By the Tooliday Team. Last updated 14 November 2026.
Interest looks like a small detail until you compare two accounts that pay the same rate but grow very differently. The difference usually comes down to one question: is interest paid only on your original money, or also on the interest you have already earned?
That is the whole difference between simple and compound interest. This guide explains both, with formulas and numbers you can check with our Compound Interest Calculator.
Simple interest
With simple interest, you earn interest only on the original amount (the principal).
Simple interest = P × r × t
- P is the principal.
- r is the yearly rate as a decimal (8% is 0.08).
- t is the time in years.
Example: ₹1,00,000 at 8% for 10 years earns 1,00,000 × 0.08 × 10 = ₹80,000. The final amount is ₹1,80,000. Every year adds the same ₹8,000.
Compound interest
With compound interest, the interest you earn is added to the balance, and the next period’s interest is calculated on the larger amount.
A = P × (1 + r ÷ n)^(n × t)
- A is the final amount.
- n is how many times a year the interest is compounded (1 for yearly, 4 for quarterly, 12 for monthly).
Example: ₹1,00,000 at 8% compounded yearly for 10 years becomes 1,00,000 × 1.08^10 = ₹2,15,892.50. That is about ₹35,892 more than the ₹1,80,000 from simple interest, from exactly the same rate.
The gap grows with time
| Years | Simple interest | Compound (yearly) |
|---|---|---|
| 10 | ₹1,80,000 | ₹2,15,892 |
| 20 | ₹2,60,000 | ₹4,66,096 |
| 30 | ₹3,40,000 | ₹10,06,266 |
After 30 years, compounding has turned ₹1,00,000 into more than ₹10 lakh, while simple interest reaches ₹3.4 lakh. The first years look similar. The difference is made in the later years, which is why time is the biggest factor in compounding.
Does the compounding frequency matter?
A little. Using the same ₹1,00,000 at 8% for 10 years:
- Compounded yearly: ₹2,15,892
- Compounded monthly: ₹2,21,964
More frequent compounding gives a higher result, but the jump is small compared with the effect of the rate and the number of years. When comparing offers, check whether the stated rate is yearly or monthly, and how often it compounds.
The Rule of 72
A quick mental shortcut: divide 72 by the interest rate to estimate how many years it takes for money to double with compounding.
At 8%, 72 ÷ 8 = 9 years. The exact answer is about 9.006 years, so the shortcut is very close. At 12% it suggests 6 years, and at 6% it suggests 12 years. It is an estimate and works best for rates between about 6% and 10%.
Compounding works against borrowers too
The same mechanism that grows savings makes some debts expensive. Credit card balances and some informal loans charge interest on unpaid interest. If you carry a balance, the amount owed can grow much faster than expected. Check how a loan’s interest is calculated before you borrow.
Common mistakes
- Mixing up the rate period. A monthly rate of 1% is not the same as 1% a year. Convert carefully.
- Entering the rate as 8 instead of 0.08. The formulas use a decimal.
- Ignoring taxes and inflation. Interest may be taxable, and prices usually rise over time, so the real value of the final amount is lower than it looks.
- Assuming returns are guaranteed. Fixed deposits state their rates, but investments linked to markets do not.
Try it yourself
Enter your own principal, rate, years and compounding frequency in the Compound Interest Calculator. Try changing only the number of years and see how much the result moves.
This article is for general information, not financial advice. Real products have fees, taxes and conditions that these formulas do not include.