What compounding actually does
Compound interest is interest earned on interest. In the first period you earn a return on your principal; in every period after that you earn a return on the principal and on everything it has already earned. The balance does not climb in a straight line — it curves, gently at first and then steeply, because the amount doing the earning keeps getting larger.
That curve is why time matters more than rate for most savers. Money left for thirty years at a modest rate routinely beats money left for ten at a much better one, and no amount of clever rate-shopping closes the gap. It is also why the early years feel disappointing: almost all of the growth in a long projection happens in its final third.
Using this compound interest calculator
Enter your starting amount, the annual rate, how long you are investing for, and — optionally — a monthly contribution. The results show the future value, the interest earned, how much you contributed beyond the original principal, and the effective annual yield.
The compounding selector matters more than its size suggests. Yearly, half-yearly and quarterly compounding of the same nominal rate produce different results, because more frequent compounding starts paying interest on interest sooner. The effective annual yield row is what makes that comparable: it converts whatever nominal rate and frequency you entered into the single annual figure that would produce the same growth.
The monthly contribution field is off by default at 0. Turn it on and the row for “extra contributed” appears, which keeps an honest line between money you added and money the investment earned — a distinction that projections often blur.
The formula, worked through
The compound interest formula is A = P(1 + r/n)^(nt), where P is the principal, r the annual rate as a decimal, n the compounding periods per year, and t the years.
Take ₹10,000 at 6% for 10 years, compounded yearly. That is 10,000 × 1.06¹⁰ = 17,908.48 — so 7,908.48 in interest, on a deposit you never topped up. Switch to quarterly compounding and the same nominal 6% becomes 10,000 × (1.015)⁴⁰ = 18,140.18, about 232 more, with an effective annual yield of 6.14% rather than 6%.
The single most common error here is comparing a nominal rate against an effective one. A headline “6%” compounded quarterly is not the same product as a flat 6% a year, and the APY figure exists precisely so you do not have to take the headline at face value.
What a projection like this cannot tell you
It assumes a constant rate. Real savings rates move, and real investments do not return a tidy percentage every year — they return a sequence, and the order matters once you are drawing money out. Treat the output as the shape of the growth, not a forecast.
It also works in nominal terms. At 3% inflation, money doubling over 24 years has not doubled in what it buys. And it is before tax: interest is usually taxable in the year it is earned, and a tax-sheltered account and a taxable one with the same headline rate do not end up in the same place. None of that is a reason to distrust the arithmetic — it is a reason to read the result as a ceiling rather than a promise.
For a loan or a deposit where interest is charged only on the original principal, use the simple interest calculator instead.
FAQ
How long will it take to double my money?
The rule of 72 gives a quick estimate: divide 72 by the annual interest rate and you get the approximate number of years to double. At 6%, that is 72 ÷ 6 = 12 years. At 8%, nine years.
It is an approximation, not the formula, and it drifts at high rates — but between about 4% and 12% it lands within a few months of the exact answer, which is close enough to do in your head. Set the starting amount and rate above and watch the future value to check it against the real calculation.
Is this calculator before or after tax and inflation?
Before both. The projection uses the nominal rate exactly as you enter it, so the future value is in today's currency units at tomorrow's date — not in today's purchasing power.
Two rough adjustments help. For inflation, subtract your expected inflation rate from the interest rate before entering it: 6% growth with 3% inflation behaves roughly like 3% in real terms. For tax, if interest is taxed annually at your marginal rate, reduce the rate by that proportion. Both are approximations, but they stop a long projection from reading as more than it is.
What is compound interest?
Compound interest is interest calculated on both the original principal and the interest already earned — "interest on interest."
P = principal, r = annual rate, n = compounds per year, t = years. Because each period's interest gets added to the balance future interest is calculated on, growth accelerates instead of staying flat like simple interest — the table below shows $10,000 at 6% compared to simple interest over time:
| Year | Simple interest | Compound interest |
|---|---|---|
| 5 | $13,000 | $13,382 |
| 10 | $16,000 | $17,908 |
| 20 | $22,000 | $32,071 |
How does compounding frequency change my return?
More frequent compounding means interest starts earning interest sooner, which raises your effective annual yield (APY) above the stated nominal rate. The gap is real but shrinks quickly as compounding gets more frequent — here's a 6% nominal rate at each frequency this calculator supports:
The APY figure in the results shows the true annualized return for whichever frequency you've selected in the form.
Does adding monthly contributions really make a difference?
Yes — often more than the interest rate itself over long horizons, because each contribution gets its own runway to compound. A rough rule of thumb for how long money takes to double is the Rule of 72:
Contributing consistently effectively restarts that doubling clock on new money every month, which is why starting early tends to matter more than chasing a slightly higher rate. Set “Monthly contribution” above 0 to see the effect on your own numbers — the “Total contributed” result shows exactly how much of the final balance came from deposits versus growth.
How is this different from simple interest?
Simple interest is only ever calculated on the original principal, so it grows in a straight line. Compound interest is recalculated on the growing balance, so it grows exponentially — the longer the time horizon, the bigger the gap:
See our Simple Interest Calculator to compare the two on the same numbers.
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