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Simple Interest

Investment

Simple Interest

Interest calculated only on the original principal, not on accumulated interest — used in short-term loans and some fixed deposits.

Definition

Simple interest is a method of calculating interest where the interest earned or charged is computed only on the original principal amount — not on any previously accumulated interest. The interest is the same every period, making simple interest grow linearly (in a straight line) rather than the exponential growth of compound interest.

Simple interest is conceptually the most straightforward interest calculation: you borrow (or invest) a sum, and you pay (or earn) a fixed percentage of that sum as interest for each period. No interest accumulates on earlier interest payments.

While compound interest dominates long-term investments and most modern loan products, simple interest remains relevant for short-term calculations, income tax refund interest, and provides the baseline comparison that makes compound interest's power visible.

Formula

Simple Interest (SI) = P × R × T / 100

Where:

  • P = Principal (original amount)
  • R = Annual interest rate (in percentage)
  • T = Time (in years)

Total Amount = P + SI = P × (1 + R×T/100)

For monthly rate: SI = P × R_monthly × N_months / 100

Worked Example

₹2,00,000 deposited at 7% per annum simple interest for 3 years:

SI = ₹2,00,000 × 7 × 3 / 100 = ₹42,000

Total amount = ₹2,42,000

Compare with compound interest at the same rate (quarterly compounding):

  • Amount = ₹2,00,000 × (1 + 0.07/4)^12 = ₹2,46,044
  • Compound interest = ₹46,044 (₹4,044 more than simple interest)

Over 10 years, the same comparison:

  • Simple interest = ₹2,00,000 + (₹2,00,000 × 7% × 10) = ₹3,40,000
  • Compound interest = ₹2,00,000 × (1.0175)^40 = ₹4,00,779

The compounding advantage becomes ₹60,779 over 10 years — highlighting why long-term investors should always seek compound returns. Use the simple interest calculator to compute SI for any amount and period.

Key Things to Know

  • Flat rate loans cost more than they appear: When a lender quotes a "flat rate" of 6% on a car loan, the effective interest rate (on a reducing balance basis) is approximately 11%. This is because flat-rate interest is computed on the original principal throughout, even as you repay it — you end up paying interest on money you've already returned. Always ask for the reducing-balance equivalent rate.
  • Useful for short-term calculations: For loan tenures under 1 year or interest periods of a few months, the difference between simple and compound interest is small and simple interest is often used for its computational convenience. Invoice discounting, trade credit, and inter-corporate loans sometimes use simple interest for short tenures.
  • Income tax refund interest (Section 244A): The IT Department pays interest on delayed refunds at 0.5%/month on a simple interest basis. Similarly, interest on delayed TDS refunds is also computed as simple interest. Understanding SI helps verify whether the IT Department has paid the correct interest on your refund.
  • APR vs simple interest: Annual Percentage Rate (APR) on consumer loans is always expressed as a compound effective rate. When comparing loans, always convert any simple/flat rate to an equivalent APR for an apples-to-apples comparison. A flat 8% loan and an 8% APR loan are not the same — the flat rate is significantly more expensive.
  • Simple interest in legal settlements: Court-awarded damages and interest on delayed payments in Indian courts are typically calculated as simple interest at rates specified in the court order (often 6–12% per annum). Legal contracts often specify "interest at X% per annum" which defaults to simple interest unless "compounded" is explicitly stated.

Related Calculators

Simple Interest Calculator →Calculate simple interest on any principal amount instantly. Find total interest earned or paid and final maturity amount using a fixed rate.
Fixed Deposit Calculator →Calculate your FD maturity amount, total interest, and payout schedule instantly. Supports monthly, quarterly, and cumulative interest options for Indian bank FDs.
Personal Loan EMI Calculator →Calculate your personal loan EMI, total interest, and repayment schedule instantly. Covers all unsecured personal loans in India.

Related Terms

Compound InterestCompound InterestFDFixed DepositAPRAnnual Percentage Rate
Frequently Asked Questions
Where is simple interest actually used in India?
Simple interest is most commonly used in: (1) Short-term personal loans and payday loans from informal lenders — where the interest period is too short for compounding to be practically significant. (2) Post Office Schemes like Kisan Vikas Patra calculations. (3) Vehicle loan and personal loan early repayment statements (some banks use SI to calculate pre-closure amounts). (4) Interest on income tax refunds paid by the IT Department (Section 244A — 0.5% per month on a simple interest basis).
Do banks use simple or compound interest for FDs?
Banks use compound interest for Fixed Deposits — typically compounding quarterly. When you compare two FDs, an 8% FD compounded quarterly gives a higher effective yield than an 8% FD with annual compounding. The effective annual yield (EAY) formula — (1 + r/n)^n − 1 — converts the nominal rate to the actual annual return accounting for compounding.
What is the difference between flat rate and reducing balance interest?
A 'flat rate' loan calculates interest on the original principal throughout the tenure — equivalent to simple interest. A 'reducing balance' or 'diminishing balance' loan calculates interest on the outstanding principal each month — equivalent to compound interest applied only on the remaining balance. A 12% flat rate loan is significantly more expensive than a 12% reducing balance loan. Most Indian home, car, and personal loans use the reducing balance (compound) method.
How is simple interest calculated on income tax refunds?
Under Section 244A of the Income Tax Act, if the IT Department is delayed in issuing your refund, they pay interest at 0.5% per month (simple interest) on the refund amount from April 1 of the assessment year to the date of refund. If you filed correctly and were due a ₹30,000 refund but received it 8 months late, you're entitled to ₹30,000 × 0.5% × 8 = ₹1,200 in interest.
Is simple interest ever better than compound interest for borrowers?
For borrowers, simple interest is always better than compound interest (lower total cost). A ₹5 lakh loan at 10% simple interest for 3 years: total interest = ₹5L × 10% × 3 = ₹1.5 lakh. The same loan at 10% compound interest: total interest ≈ ₹1.65 lakh. The difference grows with higher rates and longer tenures. This is why borrowers should check whether their loan is calculated on a flat (simple) or reducing (compound) basis.