📋 In This Page
- What is compound interest and why it matters
- The compound interest formula — explained with examples
- Compound interest vs simple interest — the real difference
- Compound interest on Indian FDs — how it works
- Taxation on compound interest income in India
- 5 common compound interest mistakes
- Frequently asked questions
What is Compound Interest and Why It Matters
Compound interest is interest calculated on both the initial principal and the accumulated interest from previous periods. Unlike simple interest, which is calculated only on the original principal, compound interest grows exponentially — each period's interest becomes part of the base for the next period's calculation.
Albert Einstein is often (though probably incorrectly) credited with calling compound interest the "eighth wonder of the world." Whether or not he said it, the sentiment is accurate: compound interest is the single most powerful force in personal finance. Given enough time, it turns modest savings into substantial wealth — and turns manageable debt into crushing burdens.
📈
Wealth Building
The longer you stay invested, the more dramatically compound interest works in your favour. Starting early matters far more than the amount you invest.
🏦
Fixed Deposits
Indian FDs use quarterly compounding. A cumulative FD reinvests all interest, while a non-cumulative FD pays interest out regularly.
💳
Loans & Debt
Compound interest works against you on credit card debt and loans. High-rate debt compounds rapidly — always pay more than the minimum.
⏰
Time is Everything
Starting 10 years earlier can more than double your final corpus at the same rate — the compounding effect is non-linear over time.
The Compound Interest Formula — Explained with Examples
Compound Interest Formula
A = P × (1 + r/n)^(n × t)
CI = A − P
CI = A − P
A = Final amount (maturity value)
P = Principal (initial investment)
r = Annual interest rate (as a decimal, e.g. 7% = 0.07)
n = Compounding periods per year (quarterly = 4)
t = Time in years
P = Principal (initial investment)
r = Annual interest rate (as a decimal, e.g. 7% = 0.07)
n = Compounding periods per year (quarterly = 4)
t = Time in years
Worked example — FD at SBI
Example: ₹1,00,000 FD at 7% for 5 years, compounded quarterly
P = ₹1,00,000 | r = 0.07 | n = 4 (quarterly) | t = 5 years
A = 1,00,000 × (1 + 0.07/4)^(4×5)
A = 1,00,000 × (1.0175)^20
A = 1,00,000 × 1.41478 = ₹1,41,478
Compound Interest = ₹1,41,478 − ₹1,00,000 = ₹41,478
Maturity Value = ₹1,41,478 | CI Earned = ₹41,478 | Simple Interest would have given only ₹35,000
Effective Annual Rate (EAR)
The stated interest rate (nominal rate) and the actual rate you earn (effective annual rate) differ when compounding is more frequent than annual. The EAR formula is:
Effective Annual Rate Formula
EAR = (1 + r/n)^n − 1
At 7% nominal rate: Annual EAR = 7.00% | Quarterly EAR = 7.19% | Monthly EAR = 7.23% | Daily EAR = 7.25%
Compound Interest vs Simple Interest — The Real Difference
Simple interest is calculated only on the original principal. Compound interest is calculated on the principal plus all previously accumulated interest. The difference is small over short periods but becomes dramatic over longer horizons.
| Principal | Rate | Years | Simple Interest | Compound Interest (Quarterly) | Extra from Compounding |
|---|---|---|---|---|---|
| ₹1,00,000 | 7% | 1 | ₹7,000 | ₹7,186 | ₹186 |
| ₹1,00,000 | 7% | 5 | ₹35,000 | ₹41,478 | ₹6,478 |
| ₹1,00,000 | 7% | 10 | ₹70,000 | ₹99,749 | ₹29,749 |
| ₹1,00,000 | 7% | 20 | ₹1,40,000 | ₹3,98,912 | ₹2,58,912 |
| ₹1,00,000 | 7% | 30 | ₹2,10,000 | ₹8,00,640 | ₹5,90,640 |
| ₹5,00,000 | 7% | 20 | ₹7,00,000 | ₹19,94,560 | ₹12,94,560 |
The 30-year impact: ₹1 lakh at 7% simple interest for 30 years gives ₹3.1 lakh total. At 7% compound interest (quarterly), it gives ₹9.0 lakh — nearly 3× more. This is why staying invested for the long term is the single most important financial decision.
Compound Interest on Indian FDs — How It Actually Works
Fixed Deposits (FDs) are one of the most popular investment instruments in India. Understanding how compounding works on FDs helps you make better decisions about tenure, bank selection, and reinvestment strategy.
