Compound Interest Calculator

What is Compound Interest?

Compound interest is “interest on interest.” Instead of earning interest only on your original deposit, you earn interest on the growing total — including all the interest you’ve already earned. This creates a snowball effect that makes your money grow faster over time.

How to Calculate Compound Interest

The calculator has five inputs. The currency symbol ($, EUR, KRW, etc.) adapts to your browser language automatically.

Inputs

Input	What to enter
Initial Investment	The amount you start with (e.g., $10,000)
Monthly Contribution	How much you add every month (e.g., $500)
Annual Interest Rate	Expected yearly return as a percentage (e.g., 7%)
Investment Duration	How long you invest. Toggle between Years + Months or Total Months
Compound Frequency	How often interest is calculated. Pick a unit (Days/Months/Years) and enter a number

Compound Frequency Examples

Setting	Meaning
Month 1	Monthly compounding (12 times/year) — most common
Month 3	Quarterly (4 times/year)
Year 1	Annually (1 time/year)
Day 1	Daily (365 times/year)

What the Results Mean

Result	What it shows
Total Balance	Your final amount including everything — principal, contributions, and all interest earned
Total Contributions	The money you actually put in (initial + monthly deposits)
Total Interest	The money earned purely from compound interest — this is the “free money”
Chart	Visual growth over time. The gap between the two lines shows how much interest is accelerating

Good Examples

Example 1: Retirement Savings (30 years)

Starting with $5,000, contributing$300/month at 7% annual return, compounded monthly.

Result	Value
Total Balance	~$370,000
Total Contributions	~$113,000
Total Interest	~$257,000

You put in $113K but end up with$370K — more than double came from compound interest alone.

Example 2: Emergency Fund (5 years)

Starting with $0, saving$400/month in a high-yield savings account at 5%, compounded monthly.

Result	Value
Total Balance	~$27,200
Total Contributions	$24,000
Total Interest	~$3,200

Even at a modest 5%, compounding adds an extra $3,200 over 5 years.

Example 3: Education Fund (18 years)

A parent invests $2,000 at birth and adds$150/month at 6%, compounded monthly.

Result	Value
Total Balance	~$62,000
Total Contributions	$34,400
Total Interest	~$27,600

Nearly half the final amount is pure interest.

Example 4: The Cost of Waiting 10 Years

Same $300/month at 7%, but starting at age 25 vs 35 (until age 60):

Start Age	Years	Total Balance
25	35 years	~$570,000
35	25 years	~$270,000

Starting 10 years earlier more than doubles the outcome. Time is the most powerful factor.

Compound Interest vs Simple Interest

Comparison	Simple Interest	Compound Interest
How it works	Interest only on the original amount	Interest on the original + all accumulated interest
$10,000 at 5% for 10 years	$15,000	~$16,289
$10,000 at 5% for 30 years	$25,000	~$43,219
Best for	Short-term, predictable returns	Long-term growth (savings, investments)

The gap widens dramatically over longer periods — that’s the power of compounding.

Key Tips

The Rule of 72. Divide 72 by your annual interest rate to estimate how many years it takes to double your money. At 7%, your money doubles roughly every 10 years.

Contributions matter more than you think. $10,000 at 7$38,700. But adding just $200/month turns it into$142,000.

Start early, even if it’s small. $100/month starting at age 22 beats$300/month starting at age 32 — by age 60.

Compound interest works against you with debt. Credit card balances at 20% APR compounded daily can take over 10 years to pay off with minimum payments. Paying off high-interest debt is often the best “investment” you can make.

Formula

$A = P \left(1 + \frac{r}{n}\right)^{nt} + PMT \times \frac{\left(1 + \frac{r}{n}\right)^{nt} - 1}{\frac{r}{n}}$

Variable	Meaning
A	Future value (total balance)
P	Principal (initial investment)
PMT	Monthly contribution
r	Annual interest rate (as decimal)
n	Compounds per year
t	Number of years

Frequently Asked Questions

How often should interest compound?

More frequent = slightly more return. But the practical difference between daily and monthly compounding is very small. Monthly is the standard for most accounts. The biggest jump comes from annual → monthly.

What is the Rule of 72?

Divide 72 by the annual interest rate to estimate doubling time. At 6% → ~12 years. At 9% → ~8 years. Works best for rates between 2-15%.

Does compound interest work against me with debt?

Yes. A $5,000 credit card balance at 20% APR compounded daily, with only minimum payments, can take over 10 years to pay off and cost thousands in interest.