Simple Interest Calculator India — Formula, SI vs CI & Real-World Use 2026
Simple interest (SI) is calculated only on the original principal — the interest earned does not itself earn further interest. The formula: SI = (P × R × T) / 100, where P is the principal amount, R is the annual interest rate (as a percentage), and T is the time in years. Total maturity amount: P + SI = P × (1 + RT/100). On ₹1,00,000 at 10% p.a. for 3 years: SI = ₹1,00,000 × 10 × 3 / 100 = ₹30,000; total = ₹1,30,000. Compound interest (CI) on the same figures: ₹1,00,000 × (1.10)³ = ₹1,33,100 — ₹3,100 more, because the Year 1 interest (₹10,000) earns ₹1,000 in Year 2, and so on. The gap between SI and CI grows exponentially with time: at 10 years the difference is ₹59,374 (SI = ₹2,00,000; CI = ₹2,59,374), and at 20 years the CI corpus is more than double the SI corpus. This is why compound interest is universally used for long-term savings instruments (bank FDs, RDs, PPF, EPF, mutual funds), and simple interest is confined to short-duration or specific-use products.
Simple interest is used in Indian financial products in specific circumstances: personal loans and EMI schedules — the outstanding principal on a reducing-balance loan is recalculated after each EMI payment, effectively creating a daily-reducing SI calculation (not flat SI); overdraft facilities — interest is charged on the daily utilised balance; some government agriculture loans and MSME schemes — flat SI for one-time disbursements; and short-term inter-company deposits. For borrowers, a "flat rate" personal loan offering 12% p.a. flat is actually equivalent to approximately 21–22% effective annual rate on a reducing balance — because you pay interest on the full original principal throughout the tenure, even as the outstanding balance reduces with each EMI. Always compare the Effective Annual Rate (EAR) or APR, not the stated flat rate, when evaluating loan offers. For savings instruments, always prefer compound interest products over simple interest — ₹1,00,000 invested for 5 years at 7% SI returns ₹35,000 in interest; the same amount in a compounding instrument at 7% returns ₹40,255 — a ₹5,255 difference that grows with tenure. Use the FD Calculator for compound interest on fixed deposits and the RD Calculator for recurring deposit accumulation.
The Simple Interest Formula Explained — SI = P×R×T/100, Maturity Value, and How SI Differs from Compound Interest
Simple interest is calculated on the original principal amount only, for every period. The formula: SI = (P × R × T) / 100, where P is the principal (₹), R is the annual rate of interest (%), and T is the time in years. Maturity value (total amount including principal and interest): A = P + SI = P × (1 + R×T/100).
Example: ₹1,00,000 at 10% p.a. for 5 years (simple interest): SI = ₹1,00,000 × 10 × 5 / 100 = ₹50,000. Maturity value = ₹1,50,000. Under compound interest (annual compounding): A = ₹1,00,000 × (1 + 10/100)⁵ = ₹1,61,051. Difference: ₹11,051 — the CI earns ₹11,051 more because each year's interest earns further interest. At Year 1, both are identical (₹10,000 interest). At Year 2, SI earns another ₹10,000 on the original ₹1L; CI earns ₹11,000 on ₹1.1L (original + Year 1 interest). This compounding gap widens with time.
SI vs CI: the time effect — on ₹1,00,000 at 10% p.a.:
| Tenure | SI Maturity | CI Maturity | Difference |
|---|---|---|---|
| 1 year | ₹1,10,000 | ₹1,10,000 | ₹0 |
| 3 years | ₹1,30,000 | ₹1,33,100 | ₹3,100 |
| 5 years | ₹1,50,000 | ₹1,61,051 | ₹11,051 |
| 10 years | ₹2,00,000 | ₹2,59,374 | ₹59,374 |
| 20 years | ₹3,00,000 | ₹6,72,750 | ₹3,72,750 |
The gap between SI and CI is negligible in the short term (1–2 years) but enormous over 10–20 years. This is why simple interest is appropriate only for short-duration instruments or daily-accruing products where compounding frequency matters less. For any long-duration savings goal (retirement, children's education), compound interest instruments (FD, RD, PPF, mutual funds) are dramatically superior to simple interest instruments.
