Simple Interest Calculator
Flat, non-compounding interest on a principal — P × r × t with a year-by-year build-up of interest and total value.
Year-wise interest
| Year | Total interest | Total value |
|---|---|---|
| 1 | ₹8,000 | ₹1,08,000 |
| 2 | ₹16,000 | ₹1,16,000 |
| 3 | ₹24,000 | ₹1,24,000 |
| 4 | ₹32,000 | ₹1,32,000 |
| 5 | ₹40,000 | ₹1,40,000 |
What is Simple Interest?
Simple interest is the most foundational mathematical method for calculating the cost of borrowing or the return on an investment. Unlike compound interest—where accumulated interest is periodically added back into the principal to earn "interest on interest"—simple interest accrues strictly and exclusively on the original, initial principal sum for the entire duration of the agreement.
Because the principal base remains static throughout the tenure, the interest generated in Year 1 is identical to the interest generated in Year 5 or Year 10. This linear proportionality makes simple interest transparent, predictable, and straightforward to calculate for personal, business, and statutory transactions.
Simple Interest vs Compound Interest
Understanding when to use simple versus compound interest is critical across commercial and statutory contexts:
- Growth Trajectory: Simple interest grows linearly (a straight upward sloping line), whereas compound interest expands exponentially (a hockey-stick curve steepening over long horizons).
- Commercial Applications: Most consumer banking products (savings accounts, recurring deposits, fixed deposits, home loans) employ compound interest. Simple interest is utilized primarily in short-term promissory notes, auto-loans quoted on flat rates, informal peer lending, and statutory government penalties (such as GST Section 50 and Income Tax Section 234 interest).
The Simple Interest Formulation
SI = (P × R × T) ÷ 100 · Total Amount (A) = P + SI
- SI: Total Simple Interest accrued over the entire term (in ₹)
- P: Principal initial amount borrowed or invested (in ₹)
- R: Annual interest rate expressed as a percentage per annum (% p.a.)
- T: Time duration in years (fractions permitted: months ÷ 12, or days ÷ 365)
- A: Total maturity amount or repayment obligation (Principal + Simple Interest)
Worked example: ₹1,00,000 at 8% p.a. for 3 Years
Consider an investor lending ₹1,00,000 at a simple interest rate of 8% per annum over an agreement period of 3 years:
- Principal (P): ₹1,00,000
- Annual Rate (R): 8%
- Time (T): 3 years
- Annual Interest: ₹1,00,000 × (8 ÷ 100) = ₹8,000 per year
- Total Simple Interest (SI): ₹8,000 × 3 = ₹24,00,000 ÷ 100 = ₹24,000
- Total Maturity Value (A): ₹1,00,000 + ₹24,000 = ₹1,24,000
If the same ₹1,00,000 were compounded annually at 8%, the 3-year interest would be ₹25,971—yielding ₹1,971 more due to compounding effects.
Simple Interest vs Maturity Value on ₹1,00,000 Principal
| Interest Rate (p.a.) | 1 Year Interest | 3 Years Interest | 5 Years Interest | Total at 5 Years |
|---|---|---|---|---|
| 6% | ₹6,000 | ₹18,000 | ₹30,000 | ₹1,30,000 |
| 8% | ₹8,000 | ₹24,000 | ₹40,000 | ₹1,40,000 |
| 10% | ₹10,000 | ₹30,000 | ₹50,000 | ₹1,50,000 |
| 12% | ₹12,000 | ₹36,000 | ₹60,000 | ₹1,60,000 |
| 15% | ₹15,000 | ₹45,000 | ₹75,000 | ₹1,75,000 |
| 18% (GST Late Rate) | ₹18,000 | ₹54,000 | ₹90,000 | ₹1,90,000 |
Handling Partial Years: Months and Days Conventions
In real-world finance, loan tenures rarely conclude on exact year boundaries. Converting non-annual tenures into year fractions T is straightforward:
- Tenure in Months: Divide the months by 12. For 18 months: $T = 18 div 12 = 1.5$ years.
- Exact Interest Convention (365-day year): Standard under Indian statutes (e.g., GST Section 50): divide days by 365. For 73 days: $T = 73 div 365 = 0.2$ years.
- Ordinary Interest Convention (360-day year): Common in international banking (commercial year): divide days by 360.
Taxation of Simple Interest Income (Section 56)
Under the Income Tax Act, 1961, interest income earned from private lending, bonds, or bank accounts is classified as "Income from Other Sources" under Section 56 and taxed at your marginal personal income tax slab rate:
- Section 80TTA: Provides an exemption up to ₹10,000 per financial year on savings bank account interest for individuals and HUFs below 60 years under the Old Regime.
- Section 80TTB: Provides senior citizens (aged 60+) an exemption up to ₹50,000 per financial year on all interest income (savings accounts, fixed deposits, post office deposits) under the Old Regime.
Frequently Asked Questions on Simple Interest
Mathematical mechanics, statutory applications, compounding comparisons, and tax rules.
In simple interest, interest is calculated solely on the original principal throughout the loan tenure. In compound interest, accrued interest is periodically credited to the principal, so subsequent periods earn interest on both principal and past interest, leading to exponential growth over time.
Simple interest is used in short-term peer-to-peer loans, flat-rate vehicle financing, government statutory interest on delayed tax payments (Income Tax Section 234 and GST Section 50), sovereign gold bond semi-annual payouts, and court-awarded compensation claims.
For months, divide the number of months by 12 (e.g., 6 months = 6 ÷ 12 = 0.5 years). For days, divide the number of days by 365 for a standard year (e.g., 90 days = 90 ÷ 365 ≈ 0.2466 years), which is the standard method used in statutory tax interest calculations.
Both the Income Tax Department (under Sections 234A, 234B, and 234C) and the GST Department (under Section 50) levy simple interest on unpaid tax dues. No interest-on-interest is charged on tax arrears.
Interest earned on loans or investments is added to your total income under "Income from Other Sources" and taxed at your applicable personal income tax slab rate. TDS may also be deducted by the payer if statutory thresholds are crossed.
Section 80TTA allows an individual or HUF (under the Old Tax Regime) to claim a deduction of up to ₹10,000 per financial year on interest earned from savings bank accounts. Any interest earned above ₹10,000 is fully taxable.
Resident senior citizens aged 60 and above can claim a deduction of up to ₹50,000 per financial year under Section 80TTB on interest earned from savings accounts, fixed deposits, and recurring deposits with banks and post offices under the Old Tax Regime.
Exact Interest uses 365 days in a year (or 366 in a leap year) to calculate the daily interest rate. Ordinary Interest assumes a standardized 360-day financial year (twelve 30-day months), which produces slightly higher interest for lenders.
In custom private loan agreements, parties can agree to adjust simple interest rates periodically (e.g., linking to RBI Repo Rate). However, once adjusted, the new rate applies strictly to the base principal without compounding past unpaid interest.
No. Sovereign Gold Bonds pay a fixed simple interest of 2.50% per annum on the initial nominal issue value. The interest is credited semi-annually directly to the investor’s bank account and does not compound.