ToolFoxCompound Interest, Explained Simply
20 September 2026 · 5 min read
Compound interest is interest earning interest. Instead of paying you only on your original money, it pays you on your original money plus all the interest that has already piled up. That small difference, repeated for years, is what turns steady saving into serious wealth — and what makes debt so hard to escape when it works against you.
This guide explains the idea, the formula, and one shortcut for estimating it in your head. Try real numbers in the compound interest calculator.
Simple vs compound
With simple interest, you earn a fixed amount each period on your original sum only. Put ₹1,00,000 at 10% simple interest and you earn ₹10,000 every year, forever — ₹10,000 in year one, ₹10,000 in year ten.
With compound interest, each year's interest is added to your balance and the next year's interest is calculated on the larger total. That same ₹1,00,000 at 10% compounded earns ₹10,000 the first year, but ₹11,000 the second (10% of ₹1,10,000), and more each year after. The gap between the two widens slowly at first, then dramatically.
The formula
The future value is A = P × (1 + r/n)^(n×t), where P is the starting amount, r is the annual rate as a decimal, n is how many times a year interest is compounded, and t is the number of years. The more frequently interest compounds — yearly, quarterly, monthly — the faster it grows, though the difference between monthly and yearly is smaller than most people expect.
The real engine in that formula is the exponent. Time is raised to a power, which is why the number of years matters more than almost anything else.
The Rule of 72
Here is a shortcut worth memorising: divide 72 by your annual return to estimate how many years it takes your money to double. At 8% a year, money doubles in about nine years (72 ÷ 8). At 12%, in about six. It is an approximation, but a remarkably good one for typical rates, and it lets you sanity-check any investment claim in your head.
Why starting early beats investing more
Because time is the exponent, the years you give your money matter more than the amount you add. A person who invests a modest sum for thirty years often ends up ahead of someone who invests far more but starts ten years later, simply because the early money had a decade of extra compounding.
This is the single most valuable lesson in personal finance: start now, even small. A monthly SIP started today and left alone will usually beat a larger one started later. Model it yourself in the compound interest calculator or the SIP calculator.