Cumulative vs Non-Cumulative FDs
| Feature | Cumulative FD | Non-Cumulative FD |
|---|---|---|
| Interest payout | At maturity | Monthly / Quarterly / Annually |
| Compounding | Yes — interest reinvested | No — interest paid out |
| Final corpus | Higher (due to compounding) | Lower |
| Best for | Long-term wealth building | Regular income (retirees) |
| TDS | At maturity on full interest | Each year on interest paid |
FD rates across tenures — 2026
| Bank | 1 Year | 2 Years | 3 Years | 5 Years | Senior Citizen Extra |
|---|---|---|---|---|---|
| SBI | 6.80% | 7.00% | 6.75% | 6.50% | +0.50% |
| HDFC Bank | 6.60% | 7.00% | 7.00% | 7.25% | +0.50% |
| ICICI Bank | 6.70% | 7.00% | 7.00% | 7.10% | +0.50% |
| Axis Bank | 6.70% | 7.10% | 7.10% | 7.20% | +0.75% |
| Kotak Mahindra | 7.10% | 7.20% | 7.30% | 7.40% | +0.50% |
| Small Finance Banks | 7.50%+ | 8.00%+ | 8.00%+ | 8.05%+ | +0.25–0.50% |
⚠️FD rates change frequently. Always verify the current rate directly with your bank before making a deposit. Small Finance Banks offer higher rates but carry higher risk — ensure your deposit is within the ₹5 lakh DICGC insurance limit.
Taxation on Compound Interest Income in India
Interest earned on FDs and savings accounts is fully taxable in India as "Income from Other Sources." Understanding the tax implications is critical for accurate post-tax return calculations.
| Scenario | TDS Rate | TDS Threshold | Action Required |
|---|---|---|---|
| FD interest (general) | 10% | ₹40,000/year | Submit Form 15G if income below taxable limit |
| FD interest (senior citizen) | 10% | ₹50,000/year | Submit Form 15H if income below taxable limit |
| Without PAN card | 20% | ₹40,000/year | Always link PAN to your bank account |
| Savings account interest | No TDS | — | Deduction under Section 80TTA (up to ₹10,000) |
| Senior citizen savings interest | No TDS | — | Deduction under Section 80TTB (up to ₹50,000) |
💡For a 5-year cumulative FD, banks report the interest earned every year to the Income Tax Department even though it is paid only at maturity. You must declare this interest in your ITR annually, not just in the year of maturity — otherwise it results in a discrepancy that can trigger a tax notice.
5 Common Compound Interest Mistakes
Mistake 1 — Confusing nominal rate with effective annual rate
✗ Wrong: "My FD pays 7% so I earn exactly ₹7,000 on ₹1 lakh per year"
✓ Right: At 7% quarterly compounding, effective rate is 7.19% — you earn ₹7,186, not ₹7,000
When an FD compounds quarterly, the effective annual rate (EAR) is higher than the stated rate because each quarter's interest earns interest in subsequent quarters. A 7% FD compounding quarterly gives an EAR of 7.19%. This difference becomes more significant over longer terms — always compare FDs using their effective rate, not just the headline rate.
Mistake 2 — Withdrawing FD interest instead of letting it compound
✗ Wrong: Choosing a non-cumulative FD to receive quarterly interest payouts
✓ Right: For wealth building, use a cumulative FD — reinvesting interest dramatically increases the final corpus
On a ₹5 lakh FD at 7% for 10 years, a cumulative FD gives approximately ₹9.97 lakh at maturity. A non-cumulative FD paying interest quarterly gives you ₹87,500 in interest payments over 10 years, but your principal remains ₹5 lakh — a total of ₹5,87,500. The cumulative FD gives ₹4.1 lakh more. Withdraw interest only if you need regular income.
Mistake 3 — Ignoring inflation when evaluating FD returns
✗ Wrong: "My FD returns 7%, which is great — I'm growing my wealth"
✓ Right: At 6% inflation, a 7% FD gives only ~1% real return — barely beating inflation
India's average CPI inflation has run at 5–7% in recent years. An FD yielding 7% at 6% inflation delivers only approximately 1% real return after adjusting for purchasing power. Additionally, TDS at 10% reduces your effective return further — a 7% pre-tax FD becomes approximately 6.3% post-tax (at 10% TDS), giving a real return near zero. For long-term wealth building, equity-linked instruments (mutual funds, NPS) typically provide better inflation-adjusted returns.
Mistake 4 — Not declaring FD interest in ITR every year (for cumulative FDs)
✗ Wrong: "My FD matures in 5 years — I'll declare the interest only in year 5"
✓ Right: Accrued FD interest must be declared annually in your ITR under "Income from Other Sources"
Banks issue Form 16A each year showing interest accrued on your FD, even if not paid out yet. The Income Tax Department cross-references this with your ITR. Not declaring it annually means underreporting income, which can result in tax notices, interest on underpaid tax, and penalties. Declare FD interest on an accrual basis every year — your CA or tax software can help calculate the annual accrual amount.
