QuietCalcsThe formula, and why frequency barely matters
For a lump sum, compound interest follows A = P (1 + r/n)nt — principal P, annual rate r, compounded n times a year for t years. Monthly contributions are layered on top, each one compounding from the month it lands.
People often agonize over compounding frequency, but at everyday rates it's nearly a rounding error: $10,000 at 7% for 30 years is $76,123 compounded annually and $81,165 compounded monthly — while doubling the time or the rate changes the result by multiples, not percents. Time in, rate, and contributions are the levers that matter, in that order.
The rule of 72
A useful head-math shortcut: money doubles roughly every 72 ÷ rate years. At 7%, that's about every 10 years — so a 30-year horizon is three doublings, or 8× your starting money. It's also why starting ten years earlier roughly doubles the end result, no extra saving required.
What rate should you use?
Match the rate to the actual vehicle: a high-yield savings account pays whatever it advertises (and that changes with central-bank rates); broad stock index funds have averaged roughly 10% per year nominally over the last century, with brutal individual years along the way. If you want the result in today's buying power, use a real rate (return minus inflation — about 7% for the long-run stock average) or sanity-check the output with the inflation calculator.
Frequently asked questions
Are contributions added at the start or end of each month?
End of month, the common convention. The difference versus start-of-month is one month of interest on each contribution — visible but small.
What does compounding frequency actually change?
How often earned interest starts earning its own interest. Daily versus monthly at 7% changes a 30-year result by well under 1% — it matters far less than rate or time.
Does this account for taxes or fees?
No. In taxable accounts, taxes on interest or dividends reduce the effective rate; fund fees do too. Subtract them from the rate input for a more honest projection.
Is my data stored?
Only in your own browser — nothing is uploaded, and your inputs are simply there when you come back.
Related calculators
Coast FIRE Calculator — compound growth pointed at a retirement target.
How Long Will My Money Last — the same math running in reverse.
Inflation Calculator — what that future balance buys in today's money.