Three Simple Interest Scenarios — Short-Term Deposit, Personal Loan True Cost, and Agriculture Loan Comparison
Scenario 1: Radha, ₹2 lakh short-term deposit at 6% simple interest for 90 days
A small cooperative bank offers a 90-day deposit at 6% p.a. on simple interest basis. P = ₹2,00,000. R = 6%. T = 90/365 = 0.2466 years. SI = ₹2,00,000 × 6 × 0.2466 / 100 = ₹2,959. Maturity value: ₹2,02,959. Note: if the bank compounds quarterly instead of using SI, the maturity value at 6% quarterly compounding (6/4 = 1.5% per quarter) for 1 quarter: ₹2,00,000 × (1 + 0.015)¹ = ₹2,03,000 — an additional ₹41. Over short durations like 90 days, the SI vs CI difference is minimal. SI instruments are perfectly acceptable for parking money for periods under 6 months where the compounding gap is small.
Scenario 2: Priya, ₹5 lakh personal loan at 12% 'flat rate' for 3 years — actual cost revealed
Priya takes a ₹5L personal loan at 12% flat rate for 3 years. The bank calculates EMI using the flat rate: Total interest = ₹5,00,000 × 12% × 3 = ₹1,80,000. Total repayment: ₹6,80,000. Monthly EMI: ₹6,80,000 / 36 = ₹18,889. But the outstanding principal reduces with each EMI — by the midpoint of the loan, Priya has repaid approximately ₹2.5L of principal. Yet she is paying interest on the original ₹5L all through. The effective annual rate (EAR) on a reducing balance = approximately 2 × flat rate × n / (n+1) where n = number of periods. Approximate EAR: 2 × 12% × 36 / 37 = 23.4%. Priya is effectively paying 23–24% on a reducing balance loan, not 12%. This is the standard flat-rate personal loan trap. Always ask banks for the APR (Annual Percentage Rate) or the reducing balance effective rate, not the flat rate.
Scenario 3: Mohan, agriculture loan at 7% simple interest vs 7% compound interest — 2-year comparison
Mohan takes a ₹3 lakh Kisan Credit Card (KCC) loan at 7% p.a. for 2 years. Government agriculture loans are often calculated on simple interest for short tenures. SI over 2 years: ₹3,00,000 × 7 × 2 / 100 = ₹42,000. Total repayment: ₹3,42,000. If the same loan were at 7% compound (annual): ₹3,00,000 × (1.07)² = ₹3,43,470 — additional ₹1,470. Over 2 years at 7%, the SI-CI difference is small (₹1,470). The government subsidy on KCC interest (interest subvention scheme reducing effective rate to 4% or below for timely repayment — verify at nabard.org.in) makes the absolute interest cost secondary to accessing credit. The SI structure is appropriate for short-duration agriculture working capital — the simplicity of the SI calculation also makes it easier for rural borrowers to understand.
Where Simple Interest Is Used in Indian Finance — Loans, Overdrafts, Government Schemes, and Where CI Is Always Better
Where simple interest is commonly used in India:
1. Personal loans on reducing balance (misnamed as 'flat rate' SI): Almost all personal loans from banks and NBFCs quote a 'flat rate' based on simple interest on the original principal — but the EMI is calculated on this basis and the loan actually amortises on a reducing balance. The stated flat rate of 12–18% translates to an effective annual rate of 22–33% on a reducing balance. The SI formula is used to calculate total interest, which is then divided into equal monthly EMIs — not true reducing balance compound interest. The practical impact: always compare APR, not flat rate, across loan offers.