Mistake 5 — Spreading FDs across multiple banks to avoid TDS without submitting 15G/H
✗ Wrong: Opening many small FDs across multiple banks to keep each below ₹40,000 interest/year
✓ Right: Submit Form 15G (or 15H for seniors) at each bank where your interest exceeds the threshold
The ₹40,000 TDS threshold applies per bank, not across all banks combined. Spreading FDs helps delay TDS — but the interest is still fully taxable and must be declared in your ITR regardless of whether TDS was deducted. If your total income is below the taxable limit, the correct approach is to submit Form 15G (or 15H for senior citizens) to each bank — not to artificially split deposits.
💰 Calculate Your Compound Interest
Use the free calculator above — enter your principal, rate, and tenure to see your maturity amount, yearly breakdown, and how compounding frequency affects your returns.
Calculate Now →Frequently Asked Questions
The compound interest formula is: A = P × (1 + r/n)^(n×t), where A is the final amount, P is the principal, r is the annual interest rate as a decimal, n is the number of compounding periods per year, and t is the time in years. Compound Interest = A − P. For example, ₹1 lakh at 7% quarterly for 5 years: A = 1,00,000 × (1 + 0.07/4)^(4×5) = ₹1,41,478, giving CI of ₹41,478.
Simple interest is calculated only on the original principal amount throughout the investment period. Compound interest is calculated on the principal plus all previously earned interest — so your interest earns interest. Over short periods (1–2 years), the difference is small. Over longer periods, compound interest grows exponentially while simple interest grows linearly. At 7% for 20 years on ₹1 lakh: SI gives ₹1,40,000 while CI (quarterly) gives ₹3,98,912 — nearly 3× more.
Most Indian bank Fixed Deposits compound interest quarterly (4 times per year). For cumulative FDs, this compounded interest is reinvested and paid out at maturity along with the principal — producing a higher maturity amount. For non-cumulative FDs, interest is paid out regularly (monthly, quarterly, or annually) without compounding. Some banks offer monthly compounding, which gives a marginally higher effective rate than quarterly.
The Rule of 72 is a simple mental shortcut to estimate how many years it takes to double your money at a given compound interest rate. Divide 72 by the annual rate: at 8%, money doubles in about 9 years (72÷8). At 6%, it takes 12 years (72÷6). At 12%, it takes 6 years (72÷12). The rule is an approximation — the actual doubling time is calculated using logarithms, but the Rule of 72 is accurate to within a few months for most practical rates.
More frequent compounding gives higher returns, in this order from lowest to highest: Annual → Semi-annual → Quarterly → Monthly → Daily → Continuous. However, the difference becomes smaller as frequency increases. Going from annual to quarterly compounding on a 7% rate adds about 0.19% to your effective rate. Going from monthly to daily adds only about 0.02% — effectively negligible. In practice, the difference between quarterly and monthly compounding on most FD amounts is just a few hundred rupees per year.
For a cumulative FD in India, interest compounds quarterly at the bank's stated rate. For example, a ₹1 lakh FD at 7% for 5 years compounding quarterly grows to approximately ₹1,41,478, giving ₹41,478 in compound interest. A simple interest calculation would give only ₹35,000 over the same period. The difference of ₹6,478 represents the compounding benefit — the interest earned on previously earned interest over the 5 years.
Yes. FD interest income in India is fully taxable as "Income from Other Sources" at your applicable income tax slab rate. Banks deduct TDS at 10% if FD interest exceeds ₹40,000 in a financial year (₹50,000 for senior citizens). If your PAN is not linked to the FD, TDS is deducted at 20%. If your total income is below the basic exemption limit, you can submit Form 15G (or Form 15H for senior citizens) to your bank to avoid TDS deduction. Interest must be declared in your ITR annually, even for cumulative FDs where payment is at maturity.
The Effective Annual Rate (EAR) is the actual annual return you earn after accounting for compounding frequency within the year. It is always higher than the nominal (stated) rate when compounding is more frequent than annual. Formula: EAR = (1 + r/n)^n − 1. At 7% nominal rate: Annual compounding gives EAR = 7.00%, Quarterly gives 7.19%, Monthly gives 7.23%, Daily gives 7.25%. When comparing FDs from different banks, use EAR for an apples-to-apples comparison — especially if they compound at different frequencies.
More Free Tools on ToolLoom
📈
Tool
SIP Calculator
Monthly SIP returns, wealth growth and goal planner for mutual funds
🏦
Tool
EMI Calculator
Monthly loan instalments with full amortization schedule
🎂
Tool
Age Calculator
Exact age with UPSC and exam cutoff date support
📖
Guide
CIBIL Score Guide
How to improve your credit score and get better loan rates