Use this compound interest calculator to see how your savings or investments can grow over time. Enter a starting amount, a monthly deposit, an interest rate and a time frame. You’ll see your future balance, how much of it is interest, what it’s worth in today’s dollars, and a year-by-year schedule.
—
Growth by year
Year-by-year schedule
| Year | Deposits | Interest | Balance |
|---|
Estimates only. Assumes a constant interest rate, no taxes or fees, and no withdrawals. Investment returns are not guaranteed and can be negative.
What is compound interest?
Compound interest means you earn interest on your interest, not just on the money you deposit. Each time interest is added to your balance, the next interest payment is calculated on that larger balance, so growth speeds up the longer you leave the money alone.
With the calculator’s default example, $10,000 plus $200 a month at 7% for 20 years grows to $144,573. You deposit $58,000, and the other $86,573 is interest, about 60% of the final balance.
The compound interest formula
For a single deposit, the future value is:
A = P × (1 + r/n)n×t
where P is your starting amount, r is the yearly interest rate as a decimal, n is how many times a year interest compounds, and t is the number of years. Regular monthly deposits each grow the same way from the month they’re added, which the calculator adds up for you.
Time matters more than anything else
Because growth builds on itself, the last years add the most. Here’s the same $10,000 plus $200 a month at 7%:
| Years | You deposit | Final balance |
|---|---|---|
| 10 | $34,000 | $54,714 |
| 20 | $58,000 | $144,573 |
| 30 | $82,000 | $325,159 |
| 40 | $106,000 | $688,077 |
Starting early makes a big difference. Saving $300 a month at 7% from age 25 to 65 grows to about $787,444. Waiting until 35 and saving the same amount for 30 years leaves you with about $365,991, less than half. To catch up, the late starter would need to save roughly twice as much each month.
How your interest rate changes the result
Here’s the same plan ($10,000 plus $200 a month for 20 years) at different rates:
| Rate | Example | Balance after 20 years |
|---|---|---|
| 0.37% | National average savings rate | $60,581 |
| 4.2% | Top high-yield savings account | $98,157 |
| 7% | Long-term stock market assumption | $144,573 |
| 10% | Optimistic stock return | $225,155 |
The national average savings rate was 0.37% in October 2026, while top online high-yield savings accounts paid around 4.1% to 4.35% APY. Savings rates can change at any time, and stock returns aren’t guaranteed and vary widely from year to year.
Does compounding frequency matter?
Less than most people expect. At a 7% rate on the default example, compounding annually gives $140,204, monthly gives $144,573, and daily gives $144,982. That’s why banks quote APY (annual percentage yield), which already includes the effect of compounding: 7% compounded monthly is a 7.23% APY. When comparing savings accounts, compare APYs.
The Rule of 72
To estimate how long it takes money to double, divide 72 by the interest rate. At 7%, that’s about 10 years; at 4%, about 18 years. The calculator shows the exact doubling time for your rate and compounding frequency.
Inflation and “today’s dollars”
A dollar in 20 years will buy less than a dollar today. U.S. consumer prices rose 3.4% over the year to July 2026. At 3% yearly inflation, the $144,573 in the default example would buy what about $80,046 buys today. Use the inflation setting under “More options” to see your result in today’s dollars.
Ways to make compounding work harder
- Start now, even with a small amount. Time in the account does most of the work.
- Raise your deposits each year. Increasing the $200 deposit by 3% a year grows the default example to about $171,236 instead of $144,573.
- Move idle cash out of low-rate accounts. $10,000 left for 5 years grows to about $10,187 at 0.37%, compared with about $12,332 at 4.2%.
- Use tax-advantaged accounts like a 401(k) or IRA so taxes don’t slow your growth.
- Pay off high-interest debt first. Credit cards compound against you at over 20%. See how quickly you can clear a balance with our credit card payoff calculator.
Frequently asked questions
How do I calculate compound interest?
For a single deposit, use A = P × (1 + r/n)^(n×t), where P is the starting amount, r is the yearly rate as a decimal, n is compounding periods per year and t is years. For regular deposits, each deposit grows from the month it’s added. The calculator handles both.
How much will $10,000 grow in 20 years?
At 7% compounded monthly, $10,000 grows to about $40,387 in 20 years with no extra deposits. Adding $200 a month raises it to about $144,573.
Is daily compounding better than monthly?
Slightly. At 7%, daily compounding gives a 7.25% APY versus 7.23% for monthly. Over 20 years on the default example, the difference is about $410.
What is the difference between APR and APY?
APR is the yearly rate without compounding. APY includes the effect of compounding, so it shows what you actually earn in a year. Compare savings accounts by APY.
What interest rate should I use?
Use your account’s APY for savings. In October 2026, the national average savings rate was 0.37%, and top high-yield accounts paid around 4.1% to 4.35%. For long-term stock investing, many people use 6% to 7% as a conservative assumption, but returns are not guaranteed.
This calculator provides estimates for educational purposes only and is not financial advice. Sources: NerdWallet: national average and high-yield savings rates; SwitchWize: savings rate watch, October 2026; FactSet: consumer price index.