How much could your money grow? Enter what you have now, how much you add, how often and for how long. This compound interest calculator shows your future balance, how much came from you vs. from compounding, and how much waiting would cost you. You can add money daily, weekly, monthly or yearly.
I built this because compound interest is the reason I started investing at 19. I invest in VOO about once a week, so I wanted a calculator that handles weekly (and even daily) contributions, not just monthly ones.
Compound Interest Calculator
See how your money grows when your returns start earning returns.
Year-by-year table
How to read your results
You put in is every dollar you add: your starting amount plus all your contributions.
Growth is everything compounding added on top. It’s your balance minus what you put in.
From growth is the share of your final balance that came from compounding, not from you.
Doubles about every uses the Rule of 72: 72 ÷ your yearly return.
Cost of waiting runs the same plan with fewer years, as if you started later, and shows the difference.
Today’s dollars divides future amounts by inflation, so you see what they’d actually buy today. Contributions you make later are adjusted too.
How this is calculated
Compound interest is the interest you earn on interest, as the SEC’s Investor.gov puts it. Your yearly return and compounding choice are turned into an effective yearly rate: (1 + rate ÷ n)n − 1, where n is 365 for daily, 12 for monthly, 4 for quarterly or 1 for yearly.
That rate is then split evenly across your contribution schedule (daily, weekly, monthly or yearly). Each period your balance grows by that rate, and your contribution is added at the start or end of the period, depending on your setting.
With “yearly” compounding, the yearly return works like an APY, which is the easiest way to think about stock market returns like the S&P 500’s long-run average of about 10%.
Ready to start compounding? How to open your first brokerage account
Assumes a steady return every year. Real returns go up and down, and past returns don’t guarantee future results. This isn’t financial advice.
How to Use the Compound Interest Calculator
- Starting amount: the money you’re investing now. Enter 0 if you’re starting from scratch.
- Contribution and how often: how much you’ll add, and whether that’s daily, weekly, monthly or yearly.
- Yearly return: your expected average return. The stock market has averaged about 10% a year since 1926, or about 7% after inflation.
- Years: how long you’ll let it grow.
- Compounding: how often growth is added: daily, monthly, quarterly or yearly. For stock market returns, leave it on yearly.
- Add contributions at the start or end: start means each deposit grows for that whole period, so the total ends up slightly higher.
- Cost of waiting: how many years later you might start. The calculator shows what that delay costs.
- Show in today’s dollars: adjusts everything for inflation, so you see what your money would actually buy today.
Worked Example: $500 a Month for 30 Years
Say you start with $0 and invest $500 a month at 10% a year. That’s $180,000 of your own money over 30 years.
| After | Balance | You put in |
|---|---|---|
| 10 years | $99,932 | $60,000 |
| 20 years | $359,130 | $120,000 |
| 30 years | $1,031,422 | $180,000 |
$851,422 of it, about 83%, came from compounding, not from you.
Look at the last 10 years. The balance grew by $672,292, more than the first 20 years combined. That’s the snowball. It’s also why waiting hurts: if you started 10 years later and invested for only 20 years, you’d have $359,130 instead, about $672,292 less.
In today’s dollars, with 3% inflation, the $1,031,422 would buy what about $424,932 buys today. That’s still more than 3.5 times the $119,211 your deposits were worth.
Does Compounding Frequency Matter?
A little, but much less than time. Here’s $10,000 at 10% for 10 years:
| Compounding | Balance |
|---|---|
| Yearly | $25,937 |
| Quarterly | $26,851 |
| Monthly | $27,070 |
| Daily | $27,179 |
Daily beats yearly by $1,242. Adding 10 more years would roughly triple the balance. How long you invest matters far more than how often it compounds.
The Rule of 72
The calculator also shows how often your money doubles. Divide 72 by your yearly return:
- At 10%, your money doubles about every 7.2 years.
- At 7%, about every 10.3 years.
- At 3.5% (a high-yield savings account), about every 20.6 years.
Why Starting Early Wins
The money you invest first has the most time to compound. $100 a week from age 18 to 67 can grow to about $5.76 million at 10%. Start at 30 and it’s about $1.80 million. Same $100 a week, 12 fewer years, almost $4 million less.
I break this down, with my own VOO buys, in What Is Compound Interest? The Idea That Made Me Start Investing at 19.
Want to see what inflation does to cash and savings? Try my Saving vs Investing Calculator.
Ready to start? Here’s how to open your first brokerage account and how to buy your first stock. Not sure what to buy? Here’s what an ETF is and why I buy VOO.
Compound Interest Calculator FAQ
What is compound interest?
It’s earning money on your money, and then on those earnings too. Over time, your growth starts to grow by itself.
How much will $500 a month be worth in 30 years?
At 10% a year, about $1,031,422 from $180,000 of contributions. At 7%, about $589,000.
Should I choose daily or yearly compounding?
For a savings account, use whatever your bank uses, often daily. For stock market returns like 10%, use yearly, since the average return is already a yearly number.
What return should I use?
10% is the S&P 500’s long-run average before inflation. 7% is roughly the average after inflation. A high-yield savings account pays around 3.5% to 4%.
What does “today’s dollars” mean?
It’s what a future balance would actually buy today, after inflation. Turn it on to see your real buying power.
Is a 10% return guaranteed?
No. It’s a long-run average. Some years are up 25%, and some are down 20% or more.
What does the cost of waiting show?
It runs the same plan with fewer years, as if you started later, and shows how much less you’d end up with.
This calculator gives estimates for informational purposes only and isn’t financial advice. Actual returns, rates and inflation will vary.