Compound Interest Calculator

Calculate compound interest with monthly contributions, daily, monthly, quarterly, semiannual, or annual compounding. See exactly how much your savings or investment will be worth in the future.

What is Compound Interest Calculator?

A compound interest calculator shows you exactly how your money grows when interest is earned not just on your original deposit, but on all the interest you've already accumulated. This snowball effect is one of the most powerful forces in personal finance — Albert Einstein reportedly called compound interest the "eighth wonder of the world."

Whether you're saving in a high-yield savings account, growing a CD, or building a long-term investment portfolio, understanding compound growth helps you make smarter decisions about when to start saving, how much to contribute, and which compounding frequency benefits you most. This calculator lets you model different scenarios — adjusting your starting principal, monthly contributions, interest rate, time horizon, and compounding frequency — so you can see precisely how small changes today translate into significant differences decades from now.

Formula

A = P(1 + r/n)^(nt) + PMT × [((1 + r/n)^(nt) − 1) / (r/n)]

Where: A = final amount, P = principal, r = annual interest rate, n = compounding periods per year, t = time in years, PMT = periodic contribution

How to Calculate

Enter your initial principal (starting balance).
Add a monthly contribution amount, if any.
Input your expected annual interest rate.
Choose your compounding frequency and select the number of years to see your final balance.

Example

Starting with $10,000, contributing $100/month, at a 7% annual rate compounded monthly for 20 years: your balance grows to approximately $79,900. Of that, $34,000 comes from your own contributions and roughly $45,900 is pure compound interest — more than the total amount you deposited.

Key Benefits

See your exact future balance instantly
Compare compounding frequencies side by side
Understand how contributions accelerate growth
Visualize your interest versus contributions breakdown

Common Mistakes to Avoid

Waiting to start investing loses years of compounding
Underestimating how much monthly contributions matter
Ignoring how fees quietly erode compound growth

Pro Tips

Start as early as possible, even with small amounts
Automate monthly contributions to stay consistent
Reinvest all interest instead of withdrawing it

Key Terms Explained

Principal: Your original starting deposit or investment
Compounding Frequency: How often interest is calculated and added
APY: Annual percentage yield including compounding effects
Rule of 72: Quick formula estimating years to double your money

Common Use Cases

Projecting the future value of a savings account or CD
Estimating long-term investment portfolio growth
Comparing how different compounding frequencies affect returns
Planning how much to contribute monthly to reach a future balance

Frequently Asked Questions

What is compound interest and how does it work?

Compound interest is interest calculated on both your original principal and the interest that has already accumulated. Unlike simple interest, which only grows on the initial deposit, compound interest lets your earnings generate their own earnings, creating exponential growth over time. The more frequently interest compounds — daily, monthly, or quarterly — the faster your balance grows.

How does compounding frequency affect my returns?

More frequent compounding produces slightly higher returns because interest is calculated and added to your balance more often. Daily compounding earns more than monthly, which earns more than annual compounding, though the difference shrinks as rates get smaller. On a $10,000 balance at 7% for 20 years, daily compounding versus annual compounding differs by only a few hundred dollars.

How much difference do monthly contributions make?

Regular monthly contributions dramatically accelerate compound growth. Adding just $100 per month to a $10,000 investment earning 7% annually for 20 years adds roughly $52,000 to your final balance — more than double what the contributions alone total, thanks to compounding on every deposit.

What is the Rule of 72 and how does it relate to compound interest?

The Rule of 72 is a quick way to estimate how long it takes an investment to double: divide 72 by your annual interest rate. At 7% annual growth, your money doubles in roughly 72 ÷ 7 = 10.3 years. It's a handy mental shortcut, though this calculator gives you the exact figure.

Should I use compound interest for savings or investing?

Compound interest applies to both. Savings accounts and CDs use compound interest at lower, safer rates, while investment accounts (like index funds) benefit from compounding at historically higher average returns. The key principle is the same either way: start early, contribute consistently, and let time do the heavy lifting.