2. Overdraft facilities: Bank overdraft interest is typically calculated on daily outstanding balance — effectively SI on the daily balance, accrued monthly. Because the balance changes daily, the interest calculation is the sum of (daily balance × daily rate). This is SI applied to a changing principal — the most accurate form of interest calculation for revolving credit.
3. Kisan Credit Card (KCC) and agriculture schemes: Short-term agriculture loans under government schemes often use SI for the loan tenure (typically 1 crop season, 6–12 months). Interest subvention schemes reduce effective rates further — verify current subvention rates at nabard.org.in.
4. Short-term inter-company deposits and Treasury instruments: Corporate treasury deposits of 30–90 days often use SI. Commercial paper and some money market instruments accrue interest on a simple interest basis for their short tenures.
Where compound interest is always better for savers: FDs (quarterly or monthly compounding), RDs (monthly compounding), PPF (annual compounding), EPF (annual compounding), SCSS (quarterly payout, effectively quarterly CI), NSC (annual compounding with deemed reinvestment), and all mutual fund NAVs (continuous compounding through NAV appreciation). For any goal with a tenure exceeding 2 years, compound interest instruments are superior — the advantage compounds with time exactly as the formula shows.
Simple Interest Mistakes — Confusing Flat Rate with EAR, Comparing Across Loan Offers Incorrectly, and Long-Term SI Traps
Accepting the stated flat rate as the true borrowing cost. The most pervasive personal finance error in India. A 15% flat rate personal loan from an NBFC vs a 16% reducing balance loan from a bank: the NBFC flat-rate loan is actually approximately 27–28% effective rate; the bank's 16% reducing balance is 16% effective. The NBFC loan is approximately 12% more expensive in real terms, despite the lower stated number. Always ask: 'Is this a flat rate or a reducing balance rate? What is the EAR?' Lenders are required to disclose the Annual Percentage Rate (APR) under RBI Fair Practices Code — request this in writing before signing any loan agreement.
Using the SI formula for long-term savings projections. If you calculate your PPF maturity using SI (P × R × T / 100), you will dramatically underestimate the corpus — PPF compounds annually. At ₹1L/year contribution for 15 years at 7.1% (PPF rate — verify at nsiindia.gov.in): SI projection = ₹15L × 7.1% × 7.5 years average ≈ ₹7.98L interest, total ₹22.98L. Actual PPF (annual compounding): approximately ₹27.1L. The SI estimate understates by ₹4.1L. Always use the compound interest formula for long-duration instrument projections — FD, RD, PPF, NPS, mutual fund — all compound.
Not converting partial-year periods correctly. SI for 18 months: T = 18/12 = 1.5 years. SI for 45 days: T = 45/365 = 0.123 years (or 45/360 for banker's rule — check which the lender uses). Using 18 months as T = 18 (not 1.5) gives a result 12× too large — a common calculation error. Always express time in the same units as the rate: if rate is annual (p.a.), time must be in years.
Ignoring the penalty on prepayment for flat-rate loans. Prepaying a flat-rate loan does not save the full outstanding 'SI remaining' — many lenders use the Rule of 78 (sum-of-digits method) for prepayment calculations, which front-loads interest allocation. If you prepay a 3-year flat-rate loan at the end of Year 1, you have paid more than 1/3 of the total interest (because more interest is allocated to early periods in the Rule of 78). Verify the prepayment calculation method and foreclosure charge with the lender before making a prepayment decision.
Frequently Asked Questions
What is the simple interest formula in India?
SI = (P × R × T) / 100, where P is the principal amount (₹), R is the annual interest rate (%), and T is the time in years. Total maturity value: A = P + SI = P × (1 + R×T/100). Example: ₹1,00,000 at 8% for 3 years — SI = ₹1,00,000 × 8 × 3 / 100 = ₹24,000. Maturity value = ₹1,24,000. Note: this assumes T is expressed in the same unit as R (both annual).
What is the difference between simple interest and compound interest?
