The gap between simple and compound interest decides whether your savings grow in a straight line or curve upward, and whether a debt stays manageable or spirals. On a ₹1 lakh deposit at 10% for 20 years, simple interest earns ₹2 lakh while compound interest (annual) earns ₹5.73 lakh. Same principal, same rate, wildly different outcome.
Overview
Simple interest charges interest only on the original principal. Deposit ₹1 lakh at 10% simple interest and you earn exactly ₹10,000 every year, no matter how long the deposit runs. The interest itself never earns anything.
Compound interest charges interest on the principal plus whatever interest has already accumulated. After year one, your ₹1 lakh earns ₹10,000, bringing the balance to ₹1,10,000. In year two, interest gets computed on ₹1,10,000, earning ₹11,000. In year three it's computed on ₹1,21,000, earning ₹12,100. The base keeps growing, and each period's interest outpaces the last.
Use the Simple Interest Calculator and Compound Interest Calculator to model any scenario with exact numbers.
Side-by-Side Comparison
| Parameter | Simple Interest | Compound Interest |
|---|---|---|
| Formula | SI = P × r × t | CI = P × (1 + r/n)^(n×t) − P |
| Interest calculated on | Principal only | Principal + accumulated interest |
| Growth curve | Linear | Exponential |
| ₹1 lakh at 10% for 10 years | ₹1,00,000 interest earned | ₹1,59,374 interest earned (annual compounding) |
| ₹1 lakh at 10% for 20 years | ₹2,00,000 interest earned | ₹5,72,750 interest earned (annual compounding) |
| Where used | Short-term loans, some vehicle loans, education loan moratorium | FDs, savings accounts, SIPs, home loans, credit cards |
| Effect on borrower | Predictable, lower total cost on longer terms | Starts slow, then accelerates; can get very expensive if unpaid |
Simple Interest: Deep Dive
The formula for simple interest is SI = P × r × t, where P is the principal amount, r is the annual interest rate as a decimal, and t is the time in years.
Example: personal loan. On ₹50,000 borrowed at 12% per annum for 3 years:
- SI = 50,000 × 0.12 × 3 = ₹18,000
- Total repayment = ₹68,000
- Annual interest charge: exactly ₹6,000 every year
That predictability works in the borrower's favor. Every year's interest is the same fixed amount, with no acceleration. A borrower who can't repay in year 1 and waits until year 3 owes the same interest per year the whole time. The outstanding balance never balloons.
Where simple interest is used in India. Short-term personal loans (under 12 months), some vehicle loans structured on a "flat rate" basis, and the moratorium period of education loans. Under Reserve Bank of India guidelines, interest on education loans accrues on a simple basis during the course period and the 6-12 month repayment moratorium after graduation, which shields students from compounding on a large loan balance before they start earning.
Flat-rate vehicle loans and the hidden trap. Many vehicle loans get quoted as "flat rate" loans, say 10% flat on a ₹5 lakh car loan for 5 years. That looks like simple interest: 5,00,000 × 0.10 × 5 = ₹2,50,000 total interest, making total repayment ₹7,50,000 or ₹12,500 per month. But because the outstanding principal shrinks with every EMI payment, the effective annual rate (once you convert the flat rate to a reducing balance rate) works out to roughly 18-19%, nearly double the quoted rate. Always ask lenders for the reducing balance rate or APR, not the flat rate.
Simple interest for investors. Some post office schemes and small savings products historically quoted simple interest. At low rates over short terms, the gap from compound interest barely matters. Over long terms it widens a lot, which is why comparing any two financial products means looking at compounding frequency, not just the stated rate.
Compound Interest: Deep Dive
The formula for compound interest is CI = P × (1 + r/n)^(n×t) − P, where n is the compounding frequency per year (1 for annual, 4 for quarterly, 12 for monthly, 365 for daily).
