Compound Interest Calculator

How Compound Interest Is Calculated

Compound interest follows A = P × (1 + r/n)^(n×t), where P is the principal, r is the annual rate (as a decimal), n is the number of compounding periods per year, and t is time in years. The key difference from simple interest is the n and the exponent: interest earned in each period gets added back to the balance before the next period's interest is calculated, so you effectively earn interest on your interest as time goes on.

This is why compound growth is described as exponential rather than linear — the rupee amount earned in year 10 is larger than the rupee amount earned in year 1, even though the rate never changed, simply because the base it's calculated on has grown.

Compounding Frequency Changes the Outcome

The same annual rate produces a different result depending on how often it compounds. Monthly compounding earns slightly more than quarterly, which earns slightly more than yearly, because interest gets added to the principal (and starts earning its own interest) more frequently. The difference is small for short periods and modest rates, but compounds meaningfully over longer horizons.

The table below shows this on the same ₹1,00,000 at 8% p.a. for 5 years, changing only the compounding frequency:

Moving from yearly to monthly compounding adds roughly ₹2,000 here — a modest effect compared to what changing the rate itself or the time period would do. The rate and tenure remain the two biggest levers on your final value, not the compounding frequency.

Worked Example

₹1,00,000 at 8% p.a. for 5 years, yearly compounding, no additional contributions:

Compare this to simple interest on the same figures (₹1,40,000 total) — compound interest earns nearly ₹7,000 more over 5 years purely from interest-on-interest, and that gap widens substantially over longer periods.

Adding Monthly Contributions

When you add a recurring monthly contribution on top of the initial lump sum, the two grow separately in this calculator — the initial principal compounds at your chosen frequency, while the monthly contributions compound like a monthly SIP, and the two figures are added together for your total. This is a reasonable approximation, though if you want to model an initial deposit plus SIP as a single blended product, check with your specific bank or fund's own projection tool for an exact figure.

For example, a ₹1,00,000 lump sum at 8% p.a. (yearly compounding) alongside a ₹5,000 monthly contribution for 5 years works out to:

Here the ₹1,14,318 interest figure is the combined growth on both the lump sum and the running contributions — most of it comes from the monthly amounts since, cumulatively, more money passes through them over the 5-year period than sits in the original lump sum alone.

The Rule of 72: A Rough Mental Shortcut

A commonly used approximation for compound growth is the "Rule of 72" — dividing 72 by your annual interest rate gives a rough estimate of how many years it takes for a lump sum to double. At 8% p.a., that's about 9 years; at 12% p.a., about 6 years. This is a rough mental shortcut, not an exact calculation — it gets less accurate at very high or very low rates — so use the calculator above for any figure you actually plan to rely on.

Compounding on Loans Works Against the Borrower

Everything above describes compounding from a saver's or investor's perspective, where a growing balance works in your favor. The identical mathematics works against you as a borrower — interest on an outstanding loan balance that isn't paid down also compounds, meaning unpaid interest itself starts accruing further interest. This is most visible on revolving credit like credit cards, where carrying a balance and paying only the minimum due lets interest compound rapidly on an amount that barely shrinks month to month. Whenever you're evaluating a loan or credit product, ask about the compounding frequency the same way you would for a deposit — it affects your total cost exactly the way it affects a saver's total return, just in the opposite direction. Reading the compounding terms on a credit product before borrowing is just as important as reading them on a deposit before investing, and paying down more than the minimum due each month is one of the most effective ways to blunt the effect of compounding working against you.

Why Your Bank's Figure May Differ Slightly

Real-world FDs, recurring deposits, and other compounding products sometimes use day-count conventions (365 vs 360 days), compound on specific calendar dates rather than exact month boundaries, or round at each compounding step — all of which can nudge the actual maturity value slightly away from a clean formula-based estimate. Use this calculator for planning, comparison, and understanding the mechanics, and use your bank's own calculator or maturity slip for the exact figure you'll rely on for a financial decision.

Where Compound Interest Shows Up in Everyday Products

Compound interest is the default in most formal Indian savings and loan products, not the exception. Bank FDs and recurring deposits typically compound quarterly, savings accounts often compound quarterly as well (though the quoted rate is usually far lower than an FD's), PPF compounds annually and is credited at financial year-end, and most home, personal, and vehicle loans charge interest on a reducing balance, which is itself a form of compounding working in the lender's favor as your balance falls. Recognizing which frequency a specific product actually uses helps you read its brochure correctly instead of assuming a flat, simple-interest-style calculation.

On the borrowing side, compounding cuts against you: interest on an unpaid credit card balance, for instance, is typically compounded monthly and calculated daily on the outstanding amount, which is a major reason credit card debt can grow rapidly if only minimum payments are made — each unpaid interest charge itself starts accruing further interest the following month.

How Time Amplifies the Effect of Compounding

Because later years contribute more than earlier years, starting early has an outsized effect on a compounding investment's final value, even if the total amount contributed is identical. Consider two people investing the same ₹1,00,000 at the same 8% p.a.: one who starts and lets it run for 20 years ends up with roughly ₹4,66,096, while the same amount invested for just 10 years reaches only ₹2,15,893 — doubling the time period more than doubles the outcome, because the second decade compounds on a base that's already grown substantially in the first. This is the core argument behind starting to save or invest as early as possible: time in the market, not just the amount contributed, is one of the biggest drivers of a compounding outcome.

Compound Interest and Market-Linked Investments

This calculator assumes a fixed, unchanging rate applied consistently every period — that's a fair model for a fixed-rate deposit, but market-linked investments like mutual funds and equities don't grow at a steady rate. Their year-to-year return varies, sometimes sharply, and a projection using an assumed average rate is an illustration of a possible outcome, not an assured one. Use this calculator's output for fixed-rate products as a close approximation, and treat any similar projection for market-linked investments as an illustration at an assumed rate rather than a promised result.