Compound Interest Calculator
It projects how a starting balance plus regular contributions grows at a given return, compounded at your chosen frequency.
Value after 30 years
$386,158
You contribute
$100,000
Interest earned
$286,158
Growth multiple
3.86×
What it calculates
It projects how a starting balance plus regular contributions grows at a given return, compounded at your chosen frequency.
Why it matters
Compounding is non-linear. The last decade of a 30-year plan usually creates more growth than the first two combined.
Who it's for
Savers, index investors, parents building an education fund, and anyone comparing deposit accounts.
Formula
- P
- Initial principal
- PMT
- Regular contribution
- r
- Annual interest rate
- n
- Compounding periods per year
- t
- Years invested
Worked example
$10,000 start, $300/month, 7% for 20 years
- 1Future value of the lump sum: 10,000 · (1 + 0.07/12)²⁴⁰
- 2Future value of contributions: 300 · [((1 + 0.07/12)²⁴⁰ − 1) / (0.07/12)]
- 3Add both components
≈ $196,000, of which about $114,000 is growth
How the compound interest calculator works
Interest earns interest. Each period the balance is multiplied by (1 + r/n), so growth accelerates as the balance grows. Contributions made earlier compound for longer, which is why time in the market dominates timing.
P is your starting principal, r the annual nominal rate, n how many times interest compounds per year and t the number of years.
Regular contributions are added at the end of every compounding period, so the blue block below is what you put in and the green block is interest earned on top.
Common mistakes
- Using a nominal return without subtracting inflation and fees.
- Assuming a smooth annual return — real markets are volatile even when the average holds.
- Forgetting tax on interest or dividends in a taxable account.
Tips and best practice
- Increase contributions with your income; even 1% a year compounds hard.
- Compare the same scenario at 5%, 7% and 9% to see how sensitive the outcome is.
Frequently asked questions
What is the compound interest formula?
A = P(1 + r/n)^(nt) for a lump sum, plus PMT · [((1 + r/n)^(nt) − 1)/(r/n)] for regular contributions.
How often should interest compound?
More frequent compounding gives slightly more growth, but the difference between monthly and daily is small compared with the rate itself.
What return should I assume?
Long-run global equity returns have averaged roughly 6–8% nominal. Cash savings are far lower. Use a conservative figure for planning.
Does this account for inflation?
No. To see real purchasing power, subtract expected inflation from your return before entering it.
Related calculators
Further reading
Methodology & trust
- Formula source
- Compound interest and future value of an annuity formulas.
- Last updated
- 2026-07-28
- Privacy
- Every calculation runs in your browser. No inputs are sent to a server or stored.
- Accessibility
- Keyboard navigable, labeled inputs and WCAG AA color contrast.