Written & reviewed byNarasimha MakireddiLast reviewed How we verify

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About This Simple Interest Calculator

This free simple interest calculator computes the interest and total repayment on any principal, for any tenure expressed in years, months, or days — with automatic leap-year handling for day-based calculations. Enter the principal, annual rate, and duration, and it instantly shows the interest earned or owed, the maturity amount, and a period-by-period projection.

Simple interest is everywhere in Indian daily life, even if the label isn't: gold loans from NBFCs, informal lending between relatives and in chit circles, security deposits, delayed-payment penalties on invoices, court-awarded interest, and several post-office schemes all use it. When a jeweller-backed lender in Vijayawada quotes "1% per month" on a gold loan, most borrowers cannot immediately say what that costs on ₹80,000 over 100 days — and that gap is where overpaying happens. This calculator exists to close it: exact interest, any duration, no mental arithmetic.

Why You Need a Simple Interest Calculator

The two costly mistakes with simple interest are confusing monthly and annual rates (a "2% per month" hand loan is 24% a year — worse than most personal loans) and assuming all interest compounds. On the flip side, lenders sometimes quote simple interest but compute compound — verifying takes ten seconds here and can save real money on a dispute. If you lend money informally, calculating the exact amount due on a specific date (in days) keeps the arrangement clean. For the full theory, differences from compound interest, and more worked cases, read our simple interest guide.

How to Use This Calculator

  1. Principal Amount (₹): The amount lent, borrowed, or deposited.
  2. Annual Interest Rate (%): Always convert to an annual rate first — if you were quoted a monthly rate, multiply by 12 (1.5%/month = 18% here).
  3. Tenure Type: Choose Years, Months, or Days. Use Days for loans repaid on a specific date — the calculator automatically uses 366 days for leap years.
  4. Tenure Value: The duration in your chosen unit.

The result shows interest, total amount, and effective breakdown. Remember that simple interest is linear: doubling the tenure exactly doubles the interest, which makes it easy to sanity-check any lender's figure against this output.

Simple Interest vs Compound Interest — Formula and Examples

Simple Interest (SI) is calculated only on the original principal amount throughout the entire loan or investment tenure. The interest does not compound — it does not earn further interest on itself. SI is used in short-term personal loans, overdraft facilities, government bonds, post-office savings schemes, and some traditional deposit products. It is transparent and easy to verify, which is why regulatory bodies often mandate SI disclosure alongside compound interest figures.

Simple Interest Formula

SI = (P × R × T) ÷ 100

Where P is the principal amount, R is the annual interest rate (%), and T is the time in years. For months: T = M ÷ 12. For days: T = D ÷ 365 (or 366 for leap years). Total amount at maturity = P + SI.

Worked Example

You take a personal loan of ₹1,50,000 at 9% per annum for 2 years and 6 months (2.5 years):

Had this been compound interest (compounded monthly), the total interest on the same loan would be ₹36,211 — about ₹2,461 more. The difference grows larger with longer tenures, which is why banks typically use compound interest for home loans but simple interest for certain overdraft products.

Real-World Examples

Lakshmi, Vijayawada — gold loan for 100 days

Lakshmi pledges gold for ₹80,000 at "1% per month" (12% annual) and plans to repay in 100 days. Entering ₹80,000, 12%, and 100 days, the calculator shows interest of about ₹2,630 — total repayment ₹82,630. Knowing the exact figure stopped the branch's rounded-up "₹3,500 interest" estimate from going unchallenged.

Deepak, Indore — informal loan to a cousin

Deepak lends ₹2 lakh to a cousin at 9% simple interest for 3 years. The calculator shows interest of ₹54,000 and total repayment of ₹2,54,000 — a figure both parties agreed to in writing on day one. Takeaway: for family lending, a printed calculation prevents the misunderstandings that usually sour such arrangements.

Accuracy & Common Questions

Is this simple interest calculator accurate?

Yes — SI = (P × R × T) ÷ 100 is exact arithmetic with no approximation, and day-based tenures correctly use 365 or 366 days depending on leap years, computed with high-precision decimals.

When should I use this vs the FD or EMI calculator?

Use this for anything that doesn't compound: gold loans, informal lending, penalties, and short-tenure deposits. Bank FDs over 6 months compound quarterly — use the FD calculator — and amortized bank loans need the EMI calculator.

How do I convert a monthly rate to annual?

Multiply by 12. A moneylender's "2% monthly" is 24% per annum — entering 2 instead of 24 in the rate field understates the interest twelvefold, the single most common input mistake.