Simple vs Compound Interest: Formulas, Examples and Why It Matters
Interest is the price of money over time. Whether you are saving or borrowing, the way that interest is calculated — simply or with compounding — makes an enormous difference over the years. Understanding the gap between the two is one of the most valuable pieces of financial literacy you can have.
Simple interest
Simple interest is calculated only on the original amount (the principal), never on the interest already earned. The formula is: interest = P × r × t, where P is the principal, r is the annual rate as a decimal, and t is the number of years. Put ₹1,00,000 at 8% simple interest for 5 years and you earn ₹1,00,000 × 0.08 × 5 = ₹40,000. Each year adds exactly ₹8,000 — no more, no less.
Compound interest
Compound interest is calculated on the principal AND on the interest that has already accumulated. Your interest earns interest. The formula is: final amount = P × (1 + r/n)^(n×t), where n is how many times per year interest is compounded. With the same ₹1,00,000 at 8% compounded annually for 5 years, you end with ₹1,00,000 × (1.08)^5 ≈ ₹1,46,933 — about ₹6,900 more than simple interest over just five years.
Why the gap widens dramatically over time
Five years barely hints at the difference. Extend the same investment to 30 years and simple interest gives ₹3,40,000 total, while compounding gives over ₹10,06,000 — nearly three times as much from the identical rate. This is why Albert Einstein is often (probably apocryphally) said to have called compound interest the eighth wonder of the world. Time is the ingredient that turns a modest rate into serious growth.
Compounding frequency matters too
- Annually: interest added once a year.
- Quarterly: four times a year — slightly more than annual.
- Monthly: twelve times a year — more again.
- Daily: the most frequent common option, giving the highest effective return for the same nominal rate.
The more often interest compounds, the more you earn (or owe). This is why credit cards, which often compound monthly or daily on unpaid balances, can become so expensive so quickly.
The lesson for savers and borrowers
If you are investing, you want compounding working for you — start early, leave it alone, and let time do the heavy lifting. If you are borrowing, especially on high-rate credit, compounding works against you, so clearing balances fast saves a great deal. The same force builds wealth or debt depending on which side of it you stand.
The Rule of 72: estimate doubling time in your head
A handy shortcut for compounding: divide 72 by the annual rate to estimate how many years it takes money to double. At 8%, money doubles in about 72 ÷ 8 = 9 years; at 12%, in about 6 years. It is an approximation, but a remarkably good one for typical rates, and it makes the power of a higher return tangible without reaching for a calculator. The same rule shows how inflation erodes savings — at 6% inflation, prices double in about 12 years.
Where you meet each type in real life
- Simple interest: many car loans, some short-term personal loans, and fixed deposits that pay interest out periodically instead of reinvesting it.
- Compound interest: savings accounts, deposits that reinvest, mutual funds and equity growth, and — working against you — credit-card balances and unpaid EMIs.
Frequently asked questions
Which is better for me — simple or compound interest?
If you are investing or saving, compound is better because your interest earns interest. If you are borrowing, simple interest is cheaper because the debt does not snowball. In short: you want compounding on your savings and simplicity on your loans.
Does more frequent compounding really matter?
Over short periods the difference is small, but over decades it adds up. Daily compounding beats annual on the same nominal rate, which is why the 'effective annual rate' is the figure worth comparing across products.
Is the Rule of 72 accurate?
It is an estimate that works best for rates between roughly 5% and 12%. For very high or very low rates it drifts a little, but for everyday planning it is close enough to be genuinely useful.
Where does compounding hurt me most?
On revolving credit — credit cards and overdrafts — interest often compounds monthly or even daily on the unpaid balance, so a balance left to roll can grow alarmingly fast. Clearing high-rate debt is effectively a guaranteed return equal to that interest rate.
The bottom line
Simple interest grows in a straight line; compound interest grows on a curve that bends sharply upward over time. For any long horizon, compounding wins by a wide margin. Use our calculator to see exactly how your own numbers grow under each method.