# Simple Interest Calculator: SI for Years, Months or Days > Simple interest calculator: enter principal, rate and time in years, months or days for the interest and total amount, with compound interest to compare. Source: https://trykosh.com/tools/simple-interest-calculator Free tool - Investing # Simple interest calculator Calculate simple interest for any period, in years, months or days, and see the total you'll repay or receive. Rules checked 25 October 2026Free - no sign-upRuns in your browser Simple interest INR 24,000 Total INR 1,24,000 after 3 years Total amount**INR 1,24,000** Interest a year**INR 8,000** Compound interest instead (yearly)**INR 25,971** Principal **INR 1,00,000**Interest **INR 24,000** Year by year Year Interest that year Total so far Year 1 INR 8,000 INR 1,08,000 Year 2 INR 8,000 INR 1,16,000 Year 3 INR 8,000 INR 1,24,000 Simple interest is earned only on the original principal, so it grows in a straight line. ## Upload your statement and Kosh does this for you. Upload your statements and Kosh tracks every SIP, FD and deposit in one place, and shows how far you are from each goal. [Start free](/signup?via=tool-simple-interest-calculator) ## How the simple interest calculator works Simple interest is charged or earned only on the original principal. It doesn't compound, so the interest is the same every year and the total grows in a straight line. Enter the principal, the yearly rate and the time. If the time is in months, the calculator divides by 12; if in days, by 365, the convention most Indian lenders and courts use. Interest is principal x rate x time / 100. It also shows what the same terms would earn with yearly compounding, so you can see how much a compounding arrangement would change the result. Simple interest is common in informal loans between family and friends, some gold loans and short-term borrowing, interest on delayed payments, and in school and competitive-exam maths. SI = P x R x T / 100 P principal R yearly rate in percent T time in years (months / 12, days / 365) ## Worked example: A family loan for 18 months Ravi lends his cousin INR 50,000 at 12% a year simple interest for 18 months. The interest comes to INR 9,000, so his cousin repays INR 59,000. Had they agreed on yearly compounding instead, the interest would have been INR 9,265. Simple interest INR 9,000 Total INR 59,000 after 18 months Total amount**INR 59,000** Interest a year**INR 6,000** Compound interest instead (yearly)**INR 9,265** Principal **INR 50,000**Interest **INR 9,000** Simple interest is earned only on the original principal, so it grows in a straight line. [Load this example into the calculator ->](?amount=50000&rate=12&time=18&unit=months#calculator) ## Good to know - A 'flat rate' quoted on some personal and vehicle loans is simple interest on the original amount for the full tenure, which costs far more than the same reducing-balance rate. - Interest you earn on a private loan is taxable as income from other sources, at your slab. - Write down the rate, period and repayment date for family loans: it avoids disputes and helps if the tax department asks about the money. ## Frequently asked questions What is the formula for simple interest? SI = P x R x T / 100, where P is the principal, R the yearly rate in percent and T the time in years. The total amount is P + SI. How do I calculate simple interest for months? Divide the number of months by 12 to get the time in years. Interest on INR 1 lakh at 9% for 8 months is INR 1,00,000 x 9 x 8/12 / 100 = INR 6,000. How do I calculate simple interest for days? Divide the number of days by 365. Interest on INR 2 lakh at 10% for 90 days is INR 2,00,000 x 10 x 90/365 / 100 = about INR 4,932. Is a flat interest rate the same as simple interest? Yes, in effect. A flat rate charges interest on the full original loan for the whole tenure even as you repay it, so its effective rate on a reducing balance is almost double the quoted flat rate. ## Sources and assumptions - [Reserve Bank of India](https://www.rbi.org.in/) Rules and rates last checked on 25 October 2026. Results are estimates for planning and education, not tax, legal or investment advice. Kosh is an educational financial planning tool by Primedepth Labs. It is not a SEBI-registered investment adviser and does not offer personalised investment advice, hold your money or execute trades. Markets carry risk; read scheme documents carefully and consult a registered adviser before acting.