Contents
- 1 Simple & Compound Interest Calculator — SI vs CI Side-by-Side
- 1.1 Simple Interest vs Compound Interest — The Fundamental Difference
- 1.2 Worked Example — ₹1 Lakh @ 10% for 20 Years
- 1.3 Where You Encounter Simple Interest in India
- 1.4 Where You Encounter Compound Interest
- 1.5 Flat Rate vs Reducing Balance — A Loan Trap
- 1.6 Power of Compounding — Real-Life Numbers
- 1.7 Common Misconceptions
- 1.8 Rule of 72 — Quick Mental Math
- 1.9 Frequently Asked Questions
- 1.10 Related Calculators
Simple & Compound Interest Calculator — SI vs CI Side-by-Side
Compare Simple Interest and Compound Interest on the same principal. See how compounding multiplies wealth over time.
Simple Interest vs Compound Interest — The Fundamental Difference
Simple Interest (SI) charges interest only on the original principal. Compound Interest (CI) charges interest on the principal PLUS all previously earned interest — creating “interest on interest”. This seemingly small distinction explains why long-term compounding wealth grows exponentially while simple-interest savings grow linearly.
Formula Comparison
| Concept | Simple Interest | Compound Interest |
|---|---|---|
| Interest Formula | SI = P × R × T | CI = P × (1+R/m)m×T − P |
| Final Amount | A = P + SI = P(1 + RT) | A = P × (1+R/m)m×T |
| Where Used Today | Some old FDs, car loans, education loans (interest-on-interest), short-term loans | Bank FDs (most), savings, PPF, mutual funds, mortgages |
| Growth Pattern | Linear (straight line) | Exponential (curve) |
| Long-term Effect | Modest growth | Dramatic snowball effect |
Worked Example — ₹1 Lakh @ 10% for 20 Years
| Year | SI Balance (₹) | CI Balance (Annual, ₹) | CI Balance (Monthly, ₹) |
|---|---|---|---|
| 1 | 1,10,000 | 1,10,000 | 1,10,471 |
| 5 | 1,50,000 | 1,61,051 | 1,64,531 |
| 10 | 2,00,000 | 2,59,374 | 2,70,704 |
| 15 | 2,50,000 | 4,17,725 | 4,45,392 |
| 20 | 3,00,000 | 6,72,750 | 7,32,807 |
| 30 | 4,00,000 | 17,44,940 | 20,07,729 |
After 30 years, ₹1 lakh becomes ₹4 lakh under SI but ₹17-20 lakh under CI — a 5× difference. This is the “compounding miracle”.
Where You Encounter Simple Interest in India
- Education Loan EMI Holiday Period: Some banks charge simple interest during course + 1-year moratorium
- Personal Loans (some NBFCs): Flat-rate personal loans use SI internally, then add charges making effective IRR much higher
- Car Loans (Old Style): Flat-rate car loans use SI; modern reducing-balance loans use CI
- Hundi / Informal Lending: Traditional moneylenders charged SI; many still do at high rates
- Tax Refunds: Income Tax Dept pays SI on refunds @ 6% p.a. (Sec 244A)
- Section 234 Tax Penalty: Interest on late ITR filing = 1% per month SI
- EPF Withdrawals: Interest on withdrawal-allowed amounts uses SI from withdrawal date till credit
Where You Encounter Compound Interest
- Bank FDs: All banks compound quarterly (some monthly). Interest earned in Q1 earns more interest in Q2.
- PPF: Annual compounding at 7.1% (calculated on lowest balance between 5th and last day)
- Mutual Funds: NAV growth IS compounding — gains generate more gains
- Recurring Deposits: Quarterly compounding by most banks
- Savings Accounts: Quarterly compounding (rate 2.5-4% in major banks)
- Credit Cards: Daily compounding on unpaid balance — 36-48% effective annual rate
- Home Loans: Monthly compounding on reducing balance
- Bonds: Coupon reinvested grows at YTM (yield-to-maturity), assuming reinvestment
Flat Rate vs Reducing Balance — A Loan Trap
Many Indian lenders quote “flat interest rates” for loans, especially personal and vehicle loans. This is SI logic that misleads borrowers about true cost.
| Loan Type | Flat Rate Quoted | Equivalent Reducing Balance |
|---|---|---|
| Personal Loan (5 yrs) | 10% flat | ~18.27% reducing |
| Car Loan (5 yrs) | 7% flat | ~12.83% reducing |
| Two-Wheeler Loan (3 yrs) | 9% flat | ~16.36% reducing |
| Consumer Durable EMI (2 yrs) | 0% (No Cost EMI) | Hidden in inflated price + processing fee |
Rule of thumb: Flat rate × ~1.85 ≈ Reducing balance equivalent (for typical loan tenures). RBI/SEBI mandate disclosure of “Annual Percentage Rate” (APR) including all charges — always ask for APR, not flat rate.
