Private loans
Agree a fair figure when lending to or borrowing from family or a supplier.
Simple Interest Calculator
Enter the principal, the yearly rate and the time to get the interest and the total.
Your files stay with you. Your files never leave your browser. This tool runs on your device: nothing is uploaded to our servers, so there is nothing for us to store or see.
How to use it
- Enter the principal and the yearly interest rate.
- Enter the time and choose years, months or days.
- Read the interest, the total and the working.
Where it helps
Homework and exams
Check answers to school and competitive exam questions.
Late payments
Work out interest on an overdue bill at an agreed yearly rate.
Good to know
Stays in your browser
Numbers you type are worked out on your device and never sent to our servers.
Most products compound
FDs, savings and most loans use compound interest instead.
Days use 365
73 days counts as 0.2 of a year.
Questions people ask
How do you calculate simple interest?
Simple interest is principal times rate times time, divided by 100, with the rate per year and the time in years. Enter the principal, the yearly rate and the time in years, months or days, and the tool shows the interest, the total amount and the working. ₹1 lakh at 9% for three years earns ₹27,000.
- P x R x T ÷ 100. The formula.
- ₹27,000 ₹1 lakh, 9%, 3 yrs. Worked example.
- Y, M or D Time units. Converted for you.
- Working Shown below. Check it on paper.
What is the simple interest formula?
Simple interest equals principal times rate times time, divided by 100, with the rate in percent a year and the time in years. ₹1,00,000 at 9% for three years earns ₹27,000, so you'd repay ₹1,27,000 in total. The tool writes this working out under the answer so you can follow every step.
How are months and days turned into years?
Months are divided by 12 and days by 365. So six months is half a year and 73 days is 0.2 of a year. Choose the unit from the 'Time in' list and type the number as it is. That saves you doing the conversion yourself and avoids small mistakes.
When should I use compound interest instead?
Savings accounts, fixed deposits, recurring deposits, mutual funds and most bank loans compound, so use the compound interest or FD calculator for them. Simple interest is used for some short private loans, gold loans, certain government schemes and many school and exam problems. Ask the lender which method applies if you're unsure.
What is the difference between simple and compound interest?
Simple interest is earned only on the original principal. Compound interest is also earned on interest already added. ₹1 lakh at 10% for five years gives ₹50,000 simple interest but about ₹61,051 compounded yearly. The gap grows quickly as the years go by. Try both tools with the same figures to compare.
How do I find the rate if I know the interest?
Rearrange the formula: rate equals interest times 100, divided by principal times time. If ₹50,000 earned ₹6,000 in two years, the rate is 6,000 times 100 divided by 1,00,000, which is 6% a year. You can test the answer by entering it in the tool.
How do I find the time needed for a target?
Time equals interest times 100, divided by principal times rate. To earn ₹10,000 on ₹40,000 at 10% a year, you need 10,000 times 100 divided by 4,00,000, which is 2.5 years. Put 2.5 in the tool as years to check the result matches your target.
How do I charge interest on a late customer payment?
Agree a yearly rate in your terms first, such as 18% a year. Then enter the overdue amount, that rate and the number of days late. A ₹50,000 bill paid 30 days late at 18% a year attracts about ₹740 interest. Show the working on the reminder so the customer can check it.
Is interest on a gold loan simple or compound?
Many gold loans charge simple interest for the agreed term and add penalty interest if you pay late, but it varies by lender. Read the sanction letter for the rate type and when interest must be paid. If it's simple, this tool gives you the cost of the loan.
What does 'per annum' mean?
Per annum means per year. A rate of 12% per annum on ₹10,000 adds ₹1,200 a year as simple interest. Some private lenders quote rates per month, such as 2% a month, which is 24% a year. Always convert a monthly rate to yearly before entering it here.
Can I use it for a loan from a friend or relative?
Yes. Agree the principal, the yearly rate and the period, and enter them here to get a clear figure you both accept. Write the terms down and keep a copy of the working. A small written record avoids misunderstandings later, even between family members. Both sides can open the tool to check.
Can I use it for exam questions?
Yes. Most simple interest questions in school, bank and SSC exams give three of the four values: principal, rate, time or interest. Enter the known values to check your answer, and read the working shown to see whether your method matches the standard formula. It's quicker than checking with a calculator.
Is the interest rounded?
The tool shows the interest to the paisa so you can see the exact figure. Banks and lenders usually round to the nearest rupee on statements. If your agreement says how rounding works, follow that, because a few paise either way can add up over many loans.
Is anything I enter stored?
No. The calculation runs in your browser and the figures never reach our servers. There's no sign-up, and nothing is saved after you close the page. You can work out private loan amounts or customer interest charges without that information going anywhere else. Close the tab and the figures are gone.
Does it work on a phone?
Yes. The fields open the number keypad, the time unit is a quick dropdown and the interest appears as soon as the numbers are in. It works in any mobile browser with nothing to install, so you can settle a figure while you're talking to the other person.
Is the simple interest calculator free?
Yes. It's free with no sign-up and no limit on use. It's one of the free finance calculators from Marketiism, along with the compound interest, FD and loan prepayment tools. Marketiism, our paid software, handles invoices and payment reminders for small businesses. Use it as often as you like, for study or business.
Want this inside your own account?
Marketiism keeps your invoice numbers, GST reports, UPI payment links and WhatsApp reminders in one place. Leave your details and we'll call to set it up for your business, or start the trial yourself.
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