Compound Interest Calculator

What Is Compound Interest?

Compound interest is interest calculated on both the original principal and the accumulated interest from previous periods. This is often called "interest on interest" — and it is the reason small, consistent investments can grow into substantial sums over time.

Unlike simple interest, where you earn a fixed amount each year, compound interest accelerates your returns because each period's interest is added to the principal, and the next period's interest is calculated on this larger base.

The Power of Compounding — Rule of 72

The Rule of 72 is a quick mental 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.

This rule is an approximation, but it gives you a powerful way to quickly gauge the impact of different interest rates on your investments.

Example Calculation

Suppose you invest ₹1,00,000 at 12% per annum, compounded monthly, for 10 years.

For comparison, the same ₹1,00,000 at 12% simple interest for 10 years would give you only ₹2,20,000 (interest of ₹1,20,000). Compound interest earned ₹1,10,039 more — that is the power of compounding.

How Compounding Frequency Matters

The more frequently interest is compounded, the more you earn — because interest gets added to the principal more often, and subsequent interest calculations use this larger base. Here is the same ₹1,00,000 at 12% for 10 years with different frequencies:

Going from yearly to monthly compounding adds roughly ₹19,454 in extra interest over 10 years — without any additional investment. The difference between monthly and daily is smaller, but still meaningful for large sums.

How to Use This Calculator

1. Enter the principal amount — the initial sum you plan to invest.
2. Enter the annual interest rate — the expected yearly return as a percentage.
3. Enter the time period — how many years you plan to stay invested.
4. Select compounding frequency — choose yearly, half-yearly, quarterly, or monthly depending on your investment instrument.

Click "Calculate" to see the maturity amount, total interest earned, a visual principal-vs-interest bar, and a detailed year-by-year breakdown table.

Where Is Compound Interest Used?

Compound interest is the standard for most long-term financial instruments. It applies to:

• Fixed deposits (FDs) — banks compound interest quarterly or monthly on term deposits.
• Savings accounts — interest is typically compounded daily or monthly.
• Mutual funds — returns from SIPs and lump-sum investments grow through compounding.
• Public Provident Fund (PPF) — compounded annually, with a 15-year lock-in period.
• National Pension System (NPS) — long-term retirement savings that benefit heavily from compounding.
• Recurring deposits (RDs) — monthly contributions that earn compound interest.

Almost every long-term savings or investment product uses compound interest — making it the most important concept in personal finance.

Key Takeaways

• Start early — time is the single biggest factor in compounding. A 25-year-old investing ₹5,000/month will accumulate far more than a 35-year-old investing ₹10,000/month, even though the latter invests more money in total.
• Small rate differences matter — the difference between 10% and 12% might seem small, but over 20 years it compounds into a significant gap. Always compare the effective annual rate across instruments.
• Compare on effective annual rate — a product offering 12% compounded monthly gives a higher effective return than one offering 12% compounded yearly. Use the effective annual rate (EAR) to make fair comparisons.