Power of Compounding — Real-Life Numbers
SIP of ₹10,000/month at 12% p.a.
| Years | Total Invested | Final Corpus | Wealth from Compounding |
|---|---|---|---|
| 10 | ₹12 L | ₹23.23 L | ₹11.23 L |
| 20 | ₹24 L | ₹99.91 L | ₹75.91 L |
| 30 | ₹36 L | ₹3.53 Cr | ₹3.17 Cr |
| 40 | ₹48 L | ₹11.88 Cr | ₹11.40 Cr |
Notice: over 40 years, ~96% of your wealth comes from compounding (not from your principal). This is why “start early, stay invested” is repeated endlessly by financial planners — it’s mathematically true.
Common Misconceptions
- “Higher rate is always better”: A 15% rate with annual compounding beats 14% with monthly compounding only marginally; true effective rates differ less than they appear.
- “Daily compounding is much better than monthly”: Only marginally — 10% daily ≈ 10.516%, 10% monthly ≈ 10.471%. Difference is 0.045% EAY.
- “Simple interest is always fairer”: Not really — SI for savings is bad (less return), SI for loans is good (less cost). It depends on which side you’re on.
- “Compounding only matters in long term”: Even in 3-5 years, monthly compounding adds noticeable extra return (~0.5-1% over annual). Adds up across multiple deposits.
- “Tax doesn’t affect compounding”: Wrong — annual tax leakage destroys compounding. FD at 7% with 30% tax effectively compounds at ~4.9% post-tax. PPF/ELSS (tax-deferred or tax-free) compound at full pre-tax rate.
Rule of 72 — Quick Mental Math
Years to double money = 72 / Interest Rate
| Rate | Doubles in (Years) | Actual (CI Annual) |
|---|---|---|
| 4% | 18.0 | 17.67 |
| 6% | 12.0 | 11.90 |
| 8% | 9.0 | 9.01 |
| 10% | 7.2 | 7.27 |
| 12% | 6.0 | 6.12 |
| 15% | 4.8 | 4.96 |
| 20% | 3.6 | 3.80 |
For tripling: use Rule of 114. For quadrupling: use Rule of 144. These are remarkably accurate up to ~20% rates.
Frequently Asked Questions
Is FD compound or simple interest?
Most Indian bank FDs use COMPOUND interest, typically compounded quarterly. Some specialty FDs (“simple interest FDs” or “interest payout FDs”) credit interest periodically without compounding — these have lower effective yield. Check the product terms carefully.
Why do home loans cost so much more than the principal?
Compound interest over 20-30 years on declining balance. A ₹50L home loan at 8.5% for 20 years has EMI of ~₹43,391 and total payment of ~₹1.04 crore — so you pay ₹54L in interest alone. Prepayment in early years (when interest portion of EMI is highest) saves the most.
How is monthly EMI calculated using compounding?
EMI = P × r × (1+r)n / [(1+r)n − 1], where r is monthly rate (annual ÷ 12) and n is total months. Each EMI has interest + principal portions; interest is calculated on outstanding balance (which decreases each month).
Does the “Rule of 72” work for all rates?
Reasonably well for rates between 4% and 20%. For higher rates (30%+), use ln(2)/ln(1+r) for exact answer. For continuous compounding, doubling time = 0.693 / r (since e0.693 = 2).
What’s the difference between APR and APY?
APR (Annual Percentage Rate) is the nominal rate without compounding adjustment. APY (Annual Percentage Yield) is the effective rate after compounding. APY ≥ APR always; difference grows with frequency. Most US bank disclosures show both; Indian banks usually disclose only nominal rate.
Why are credit card rates so high in compound terms?
Credit cards charge 3-4% monthly = 36-48% effective annual rate (after monthly compounding). Add late fees, penal interest, GST on charges — effective cost often exceeds 60%. This is why credit card debt is the most expensive form of borrowing for retail consumers.
Can compounding be negative (i.e., wealth shrinking)?
Yes — if your investment returns are negative for sustained periods. Example: real estate down 5% per year for 5 years compounds to 22.6% loss. Inflation also compounds against your purchasing power. This is why diversification across asset classes is critical.
Which is better: high principal, low time vs low principal, high time?
Time almost always wins. ₹10,000/month for 30 years at 12% (₹36L invested → ₹3.53 Cr) beats ₹50,000/month for 10 years (₹60L invested → ₹1.16 Cr). Starting earlier with smaller amounts is mathematically superior to delayed larger investments.
Does taxation reduce compound interest benefit?
Significantly. FD at 7% with 30% tax = 4.9% post-tax compounding. PPF at 7.1% tax-free = 7.1% compounding. Over 25 years, ₹1L grows to ₹3.3L in taxed FD vs ₹5.4L in PPF — a 64% difference purely from tax efficiency.
Is compounding more important than picking the right investment?
Both matter, but time horizon often beats security selection. A boring index fund (12% CAGR) over 30 years beats a “smart” actively-managed fund (15% gross, 14% net) over 15 years. Compounding rewards consistency more than brilliance.