Compound interest is interest earning interest. Instead of paying out each period, the interest is added to your balance so the next period's interest is calculated on a bigger number. Over time that snowball becomes the single most powerful force in personal finance. This guide shows the formula, a worked example, and the two levers that matter most — with a free compound interest calculator to run your own numbers.
What compound interest is
With simple interest you earn the same amount every year, because it's always calculated on the original principal. With compound interest, each year's interest is added to the balance, so next year you earn interest on the interest too. The gap between the two starts small and grows dramatically the longer you leave it.
The compound interest formula
A = P × (1 + r ÷ n)n × t
- A = final amount (principal + interest)
- P = principal (starting amount)
- r = annual interest rate as a decimal (8% = 0.08)
- n = times interest compounds per year
- t = number of years
The interest earned is simply A − P.
Worked example
₹1,00,000 at 8% for 10 years, compounded annually (n = 1):
| Principal (P) | ₹1,00,000 |
| Rate (r) | 0.08 |
| Years (t) | 10 |
| Final amount (A) | 1,00,000 × 1.08¹⁰ ≈ ₹2,15,892 |
| Interest earned | ≈ ₹1,15,892 |
Your money more than doubles without you adding a rupee. Simple interest at the same rate would earn only ₹80,000 — compounding adds an extra ₹35,892 purely from interest-on-interest.
Why compounding frequency matters
The more often interest compounds, the more you earn, because interest starts earning sooner. On the same ₹1,00,000 at 8% for 10 years:
- Annually (n = 1) → ≈ ₹2,15,892
- Quarterly (n = 4) → ≈ ₹2,20,804
- Monthly (n = 12) → ≈ ₹2,21,964
The jumps shrink as frequency rises, but more frequent compounding always wins. It's why the same “8%” can mean different real returns depending on the fine print.
Compound vs simple interest
Compounding rewards time above all. Doubling the years far more than doubles the interest, because the later years grow the largest balances. This is exactly why starting to invest early beats investing more later — the same logic that powers a SIP, where each monthly instalment compounds until you withdraw.
FAQ
What's the difference from simple interest? Simple interest is always on the original principal; compound interest is on the growing balance, so it accelerates over time.
Does compounding frequency change the result? Yes — monthly compounding beats annual at the same rate, though the extra gain shrinks at higher frequencies.
How can I double my money? Roughly, divide 72 by the interest rate for the years to double (the “Rule of 72”) — 8% ≈ 9 years.
Is my data private? Yes — the calculator runs entirely in your browser and stores nothing.