What Is Compound Interest?
Compound interest is interest calculated on the original principal plus all interest earned in previous periods. It is expressed as an annual percentage rate, applied at a chosen frequency such as yearly, quarterly, or monthly.
Simple interest only ever grows on the original amount. Compound interest grows on a rising base each period, so the same rate produces a steadily larger rupee return every year the money stays invested.
Most long-term Indian investment products, from PPF and EPF to fixed deposits and mutual funds, rely on this mechanism. Understanding how it behaves at different rates and frequencies is the starting point for any long-term financial plan.
Compound Interest Formula: How to Calculate It
The maturity value of a compounding investment is calculated as:
A = P × (1 + r/n)^(n × t)
| Symbol | Meaning |
|---|---|
| A | The maturity amount: principal plus all interest earned |
| P | The principal, your original lump-sum investment |
| r | The annual interest rate, in decimal form (10% = 0.10) |
| n | Number of times interest compounds per year |
| t | Total time the money stays invested, in years |
Worked example
₹1.00 L at 10% per annum for 10 years grows to roughly ₹2,59,374 with yearly compounding, or about ₹2,70,704 with monthly compounding. Enter the same numbers above to verify.
Compound Interest Formula in Excel: Three Methods
The same maturity value can be built in a spreadsheet three different ways.
| Method | Formula | Notes |
|---|---|---|
| Direct formula | =P*(1+R/N)^(N*T) | Matches the formula above exactly. |
| POWER function | =P*POWER(1+R/N,N*T) | Identical result, more readable in a shared workbook. |
| RRI function | =RRI(N*T,P,A) | Solves for the compound rate per period given P, a known maturity value A, and total periods. |
Compound Interest vs Simple Interest: What Is the Difference?
Simple interest is calculated only on the original principal for every period, using A = P × (1 + r × t). Compound interest recalculates on principal plus prior interest, so the gap between the two widens every additional year.
| Method | Total Interest | Maturity Value |
|---|---|---|
| Simple interest | ₹5,00,000 | ₹15,00,000 |
| Compound interest | ₹6,10,510 | ₹16,10,510 |
The extra amount from compounding, about ₹1,10,510 in this example, is interest earned on interest that had already accumulated in earlier years. The Simple and Compound Interest Calculator runs both formulas on your own numbers side by side and includes a converter between simple and compound rates.
How Compounding Frequency Changes Your Returns
More frequent compounding produces a higher maturity value at the same stated annual rate, because interest gets added to the principal sooner and starts earning its own interest sooner. The gap is small over a few years but becomes meaningful over a decade or more.
| Frequency | Maturity Value | Effective Annual Rate |
|---|---|---|
| Yearly | ₹2,15,892 | 8.00% |
| Half-Yearly | ₹2,19,112 | 8.16% |
| Quarterly | ₹2,20,804 | 8.24% |
| Monthly | ₹2,21,964 | 8.30% |
Switch the frequency toggle in the calculator above to see this same effect on your own principal, rate, and tenure.
The Rule of 72: How Fast Your Money Doubles
The Rule of 72 estimates how many years it takes an investment to double at a given annual rate: divide 72 by the rate. It is a quick mental approximation, accurate mainly for rates between roughly 6% and 20%.
| Annual Rate | Approx. Years to Double |
|---|---|
| 6% | 12.0 yrs |
| 8% | 9.0 yrs |
| 10% | 7.2 yrs |
| 12% | 6.0 yrs |
| 15% | 4.8 yrs |
The calculator above shows this exact figure, computed precisely rather than approximated, in the "Time to double" result row.
Compound Interest in Popular Indian Investments
PPF (Public Provident Fund)
PPF compounds annually and currently offers a government-declared rate revised every quarter, typically around 7 to 8%. The 15-year lock-in makes it a natural fit for long-horizon compounding, and interest is tax-free under Section 80C.
EPF (Employees' Provident Fund)
EPF compounds annually at a rate the government declares each year, generally in the 8 to 8.5% range. Both employer and employee contributions compound over the full service period.
Fixed Deposits
Most Indian banks compound FD interest quarterly. Use the FD Calculator for a version with quarterly compounding, TDS, and senior citizen rates already built in.
Mutual Funds and Equity
Equity mutual funds grow through capital appreciation rather than a stated compounding rate, and returns are never guaranteed. Their long-term annualised growth is still best understood as a compounding process, which is why fund houses quote a CAGR rather than a fixed interest rate.
CAGR Calculator
Check the actual annualised growth rate an existing mutual fund or stock investment has delivered.
Compound Interest Calculator vs SIP Calculator: Which One to Use
This calculator compounds a single lump sum you enter once. It does not add a fresh contribution every month, so it is the wrong tool for a recurring SIP.
A monthly SIP compounds each installment separately, from the month it was invested, so the first installment compounds far longer than the last. Use the SIP Calculator for recurring investments, or the Step-Up SIP Calculator if your monthly amount increases every year.
If you already have a lump sum ready and want to compare it against investing the same money as an SIP instead, the Lumpsum Calculator runs that comparison directly.
How Compound Interest Is Taxed in India
Fixed deposit interest is added to your total income and taxed at your slab rate, with TDS deducted once annual interest from one bank crosses Rs 40,000 (Rs 50,000 for senior citizens). PPF and EPF interest is fully tax-exempt under Section 80C rules.
Equity mutual fund gains held over 12 months are taxed as long-term capital gains at 12.5%, above a Rs 1.25 lakh annual exemption. Gains held under 12 months are taxed as short-term capital gains at 20%. Confirm your exact liability with the Income Tax Calculator.
How to Use This Calculator
The calculator needs four inputs:
- Principal Amount: enter the lump sum you plan to invest, using the slider or by clicking the value to type an exact figure.
- Rate of Interest: enter the expected annual return. For equity funds, 10 to 12% is a reasonable long-term assumption.
- Time Period: set the number of years the money stays invested, or pick one of the preset year buttons.
- Compounding Frequency: choose yearly, half-yearly, quarterly, or monthly to match your actual investment.
The maturity amount, interest earned, effective annual rate, and doubling time update instantly. Expand the growth schedule to see the year-by-year build-up.
Limitations of This Calculator
Assumes a single, one-time investment.
It does not model recurring monthly contributions. For an SIP, use the SIP Calculator or Step-Up SIP Calculator instead.
Assumes a constant rate for the entire tenure.
Real rates on FDs, PPF, and EPF can change between renewal cycles. A market-linked investment will not return a fixed rate at all.
Does not calculate tax or TDS.
Use the Income Tax Calculator or FD Calculator for a version with TDS and slab-rate tax built in.
Effective annual rate assumes no withdrawals.
Any partial withdrawal or premature closure during the tenure will change the actual return realised versus what this calculator shows.
Frequently Asked Questions
Disclaimer: All calculations on this page are indicative only, based on the principal, rate, tenure, and compounding frequency you enter. This calculator does not account for tax, TDS, or any fees, and past interest rates do not guarantee future rates. This tool is for educational and planning purposes only and does not constitute financial advice. Consult a SEBI-registered investment adviser or a chartered accountant before making investment decisions.