growth
Compound interest is the core mechanism behind wealth accumulation. The earlier you start and the more consistently you contribute, the more your money works for you. Time in the market beats timing the market.
Frequently Asked Questions
Common questions about compound interest and how to use this calculator.
What is compound interest?
Compound interest is the interest calculated on both the initial principal and the accumulated interest from previous periods. Unlike simple interest, which only earns returns on the original amount, compound interest allows your money to grow exponentially over time. This is why it's often referred to as "interest on interest."
How often should interest compound?
More frequent compounding leads to slightly higher returns. Daily compounding will yield more than annual compounding over the same time period and rate. However, the difference between daily and monthly compounding is usually very small. Most savings accounts compound daily, while many investments compound annually or quarterly.
Is compound interest better than simple interest?
Yes, for growing money over time, compound interest is significantly better than simple interest. Simple interest only earns returns on the original principal, while compound interest earns returns on both the principal and previously earned interest. Over long periods, the difference becomes dramatic. For example, $10,000 at 5% simple interest earns $500/year forever. With compound interest, that same amount earns increasingly more each year.
How do monthly contributions affect compound growth?
Regular contributions dramatically accelerate compound growth. Each new contribution immediately begins earning interest, which then compounds over the remaining time period. This is why financial advisors emphasize consistent investing — even small regular amounts can grow substantially over decades due to the combined effects of contributions and compounding.
What's the difference between APR and APY?
APR (Annual Percentage Rate) is the stated annual interest rate without accounting for compounding. APY (Annual Percentage Yield) includes the effect of compounding and represents the actual annual return. APY is always equal to or greater than APR. For example, a 5% APR compounded monthly results in a 5.12% APY. When comparing savings accounts, always compare APY for an accurate picture.
How does the Rule of 72 work?
The Rule of 72 is a quick estimation for how long it takes to double your money. Divide 72 by your annual interest rate to get the approximate number of years. For example, at 6% interest, your money doubles in roughly 72 ÷ 6 = 12 years. At 8%, it takes about 9 years. This simple mental shortcut helps illustrate the power of different growth rates.