Example: fixed deposit. ₹50,000 at 12% per annum compounded monthly for 3 years:
- n = 12, t = 3
- Amount = 50,000 × (1 + 0.12/12)^(12×3) = 50,000 × (1.01)^36
- (1.01)^36 = 1.43077
- Amount = 50,000 × 1.43077 = ₹71,538
- Compound interest = ₹21,538
- Compare to simple interest at ₹18,000: compound interest earns ₹3,538 more over 3 years
The compounding frequency effect. At the same nominal rate of 12%, higher compounding frequency produces a higher effective annual rate (EAR):
| Compounding | EAR |
|---|---|
| Annual | 12.00% |
| Quarterly | 12.55% |
| Monthly | 12.68% |
| Daily | 12.75% |
| Continuous | 12.75% |
The gap between annual and monthly compounding at 12% is 0.68 percentage points, modest in the short term but significant over decades. A bank FD compounded quarterly at 7% has an EAR of 7.19%, so compare offers using EAR rather than nominal rate.
The power of time. Because compound interest grows exponentially, time matters more than rate. ₹1 lakh invested at 12% for 30 years (annual compounding) grows to ₹29.96 lakh. At 10% for 30 years: ₹17.45 lakh. Start 10 years earlier at 10% (40 years total) and you get ₹45.26 lakh, more than starting at 12% for 30 years. That's the math behind the advice to start investing early.
SIPs and compound growth. A SIP of ₹10,000/month at 12% CAGR for 20 years accumulates approximately ₹99.9 lakh. Of that, ₹24 lakh is your invested capital (₹10,000 × 240 months) and ₹75.9 lakh is compounded growth, meaning 76% of the total corpus is pure compounding. Use the SIP Calculator to model different amounts, rates, and durations.
The PPF example. PPF earns compound interest annually on the outstanding balance. At the current rate of 7.1% per annum, ₹1.5 lakh invested annually for 15 years produces approximately ₹40.68 lakh, compared to ₹22.5 lakh under simple interest at the same rate. Compound interest adds ₹18.18 lakh over 15 years on the same contributions. The Fixed Deposit Calculator applies the same quarterly compounding logic most banks use.
When Simple Interest Applies
Simple interest is the relevant calculation for a handful of cases. Short-term borrowing under 12 months barely shows a compound-vs-simple gap at all: a 3-month loan at 18% p.a. charges 4.5% under both methods. The education loan moratorium is another: RBI guidelines specify simple interest during the study period and grace period, protecting students from exponential growth on large loan balances before income begins. Understanding flat-rate loan costs also calls for it: when a lender quotes a flat rate, compute total interest using SI = P × r × t to find the absolute rupee cost, then separately work out the effective reducing balance rate to compare with market alternatives. And many government securities and bonds pay a coupon (fixed interest payment) semi-annually on face value; if coupons aren't reinvested, the effective return sits closer to simple interest.
When Compound Interest Applies
Compound interest governs nearly every long-term financial product. Bank FDs compound quarterly by convention, so always check the EAR rather than the headline rate. Savings account interest gets calculated on daily balance and credited quarterly under RBI rules, which works out to effectively monthly or daily compounding. Mutual fund returns, whether SIP or lumpsum, compound on the NAV; use the SIP Calculator for monthly SIP growth and the Compound Interest Calculator for lumpsum projections. Home loans use monthly reducing balance in the EMI structure, which is compound interest applied to a shrinking principal. Credit cards compound monthly on outstanding balances at 3%/month (36% nominal p.a.), producing an EAR of 42.58%; an unpaid ₹1 lakh credit card balance grows to ₹4.26 lakh in 4 years without a single additional purchase.
The credit card warning. Monthly compounding at 3% per month wrecks debt fast. At a 42.58% effective annual rate, the Rule of 72 predicts the debt doubles in about 72 ÷ 42.58 = 1.69 years, under 2 years. A ₹50,000 balance paying only the minimum (2% per month, which barely covers interest) would take over 10 years to clear and cost more than ₹2 lakh in interest. Pay credit card balances in full every month if you can manage it.
Our Verdict
For investors, compound interest builds wealth, but only if you give it time. Starting ₹5,000/month at age 25 (a 40-year horizon at 12%) produces approximately ₹5.29 crore. Starting at 35 (30 years at 12%) produces approximately ₹1.76 crore. That 10-year head start is worth ₹3.53 crore, purely because compounding had more time to run.
For borrowers, compound interest on high-rate debt, especially credit cards, works against you with the same force. A 42.58% EAR means every year you leave a balance unpaid, it grows by nearly half. The math is identical; only the direction changes.
Maximise time in the market for investments, pay off high-rate debt fast, and compare financial products on effective annual rate rather than nominal rate. Use the Compound Interest Calculator to model the exact difference for whatever scenario you're weighing.