What is Compound Interest?

Compound interest means earning interest on your interest. Each period, the interest you earned gets added to your principal, and the next period's interest is calculated on the bigger amount. The formula is:

A = P × (1 + r/m)^(m×t)

Here, P is your starting amount, r is the yearly rate, m is how many times a year the interest compounds, and t is the number of years. As an example, ₹1 lakh at 8% becomes ₹2.16 lakh in 10 years with annual compounding, and ₹2.22 lakh with monthly compounding. Frequency helps a little, but time is what really matters: the same money left for 20 years reaches ₹4.66 lakh.

Where You Meet Each Compounding Frequency

Most Indian bank FDs and post-office deposits compound quarterly. PPF, Sukanya Samriddhi and NSC compound annually. EPF interest accrues monthly on your balance. Knowing the frequency lets you compare products honestly, since a 7.5% rate compounded quarterly is worth slightly more than the same rate compounded annually.

Two Rules of Thumb Worth Remembering

The Rule of 72 says your money doubles in roughly 72 divided by the rate, so about 9 years at 8%. The Rule of 114 does the same for tripling. These quick estimates are handy for checking claims before you reach for a calculator. For the contrast with interest that never compounds, see the simple interest calculator.

Why Starting Early Beats Everything Else

Watch what happens to ₹1 lakh over 30 years at different rates: ₹5.74 lakh at 6%, ₹10.06 lakh at 8%, ₹17.45 lakh at 10%. Now hold the rate at 8% and change only the time: ₹2.16 lakh in 10 years, ₹4.66 lakh in 20, ₹10.06 lakh in 30. Each extra decade multiplies the outcome by more than the previous one. In practice, starting ten years earlier beats finding two extra percent of return, and both beat trying to time the market.

Compounding Can Work Against You Too

The same mathematics powers credit-card debt, which at 36% to 42% a year doubles an unpaid balance in roughly two years, and inflation, which at 5% halves your purchasing power in about 14 years. Once you start asking "what does this percentage become over a decade?", loan offers and salary increments become much easier to judge.

FAQs about Compound Interest Calculator

A = P × (1 + r/m)^(m×t): principal P, annual rate r, m compounding periods per year, t years. Interest is added to principal each period and itself earns interest thereafter.
It helps modestly: ₹1 lakh at 8% for 10 years gives ₹2,15,892 annually compounded versus ₹2,21,964 monthly — about 2.8% more. Rate and time matter far more than frequency.
Roughly 72 ÷ annual rate in years: about 10.3 years at 7%, 9 at 8%, 6 at 12%. The exact answer comes from the compound-interest formula this calculator uses.
Bank FDs typically compound quarterly; PPF, SSY and NSC annually; EPF accrues monthly. Mutual funds do not pay interest at all — their NAV growth is inherently compound.
Because compounding is convex in time: at 8%, money grows 2.2× in 10 years but 10× in 30. A 25-year-old's rupee has three doublings ahead of it that a 45-year-old's rupee does not — no realistic rate difference makes up for the lost decades.
The single annual rate equivalent to a quoted rate with intra-year compounding: EAR = (1 + r/m)^m − 1. A "7.5% compounded quarterly" FD has an EAR of about 7.71% — the honest number for comparing products with different compounding frequencies.