What is Simple Interest?
Simple interest is a method of calculating the interest charge on a loan or the return on an investment where interest is computed only on the original principal amount — not on previously accumulated interest. This distinguishes it from compound interest, where interest earns interest over time.
Simple interest is commonly used in short-term loans, car loans (flat rate), personal financing, and some government savings schemes. It is straightforward to calculate and easy to understand, making it ideal for quick financial estimations.
Simple Interest Formula
SI= Simple Interest (the interest amount earned or paid)P= Principal (the original amount borrowed or invested)R= Annual Rate of Interest (in percentage per year)T= Time (in years)Amount (A)= P + SI (total amount at end of period)
You can rearrange this formula to solve for any variable:
- Principal: P = (SI × 100) / (R × T)
- Rate: R = (SI × 100) / (P × T)
- Time: T = (SI × 100) / (P × R)
Worked Examples
Example 1: Basic SI Calculation
You deposit PKR 50,000 in a savings scheme offering 8% per year for 3 years.
SI = (50,000 × 8 × 3) / 100 = PKR 12,000
Total amount at end = PKR 50,000 + PKR 12,000 = PKR 62,000
Example 2: Flat Rate Car Loan
A bank offers a car loan of PKR 1,000,000 at a 15% flat rate for 4 years.
SI = (1,000,000 × 15 × 4) / 100 = PKR 600,000
Total repayment = PKR 1,600,000 | Monthly installment = PKR 1,600,000 / 48 = PKR 33,333/month
Note: the effective annual rate (EAR) of a flat rate loan is approximately double the stated flat rate, because you are paying interest on the original balance even as the loan reduces. At 15% flat rate for 4 years, the EAR is approximately 27–28%.
Example 3: Solving for Rate
You borrowed PKR 200,000 and paid back PKR 260,000 after 2 years. What was the annual interest rate?
SI = 260,000 − 200,000 = 60,000R = (60,000 × 100) / (200,000 × 2) = 6,000,000 / 400,000 = 15% per year
Simple Interest vs Compound Interest
The key difference is that compound interest earns "interest on interest," while simple interest does not. For the same principal, rate, and time:
| Year | Simple Interest | Compound Interest (Annual) |
|---|---|---|
| 1 | PKR 10,000 interest | PKR 10,000 interest |
| 2 | PKR 10,000 interest | PKR 11,000 interest (10% of 110,000) |
| 5 | PKR 10,000/yr = PKR 50,000 total | PKR 61,051 total interest |
| 10 | PKR 100,000 total | PKR 159,374 total interest |
(Assuming PKR 100,000 principal at 10% per year)
For lenders, compound interest generates more income. For borrowers, simple interest is cheaper over long periods. For investors, compound interest builds more wealth. Always understand which method your bank uses.
Where Simple Interest is Used in Pakistan & India
In Pakistan: Some microfinance institutions (MFIs) and committees (rotating savings groups known as "committees" or "BCCs") use simple interest. Certain DSC (Defence Savings Certificates) use SI calculations for short periods. Many informal loans use SI.
In India: Post Office Savings Accounts pay simple interest. Short-term loans between individuals often use SI. Fixed deposits calculate interest on simple basis if paid out quarterly rather than reinvested.
In the UAE/Gulf: Car loans from some dealers are quoted as flat rates — which are simple interest in structure. Always ask for the APR (Annual Percentage Rate) which reflects the true cost using compound basis.
Converting Flat Rate to Effective Rate
Since flat rate loans charge SI on the original balance throughout, the effective rate is higher. A rough conversion formula:
n= number of installments
For a 2-year (24 monthly installments) flat rate loan at 10%: Effective rate ≈ 10% × 2 × 24/25 = 19.2%. This is why comparing loan offers using APR rather than flat rate is crucial.