Simple interest is calculated only on the original principal for every period — interest does not accumulate on itself. Compound interest is calculated on the principal + accumulated interest, so each period's interest earns further interest. On ₹1,00,000 at 10% for 10 years: SI maturity = ₹2,00,000; CI maturity = ₹2,59,374 — a difference of ₹59,374. The CI advantage grows exponentially with time. For savings goals, always prefer compound interest instruments (FD, PPF, mutual funds). Simple interest is appropriate for short-duration loans and instruments where compounding period has minimal impact.
What does a flat-rate personal loan at 15% actually cost?
A '15% flat rate' personal loan is significantly more expensive than it appears. Flat rate means interest is calculated on the original principal (SI formula) for the full tenure, then divided into equal EMIs. As you repay principal, the effective balance reduces — but you keep paying interest on the original amount. The approximate effective annual rate (EAR) for a flat-rate loan: EAR ≈ 2 × flat rate × n / (n+1), where n = number of monthly installments. For a 3-year (36-month) flat-rate loan at 15%: EAR ≈ 2 × 15% × 36/37 ≈ 29.2%. Always compare loans using EAR or the Annual Percentage Rate (APR), not the stated flat rate.
Where is simple interest used in India?
Simple interest is used in: (1) Personal loan flat-rate calculations (the SI formula is used to determine total interest, then split into equal EMIs — though the effective rate is reducing balance); (2) Bank overdraft interest on daily outstanding balance; (3) Short-tenure agriculture loans (Kisan Credit Card for 1 season); (4) Short-term inter-company and corporate treasury deposits (30–90 days); (5) Some government subsidy scheme calculations for simplicity. For savings instruments in India (FD, RD, PPF, SCSS, NSC, mutual funds), compound interest always applies — use the respective calculators for accurate projections.
How do I calculate simple interest for months or days?
Convert the time to years: months ÷ 12 for monthly tenures; days ÷ 365 (or ÷ 360 for banker's rule — verify with the lender) for daily tenures. Example: ₹5,00,000 at 9% for 90 days: T = 90/365 = 0.2466 years. SI = ₹5,00,000 × 9 × 0.2466 / 100 = ₹11,096. Example: ₹2,00,000 at 7% for 18 months: T = 18/12 = 1.5 years. SI = ₹2,00,000 × 7 × 1.5 / 100 = ₹21,000. Always express T in the same unit as R — if R is annual, T must be in years.
Is the PPF interest calculated using simple interest?
No. PPF interest is calculated using compound interest — annual compounding on the balance at the end of each financial year. The government announces the PPF interest rate quarterly (verify current rate at nsiindia.gov.in); interest is credited to the account at the end of the financial year. Using the SI formula for PPF will significantly underestimate the maturity value. Use a PPF Calculator with the compound interest formula for accurate maturity projections.
What is the rule of 78 for simple interest loans?
The Rule of 78 (also called sum-of-digits method) is a front-loading method for allocating flat-rate loan interest across monthly installments. It allocates more interest to the early periods and less to the later periods. For a 12-month loan: sum of digits = 1+2+...+12 = 78. Month 1's interest = 12/78 of total interest; Month 12's = 1/78. This means if you prepay a 12-month loan after 6 months, you have already paid more than half the total interest — prepayment saves less than proportional. Many flat-rate loan lenders use Rule of 78 for prepayment calculations. Verify with your lender whether they use straight-line allocation or Rule of 78 before prepaying.
What is a better alternative to a simple interest deposit?
For any tenure beyond 3–6 months, compound interest instruments always generate more returns at the same rate. Alternatives: Bank Fixed Deposits (FD) — quarterly compounding, senior citizen rates higher (verify at your bank); Post Office FDs — quarterly compounding (verify at nsiindia.gov.in); SCSS (Senior Citizens Savings Scheme, 60+) — approximately 8.2% quarterly payout (verify current rate); NSC — annual compounding with deemed reinvestment for 80C. For 5+ year tenures: PPF, equity SIP, or ELSS generate significantly higher returns than any fixed-income SI instrument. Use the FD Calculator for compound interest projections.