Calculate compound interest on your investments with this free calculator. Enter your principal amount, interest rate, time period, and compounding frequency to see how your money grows year by year with Indian rupee formatting.
Last updated: March 2026
Compound interest is the interest calculated on both the initial principal and the accumulated interest from previous periods. Unlike simple interest, which only grows linearly, compound interest grows exponentially — making it a powerful wealth-building tool.
The standard compound interest formula is:
Where:
| Variable | Meaning |
|---|---|
| A | Final maturity amount (principal + interest) |
| P | Principal (initial investment), e.g., &rupee;1,00,000 |
| r | Annual interest rate (in decimal), e.g., 8% = 0.08 |
| n | Number of times interest is compounded per year (4 for quarterly) |
| t | Time period in years |
The compound interest earned is: CI = A − P
Suppose you invest &rupee;1,00,000 at 8% per annum, compounded quarterly, for 5 years:
Compare this with simple interest on the same investment: SI = 1,00,000 × 0.08 × 5 = &rupee;40,000. That means compounding earned you an extra &rupee;8,595 — interest on your interest.
Understanding the difference between simple and compound interest is fundamental to making smart financial decisions in India, whether you are opening a bank FD, investing in PPF, or taking a loan.
Simple Interest (SI) is calculated only on the original principal: SI = P × r × t. The interest amount remains the same each year. If you invest &rupee;1,00,000 at 8%, you earn &rupee;8,000 every year — flat, with no growth.
Compound Interest (CI) is calculated on the principal plus all previously earned interest. Each year, you earn interest on a larger amount. In the first year you earn &rupee;8,000, but in the second year you earn interest on &rupee;1,08,000, giving you &rupee;8,640 — and this gap keeps widening over time.
Over short periods (1–2 years), the difference is modest. But over 10, 20, or 30 years, compound interest can generate several times more wealth than simple interest. This exponential growth is why financial advisors urge you to start investing early and let compounding do the heavy lifting.
The Rule of 72 is a simple and widely used mental math trick to estimate how many years it takes for your investment to double at a given compound interest rate. Just divide 72 by the annual interest rate:
For example:
This rule works best for interest rates between 6% and 10% but provides a reasonable approximation across a wider range. It is a handy tool for quick comparisons when evaluating different investment options in India — whether it is a bank FD, PPF, NPS, or mutual funds.
Compounding is often called the eighth wonder of the world, and for good reason. The earlier you start investing, the more time your money has to compound and grow exponentially. Consider this example with two investors:
Investor A earns more than &rupee;10 lakh extra — simply by starting 10 years earlier. Both invested the same principal and earned the same rate, but the extra decade of compounding made an enormous difference. This is the core lesson: time in the market beats timing the market.
In the Indian context, instruments like PPF (7.1%), ELSS funds (historically 12–15%), and even bank FDs all benefit from compounding. The key is to start as early as possible, reinvest your returns, and let time work in your favour.
Simple interest is calculated only on the original principal amount throughout the investment period, using the formula SI = P × r × t. Compound interest is calculated on the principal plus all previously accumulated interest, using A = P(1 + r/n)^(nt). For example, if you invest &rupee;1,00,000 at 8% for 5 years, simple interest gives you &rupee;40,000 in interest, while compound interest (compounded quarterly) gives you &rupee;48,595 — that extra &rupee;8,595 is the interest earned on interest. The longer the duration, the wider this gap becomes.
The Rule of 72 is a quick mental math shortcut to estimate how long it takes for your money to double at a given interest rate. Simply divide 72 by the annual interest rate. For example, at 8% annual return, your money doubles in approximately 72 / 8 = 9 years. At 12%, it doubles in about 6 years. This rule works best for rates between 6% and 10% and is widely used by financial planners for quick estimates.
The power of compounding refers to the exponential growth that occurs when your investment returns start generating their own returns. Over time, compounding creates a snowball effect where your wealth accelerates dramatically. For instance, &rupee;1,00,000 invested at 12% grows to about &rupee;3,10,585 in 10 years but &rupee;29,95,992 in 30 years — nearly 10 times more despite only 3 times the duration. This is why starting early is the single most important factor in building long-term wealth.
More frequent compounding is always better for investors and depositors. Daily compounding yields slightly more than monthly, which yields more than quarterly, and so on. However, the practical difference between monthly and daily compounding is quite small. For savings accounts, Indian banks typically compound quarterly or daily. For fixed deposits, most banks compound quarterly. When borrowing money, less frequent compounding is better for the borrower as it reduces the total interest payable.
Indian banks follow RBI guidelines for interest calculation. Savings accounts typically earn interest compounded daily or quarterly on the daily closing balance. Fixed deposits usually compound interest quarterly. The formula used is A = P(1 + r/n)^(nt). Interest on loans (home loans, personal loans) is generally calculated on a reducing balance basis. Since April 2010, RBI mandated that banks must use the daily balance method for calculating savings account interest.
The compound interest formula is A = P(1 + r/n)^(nt), where A is the final amount, P is the principal (initial investment), r is the annual interest rate as a decimal (e.g., 8% = 0.08), n is the number of times interest is compounded per year (1 for annually, 4 for quarterly, 12 for monthly, 365 for daily), and t is the time in years. The compound interest earned is CI = A − P. For quick calculations, you can use this ToolsHub compound interest calculator which handles the math for you.