See how your money grows when interest earns interest. Add a monthly deposit, choose how often interest compounds (even daily), and get a year-by-year breakdown.
The compound interest formula
A = P × (1 + r/n)^(n × t)
P is the starting amount, r the yearly rate as a decimal, n how many times a year interest is added, and t the number of years. For $10,000 at 5% compounded monthly for 10 years: 10,000 × (1 + 0.05/12)^120 = 16,470.09.
With a regular deposit (PMT) at the end of each period, the deposits grow too:
A = P(1 + i)ᴺ + PMT × ((1 + i)ᴺ − 1) ÷ i
Adding $200 a month to the example above gives 47,526.55, of which $34,000 is money you put in and the rest is interest.
How much does compounding frequency matter?
$10,000 at 5% for 10 years:
| Compounded | Balance |
|---|
| Yearly | $16,288.95 |
| Monthly | $16,470.09 |
| Daily | $16,486.65 |
| Continuously | $16,487.21 |
More frequent compounding helps, but much less than the rate and the time. Going from yearly to daily adds about $197.70 here; adding 1% to the rate or a few more years adds far more.
APR vs APY
Banks quote savings accounts with an APY (annual percentage yield), which already includes compounding. A 5% rate compounded monthly is an APY of 5.116%. Loans are usually quoted with an APR, which doesn't include compounding. When comparing accounts, compare APY to APY.
The rule of 72
Divide 72 by the interest rate to estimate how many years it takes to double your money. At 6%, it takes about 12 years; at 9%, about 8.
How we checked this
Before every update, this calculator is run in a real browser with these known answers, and it must match all of them:
- $10,000 at 5% yearly for 10 years = $16,288.95 ✓
- $10,000 + $200/month at 5% monthly, 10 years = $47,526.55 ✓
Last checked October 26, 2026. Spotted something wrong? The math is done in your browser with the formula shown on this page, so you can check any result by hand.
Questions people ask
How do I calculate compound interest?
Use A = P(1 + r/n)^(nt). For $1,000 at 6% compounded yearly for 5 years: 1,000 × 1.06⁵ = $1,338.23, so the interest is $338.23.
Is daily compounding much better than monthly?
Only slightly. On $10,000 at 5% for 10 years, daily compounding gives $16,486.65 versus $16,470.09 monthly, a difference of $16.55.
Should deposits be at the start or end of the month?
Deposits at the start of each month earn one extra month of interest each time, so the total is a little higher. Choose whichever matches when you actually save.
What's a realistic interest rate?
High-yield savings accounts and CDs pay what the market offers at the time; check your bank's current APY. Stock market returns aren't fixed and vary a lot from year to year.
Does this include taxes or inflation?
No. Interest in a regular savings account is usually taxable, and inflation reduces what the money will buy. Both are left out to keep the math clear.