Last updated 3 September 2026
The formula
A = P(1 + r/n)^(nt) — the starting principal P, growing at annual rate r, compounded n times a year, over t years. Interest earns its own interest at every compounding point, which is what makes growth accelerate over time rather than staying flat.
How much frequency actually matters
Going from annual to monthly compounding makes a real difference over a decade or more. Going from monthly to daily makes very little difference at typical rates — the gain from more frequent compounding shrinks quickly once you're past a handful of times per year.
The rule of 72
A quick way to sanity-check any compounding figure without a calculator: divide 72 by the annual rate and you get roughly the number of years for the money to double. At 6% that is twelve years, at 8% it is nine, at 12% it is six. The approximation is close enough to be useful between about 4% and 15%, and it gives you an immediate feel for whether a projected figure is plausible before you trust it.
It also makes the cost of a small rate difference obvious. Money at 5% doubles in a little over fourteen years; at 7% it doubles in about ten. Over a forty-year horizon that is roughly two and a half doublings versus four — the difference between six times the starting amount and sixteen times it, from two percentage points.
Assumptions and limitations
What the maths does. It applies the standard compound interest formula at the compounding frequency you choose, with any regular contribution added at the same interval and the rate held constant for the whole period.
A constant rate is the big simplification. Savings rates change, and investment returns vary year to year and can be negative. A single fixed rate produces a smooth curve that no real account follows.
Tax, fees and inflation are not deducted. Interest may be taxable depending on the account type and your country. Platform or fund fees reduce the return directly. And the total is in future currency — inflation reduces what it will buy, so a figure that looks large in thirty years is worth less than it appears.
Assumptions that apply to every finance calculator here
Currency is a display choice only. Changing the currency symbol relabels the output; it does not convert anything and no exchange rate is used anywhere in this tool.
Tax and lending rules vary by country. Nothing here is adjusted for the rules where you live — income tax, capital gains tax, stamp duty, lending caps, affordability tests and consumer-credit regulation all differ, and several of them can change the real answer materially.
Rates and returns are not guaranteed. Any rate you enter is treated as fixed for the whole period. Real interest rates move, real investment returns vary year to year and can be negative, and past performance does not predict future results.
This is an estimate, not advice. The result is arithmetic on the numbers you typed. It is not an offer, a quote, an approval, an investment recommendation or financial advice. Before committing to anything, get figures from the actual lender, provider or a qualified adviser in your country.
Common questions
Why does compounding frequency matter?
More frequent compounding means interest starts earning its own interest sooner. The difference between annual and monthly compounding is real but modest at typical rates — daily versus monthly barely matters, while annual versus monthly is worth noticing over many years.
What's the formula being used?
A = P(1 + r/n)^(nt) — principal, growing at rate r, compounded n times per year, for t years. It's the standard compound interest formula used for savings accounts, fixed deposits and similar products.
Does this account for taxes on the interest?
No — this shows gross growth before any tax on interest income, which varies by country and account type. Check your local rules for what you'd actually keep.