Simple Interest Calculator

What is Simple Interest?

Simple interest (SI) is a method of computing interest where the charge is always based on the original principal — never on accumulated interest. The formula is straightforward: SI = (P × R × T) / 100, where P is the principal, R is the annual rate and T is the time in years. The total amount at the end of the period is A = P + SI.

Simple interest is intuitive: if you lend ₹1 lakh at 8% for 3 years, the interest is exactly ₹24,000 regardless of when it's paid. In contrast, compound interest recalculates on the growing balance — making it more rewarding for investors but more expensive for borrowers. Use our Compound Interest Calculator to directly compare both methods with your specific numbers.

How the Simple Interest Formula Works

The SI formula is one of the most fundamental in finance: SI = P × R × T ÷ 100. Each variable is independent — changing any one of them proportionally changes the result. Double the principal → double the interest. Double the time → double the interest. Double the rate → double the interest. This linear relationship is what makes SI predictable and fair in short-term lending.

For partial year calculations, express T as a decimal: 6 months = 0.5, 9 months = 0.75, 18 months = 1.5. Most short-term personal loans and trade credit are computed this way. Our calculator shows both per-year and per-month interest so you can verify any repayment schedule.

Where is Simple Interest Used?

Despite compound interest dominating modern banking, simple interest hasn't disappeared — it survives wherever a lender wants the math to be easy to verify at a glance, or where regulation specifically mandates it. Recognizing which category a product falls into is the first step to knowing whether the SI formula actually applies to what you're looking at.

• Short-term personal loans — many informal and NBFC loans quote interest as flat SI rates
• Gold loans — most banks and NBFCs charge SI on gold loans with monthly payment options
• Trade credit — businesses extending credit between themselves often use SI for invoice financing
• Government bonds — coupon payments on government securities are fixed amounts (effectively SI on face value)
• Court judgments — interest on legal awards and compensation is often calculated as simple interest
• Savings schemes — some post office schemes and RDs in older formats used SI for partial periods

SI vs CI — When Does the Difference Really Matter?

For periods under 1 year, the difference between SI and CI is negligible. At 10% for 1 year: SI = 10%, CI (annual) = 10% — exactly the same. For 2 years: SI = 20%, CI = 21%. For 5 years: SI = 50%, CI = 61%. For 20 years: SI = 200%, CI = 573%. The longer the period, the larger the divergence.

This is why compound interest tools like SIP Calculator and FD Calculator are more relevant for long-term wealth building, while SI calculations are most useful for understanding short-term borrowing costs.

A useful mental shortcut: under annual compounding, CI ≈ SI + (SI² adjustment growing with each extra year), so the divergence accelerates rather than growing in a straight line. That's why the jump from year 5 to year 20 above (61% to 573% for CI) looks so much steeper than the SI figures (50% to 200%), which just scale linearly with time.

Real-World Simple Interest Examples

Example 1 — Gold loan: You pledge gold worth ₹2,00,000 and take a loan of ₹1,50,000 at 10% p.a. SI. Monthly interest = ₹1,50,000 × 10 / 1200 = ₹1,250. If you repay in 8 months: total SI = ₹10,000. Total repayable = ₹1,60,000.

Example 2 — Trade credit: A retailer buys goods worth ₹5,00,000 on 90-day credit at 2% flat. Interest = ₹5,00,000 × 2 × 0.25 / 100 = ₹2,500. Total to pay in 90 days = ₹5,02,500. Understanding this helps buyers compare credit terms from different suppliers.

Example 3 — Court-awarded compensation: A tribunal awards ₹8,00,000 compensation with 6% simple interest per annum from the date of the claim until payment, and the case takes 4 years to resolve. SI = 8,00,000 × 6 × 4 / 100 = ₹1,92,000. Total payable = ₹9,92,000. This is a common structure in Indian motor accident claims and consumer court awards, where the interest period is often longer than either party expects, and the SI formula makes it easy to verify the tribunal's math independently.

Finding Principal, Rate, or Time from SI

The SI formula can be rearranged for different unknowns:

• Find Principal: P = (SI × 100) / (R × T)
• Find Rate: R = (SI × 100) / (P × T)
• Find Time: T = (SI × 100) / (P × R)

These rearrangements are frequently tested in competitive exams (CAT, bank exams, SSC). Conceptually, they let you work backwards from known outcomes to find unknowns — a critical skill for financial problem solving.

A practical use for the rate formula: if you know a lender quoted you ₹18,000 interest on a ₹1,50,000 loan over 2 years, R = (18,000 × 100) / (150,000 × 2) = 6% flat per year — instantly comparable against offers from other lenders quoting different structures, without needing to trust their advertised headline number.

Limitations of Simple Interest

In the real world, most formal financial products — Fixed Deposits, loans, mutual funds — use compound interest or reducing balance calculations. Quoting a loan as “12% flat” (simple interest) vs “12% reducing balance” (EMI calculation) results in very different effective costs. A flat 12% simple interest loan for 5 years is actually equivalent to approximately 22% effective annual rate under reducing balance — a common trap in informal lending.

Always ask whether a rate is flat (SI) or reducing (EMI/compound) before accepting any loan. For reducing balance calculations, use our EMI Calculator.

The gap widens with tenure: a 5-year flat-rate loan roughly doubles the effective rate versus reducing balance, but a 1-year flat loan at 12% is closer to 21-22% effective — still a meaningful markup, just less dramatic. Lenders that quote flat rates almost always know this gap works in their favor, which is exactly why gold loan and personal loan marketing tends to lead with the flat rate rather than the effective rate.

When This Calculator Works Best

This calculator is ideal for understanding the basic P×R×T relationship, comparing two flat-rate loan offers, computing expected returns on simple government scheme deposits, and solving textbook or exam problems. It is not the right tool for EMI loans, FDs, or SIP investments — all of which involve compounding. Use the relevant specialized calculator for those scenarios.

Typical users include students preparing for competitive exams that test P-R-T problems directly, gold loan and personal loan borrowers checking whether a quoted flat rate is actually reasonable, small business owners setting invoice-financing or trade-credit terms with suppliers and customers, and anyone reviewing a court order or tribunal award that specifies simple interest on a compensation amount. It is less useful for anyone evaluating a bank FD, mutual fund, or EMI loan, where the real math is compounding-based even if a lender's marketing occasionally simplifies it into flat-sounding language.