How to Use the Compound Interest Calculator
Enter principal, annual interest rate, time period, and compounding frequency. Optionally add regular contributions. The calculator shows future value, APY, time to double, and a year-by-year breakdown table.
Compound Interest Calculator Formula
A = P × (1 + r/n)^(n×t)A= Final amountP= Principal (initial investment)r= Annual interest rate (decimal)n= Compounding frequency per yeart= Time in years
Example Calculation
₹1 lakh at 8% p.a. compounded monthly for 10 years:
A = 100000 × (1 + 0.08/12)^(12×10) = 100000 × (1.006667)^120
Future Value ≈ ₹2,21,964; Interest earned ≈ ₹1,21,964; APY ≈ 8.30%
Simple vs Compound Interest
Simple interest is calculated only on your original principal. Compound interest is calculated on the principal plus all the interest already earned — so your interest itself starts earning interest. Over short periods the difference is small, but over long periods it becomes enormous. ₹1 lakh at 8% simple interest earns ₹80,000 over 10 years; the same amount compounded monthly grows by about ₹1.22 lakh. The gap only widens with time, which is why compounding is often called the most powerful force in personal finance.
Why Compounding Frequency Matters
The same annual rate produces different results depending on how often interest is compounded. More frequent compounding means interest is added to the balance sooner, so it starts earning sooner. An 8% rate compounded annually yields exactly 8%; compounded monthly it yields about 8.30% (the APY); compounded daily, marginally more again.
- Annually: interest added once a year (many fixed deposits).
- Quarterly: the standard for most Indian bank FDs.
- Monthly: common for recurring deposits and many loans.
- Daily: used by some savings products and credit-card interest (which is why card debt grows so fast).
When comparing two products, always compare their APY (effective yield), not the headline rate — a slightly lower rate compounded more often can beat a higher rate compounded annually.
The Rule of 72
A handy mental shortcut: divide 72 by the annual return to estimate how many years it takes your money to double. At 8%, money doubles in about 9 years; at 12%, in about 6 years; at 6%, in 12 years. The rule also works in reverse for inflation — at 6% inflation, prices double (and your money's purchasing power halves) in roughly 12 years. It is an approximation, but accurate enough for quick planning without a calculator.
Compounding Works Against You Too
The same mathematics that grows your investments also grows your debts and quietly erodes your savings through inflation. Credit-card balances compound daily at 30–45% annually, which is why a small unpaid balance balloons within months — the most valuable "investment" most people can make is clearing high-interest debt before investing anywhere else. And money left idle in a low-interest account loses real value as inflation compounds against it. Use this calculator both ways: to see how investments grow, and to understand how fast debt and inflation work in the opposite direction.
Frequently Asked Questions
What is the compound interest formula?
A = P × (1 + r/n)^(nt), where A = final amount, P = principal, r = annual interest rate (decimal), n = compounding frequency per year, t = time in years.
What is the Rule of 72?
Divide 72 by the annual interest rate to estimate years to double your money. At 8%, your money doubles in roughly 9 years (72 ÷ 8 = 9).
What is APY vs APR?
APR is the stated annual interest rate before compounding. APY (Annual Percentage Yield) is the effective rate after compounding is applied. APY is always higher than APR when compounding occurs more than once per year.
Sources
All calculations run in your browser and are provided for information only — they are not investment, tax or legal advice. Verify current rates and rules with the official source above before acting.