Compound Interest Calculator
See how your savings and investments snowball over time with compound interest, and test how rates and timelines shape your final balance.
Enter your initial deposit amount, interest rate, and years above to project your compound growth instantly.
How to Use This Calculator
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1
Enter Your Starting Deposit
Start with the cash balance you're putting into the account today (e.g. $10,000).
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2
Enter Expected Annual Return
Type in your anticipated yearly rate of return or annual percentage yield (APY), such as 7.0%.
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3
Set Timeline & Compounding Schedule
Pick how many years you'll let it grow and choose how often interest gets added back: annually, monthly, or daily.
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4
Watch Your Money Grow
Your future balance, interest earnings, and visual growth bar show up right away. Tap Try an example to see a common baseline.
The Formula
Compound interest is computed mathematically using the standard compound growth formula:
Here is what each parameter represents in plain English:
- A Final Accrued Amount: The future value of your money, combining original principal with all accumulated interest.
- P Principal Investment: Your starting deposit or current account balance.
- r Annual Nominal Rate: The annual interest rate as a decimal (so 7% becomes 0.07).
- n Compounding Frequency: How many times per year interest is credited (1 for annually, 4 for quarterly, 12 for monthly, 365 for daily).
- t Time in Years: How many years your balance compounds uninterrupted.
Why compounding creates a snowball effect: In year one, you only earn interest on your original deposit. In year two, you earn interest on your deposit plus the previous year's interest. Over decades, that snowball effect takes over. Earned interest eventually dwarfs your original deposit.
Example
Here is a worked scenario using realistic figures. We run this test scenario to verify compounding math. I plan around 7% a year for long-term money — roughly what broad stock indexes have averaged historically — because I'd rather be pleasantly surprised than burned. Here is the exact calculation:
Applying the compound growth formula step by step:
- Calculate base rate per period:
1 + (0.07 ÷ 12) ≈ 1.005833 - Calculate growth factor:
(1.005833)¹²° ≈ 2.009661 - Calculate final future value:
A = $10,000 × 2.009661 = $20,096.61 - Calculate total interest earned:
$20,096.61 − $10,000 = $10,096.61
Simple vs. compound interest: With simple interest, you'd end up with just $10,000 × (1 + 0.07 × 10) = $17,000.00. Monthly compounding hands you an extra $3,096.61 in pure compound growth because your interest kept earning interest.
Frequently Asked Questions
What's the difference between simple and compound interest?
Simple interest only pays out on the money you originally deposited. Compound interest pays out on your original cash plus every dollar of interest already earned. Over time, that compounding turns a steady trickle into an exponential snowball.
What is the Rule of 72?
The Rule of 72 is a handy mental math shortcut to estimate when your money doubles. Divide 72 by your annual return rate (72 ÷ 7 ≈ 10.3 years) to get a fast, remarkably close estimate of your doubling timeline.
Does compounding frequency really matter?
Most finance gurus obsess over daily versus monthly compounding, but honestly, what you save and how long you leave it alone matters ten times more. Compounding daily on $10,000 at 7% over 10 years only earns about $40 more than monthly compounding. Your interest rate and time in the market do the real heavy lifting.
Is compound interest always a good thing?
Compound interest is amazing when you're the one investing, but brutal when you owe debt. Later than I should have — which is exactly why the doubling-time figure above bugs me: every year you wait just slides the whole curve to the right. Credit card companies use daily compounding against you, which is why unpaid balances can spiral out of control so fast.
What rate do I need to double my money in 10 years?
You need an annual compound return of around 7.2% to double your money in a decade. That comes straight from the Rule of 72 (72 ÷ 10 = 7.2%). Historically, broad-market index funds like the S&P 500 have averaged around that range before adjusting for inflation.