Compound Interest Calculator
Most PopularSee how your investments grow with the power of compounding.
Future Value
$59,029.54
Total Interest
$25,029.54
Total Invested
$34,000.00
The Compound Interest Formula
A = P(1 + r/n)^(nt), where P is the principal, r is the annual interest rate, n is the number of times interest compounds per year, and t is time in years. Unlike simple interest, each compounding period earns returns on both your original principal and every dollar of interest already credited — that reinvested interest is what makes the growth curve bend upward instead of running in a straight line.
At 8% compounded monthly with $200.00/month added in, your $10,000.00 starting balance grows to $59,029.54 over 10 years — of which $25,029.54 is interest your money earned on its own, without any extra contribution from you.
Quick shortcut: the Rule of 72 estimates doubling time by dividing 72 by your interest rate. At 8%, that is roughly 9.0 years for a lump sum to double — no calculator required.
Frequently Asked Questions
What is the difference between compound interest and simple interest?
Simple interest is calculated only on the original principal, so it grows at a flat, straight-line rate. Compound interest is calculated on the principal plus all interest already earned, so growth accelerates over time. On $10,000 at 8% for 20 years, simple interest earns $16,000 while compound interest (annual) earns over $36,000 — more than double.
What is the Rule of 72 and how do I use it?
The Rule of 72 is a quick mental-math shortcut: divide 72 by your annual interest rate to estimate how many years it takes money to double. At 8% annual returns, 72 ÷ 8 = 9 years to double. At 6%, it takes 12 years. It is an approximation, but it is accurate within a few months for rates between 4% and 15%.
Does compounding frequency (daily vs. monthly vs. annually) really matter?
It matters, but less than most people expect. On $10,000 at 8% for 10 years, annual compounding grows to $21,589 while daily compounding grows to $22,253 — a difference of about $664, or roughly 3%. Switching from annual to monthly or daily compounding gives a small boost; increasing your rate or contribution amount matters far more.
How much difference does starting 10 years earlier make?
A huge one, because compounding needs time to do its work. Investing $300/month at 8% for 30 years builds to roughly $447,000. Waiting 10 years and investing the same $300/month for only 20 years builds to about $177,000 — well under half, even though you contributed two-thirds as much money. The first decade of contributions does most of the heavy lifting because it compounds the longest.
How much do monthly contributions add compared to a lump sum alone?
Regular contributions compound alongside your original principal and often end up contributing more to your final balance than the initial deposit itself. $10,000 invested once at 8% for 20 years grows to about $46,600. Add just $200/month on top of that same $10,000, and the total future value climbs to roughly $167,000 — the monthly contributions and their compounded interest account for the majority of the extra growth.
What real-world accounts actually use compound interest?
Savings accounts, certificates of deposit (CDs), fixed deposits, money market accounts, bonds, and most retirement accounts (401(k), IRA, PPF, NPS) all compound interest or returns. On the flip side, credit card debt and many loans also compound against you — which is why carrying a balance at 20%+ APR grows so quickly.
Is the interest rate I enter before or after taxes and inflation?
This calculator uses your entered nominal rate before taxes and inflation. If your investment gains are taxable, your real after-tax growth will be lower. To estimate purchasing power in today's money, run your future value through the Inflation Calculator using your expected inflation rate.
Why does my total interest look larger than my total contributions?
This happens naturally over long time horizons because each year's interest itself starts earning interest. Over 25-30+ years at a healthy rate, it is common for accumulated interest to exceed the amount you personally contributed — this is compounding working as intended, not a calculation error.
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