Free Compound Interest Calculator (+ Guide)

What is a compound interest calculator?

A compound interest calculator works out how a balance grows when each period's interest is added to it and starts earning interest too.

The textbook formula is A = P(1 + r/n)^(nt), where P is what you start with, r the yearly rate, n how often it compounds a year and t the years.

Deposits make it harder to do by hand, since each one starts compounding from the month it goes in. The calculator does that month by month, so a plan with a starting amount and monthly deposits comes out exact.

How to use the compound interest calculator

You only need two numbers to start, and everything else has a sensible default you can change.

Your numbers stay in this browser, and nothing is sent anywhere.

What the calculator shows you

The result is more than the end balance, and each part answers a different question.

The chart stacks your deposits under the interest, year by year, so you can see the interest band take over.

A worked example

Here's the example the calculator loads when you press Try an example, worked out in October 2026.

You start with $10,000, add $500 a month and earn 7% a year, compounded monthly, for 30 years.

After 30 years you'd have $691,150. You put in $190,000, so $501,150 is interest.

In year 9, the interest earned that year is bigger than the $6,000 you add that year. That's the crossover, and from there your money works harder than you do.

At 3% inflation a year, the $691,150 buys what about $284,745 buys today. That's why the calculator shows both, so a big number never fools you.

What changes the result the most

Four inputs move the end balance, and they don't move it equally.

The years

Time is the biggest lever, because the interest compounds on itself every month you stay in.

Starting 10 years later costs more than half the end balance, even with the same deposit. So the best year to start is this one.

The return

The rate you earn changes the end balance a lot over 30 years. Here's the same $10,000 plus $500 a month at different returns.

No return is promised, and a higher one usually means more ups and downs on the way. So run your plan at a lower rate too, and plan on that one.

How often it compounds

Compounding more often helps, but less than banks make it sound. Here's $10,000 at 7% for 10 years.

Daily beats yearly by $464.67 over 10 years. That's worth taking when two accounts are otherwise the same, but it never beats a higher rate or more years.

What you add each month

Your deposit is the one input you control every month. Drag it up in the calculator and watch the crossover year move closer.

Mistakes to avoid

A few habits make a compound interest plan look better on paper than it turns out.

• Planning on the best year's return instead of a long-run average
• Forgetting inflation, so a future balance feels bigger than it is
• Leaving out fees: a 1% yearly fee comes off your return every year
• Stopping deposits in a bad year, which is when they buy the most
• Pulling money out early and resetting the compounding

Run your plan with a lower return and your real fees, and anything above that is a bonus.

The quiet power of starting early

Compounding rewards patience more than brilliance, and the math in the tables above shows it.

Morgan Housel makes the same point with Warren Buffett in The Psychology of Money: most of Buffett's fortune came after his mid-60s, because he started early and never stopped.

The Psychology of Money

M

by Morgan Housel

The Psychology of Money by Morgan Housel explores how emotions, biases, and behavior shape financial decisions, offering timeless lessons on wealth.

Buy now Read summary

I may earn a commission if you make a purchase, at no additional cost to you.

For me, this is the math behind freedom. I wrote about the life it pays for in what is a global citizen.

Who is the compound interest calculator for?

It's for anyone saving or investing for something years away, and each of these tools takes a plan further.

• Retirement calculator – for checking if your plan carries the life after work
• CAGR calculator – for finding the real yearly growth of something you already own
• Inflation calculator – for seeing what your future balance buys in today's money
• Net worth calculator – for adding up where you stand today
• Side hustle quiz – for finding the extra income that raises your monthly deposit

Start with the compound interest calculator, then take the number into whichever of these fits your next question.

More money calculators

These calculators share the same stage, music and save options, so your plan moves from one to the next.

Pick the one that answers your next question, and your currency and scene come with you.