Compound Interest Calculator

Compound interest is interest earned on interest. Unlike simple interest — which is always calculated on the original principal — compound interest grows faster because each period's interest is added to the balance and itself earns interest the next period. The idea is genuinely ancient: interest-on-interest appears in Mesopotamian records from thousands of years ago, and the mathematics was worked out properly in the 17th century. It remains the single most important force in personal finance — for good when you are saving, and against you when you carry debt.

The formula

A = P × (1 + r/m)^(m × t)

• P — principal amount you start with
• r — annual interest rate (as a decimal)
• m — number of times interest compounds per year (1 yearly, 4 quarterly, 12 monthly, 365 daily)
• t — number of years
• A — maturity value (principal + interest)

Worked example

Invest ₹1,00,000 at 7% per year for 10 years, compounded quarterly:

• m = 4, r = 0.07
• A = 1,00,000 × (1 + 0.07/4)^(4 × 10)
• A = 1,00,000 × (1.0175)⁴⁰ ≈ ₹2,00,160

You earn about ₹1,00,160 in interest — more than the principal itself. Compare that with simple interest, which would earn just 1,00,000 × 0.07 × 10 = ₹70,000 over the same period.

Why compounding frequency matters

The more often interest compounds, the faster your money grows — but the effect is modest. Here is the same ₹1,00,000 at 7% for 10 years under each frequency:

Switching from yearly to daily compounding adds about ₹4,647 over a decade — a real but small difference. The dominant variable is time, not frequency.

The rule of 72

To estimate how long your money takes to double, divide 72 by the interest rate. At 7%, doubling takes about 10.3 years; at 12%, around 6 years. The rule is an approximation, but it is excellent for quick comparisons: if an investment "doubles in 10 years", it is delivering roughly 7% a year; if it doubles in 4 years, roughly 18% — a red flag for risk.

The case for starting early

Extend the same example to 20 years: ₹1,00,000 at 7% compounded quarterly grows to about ₹4,00,639 — double the 10-year figure again. The second decade added ₹2,00,480 while the first added ₹1,00,160, because the interest from decade one was itself earning. Two people who save the same total but in different decades end up with very different outcomes: the one who starts at 25 with more time hands the eventual sum a far longer runway, even if the later saver contributes more each month.

Compound interest in debt — the dark side

The same mechanism works against you on credit cards, payday loans and personal loans. A card balance compounding at 36% a year doubles the debt in about two years if untouched (rule of 72: 72 ÷ 36 = 2). That is why paying down high-interest debt is almost always a better "return" than any low-risk investment you could buy with the same money.

Time is the real multiplier: the longer you stay invested, the more powerful compounding becomes — which is why starting early matters more than starting big.