Summary: a simple interest calculator that is free and private
A simple interest calculator works out the interest on a loan or deposit with SI = (P × R × T) / 100, where P is the principal, R is the annual rate in percent and T is the time in years. Enter the principal, the rate and a time in years, months or days, and this one shows the interest, the total amount (P + SI) and the working. It runs in your browser, nothing is uploaded (you can check the Network tab in DevTools), and there is no sign-up.
Simple interest is charged on the original principal only, so it adds the same amount every year. 100,000 at 8% for 2 years earns 16,000 and comes to 116,000. When interest is added to the balance and earns interest itself, use the compound interest calculator instead.
The formula and its three inputs
SI = (P × R × T) / 100
Total amount A = P + SI
The principal (P) is the amount lent, borrowed or deposited. The rate (R) is the annual percentage, so 1.5% a month is 18% a year. The time (T) must be in years: 6 months is 0.5, and 90 days is 0.2466 on a 365-day year.
Simple versus compound interest at 8% on 100,000
| After | Simple interest | Compounded yearly |
|---|---|---|
| 1 year | 8,000 | 8,000.00 |
| 2 years | 16,000 | 16,640.00 |
| 5 years | 40,000 | 46,932.81 |
| 10 years | 80,000 | 115,892.50 |
The gap widens every year, so long-term plans suit the savings calculator or the investment calculator.
Table of Use: simple interest examples
Each row uses the calculator’s formula; symbols are omitted except for yen.
| Situation | Inputs | Interest | Total amount |
|---|---|---|---|
| Fixed deposit | 100,000 at 8% for 2 years | 16,000 | 116,000 |
| Loan to a friend | 25,000 at 6% for 9 months (T = 0.75) | 1,125 | 26,125 |
| Short-term loan, 365-day year | 50,000 at 12% for 90 days | 1,479.45 | 51,479.45 |
| Same loan, 360-day year | 50,000 at 12% for 90 days | 1,500.00 | 51,500.00 |
| Monthly rate quoted | 20,000 at 1.5% a month (18% a year) for 10 months | 3,000 | 23,000 |
| Yen deposit | ¥2,000,000 at 1.5% for 3 years | ¥90,000 | ¥2,090,000 |
Rows three and four differ by 20.55 only because of the day count. If you only have two dates, the date difference calculator gives the days to type in.
Features
- Time in Years, Months or Days; for Days, a "Days in a year" menu switches between a 365-day and a 360-day year.
- Eight currencies (US dollar, euro, pound, rupee, Canadian, Australian and New Zealand dollar, yen), with the starting one guessed from your browser language.
- A Results panel with Simple Interest, Total Amount and Interest per year, plus an eight-line Step-by-step Calculation.
- Amounts typed as 1,500.75, 1.500,75, 1500,75 or 1,00,000 are all read correctly.
- Limits with plain messages: principal up to 1,000,000,000,000,000, rate up to 1,000%, time up to 1,000 years, 12,000 months or 365,000 days.
- Copy puts a text summary on the clipboard; Print opens a printable page in a new window, so allow pop-ups if your browser blocks it.
It solves one period at a time. For monthly instalments use the loan calculator, the EMI calculator or, for home loans, the mortgage calculator.
How to Use the Simple Interest Calculator
- Pick the Currency, or leave the one chosen for your browser language.
- Type the Principal Amount. A line under the box repeats it as formatted money, catching a misplaced separator.
- Type the Interest Rate (% per annum). Multiply a monthly rate by 12 first.
- Type the Time Period and press Years, Months or Days. For Days, choose 365 or 360 in Days in a year.
- Read Simple Interest and Total Amount in the Results panel, and open the working in Step-by-step Calculation.
- Press Copy for a text summary or Print for a printable page.
The general interest calculator switches between simple and compound interest and adds a year-by-year balance table, and the retirement calculator projects long horizons.
Who Can Use It
- Students checking homework, since the working reads like a textbook solution.
- Borrowers and lenders estimating the cost of a short loan or a loan between friends.
- Savers comparing a deposit that quotes simple interest with one that compounds.
- Small-business owners checking interest on an overdue invoice, where the contract sets the rate and day count.
FAQs
How do I calculate simple interest?
Multiply the principal by the annual rate in percent and by the time in years, then divide by 100: SI = (P × R × T) / 100. For 100,000 at 8% for 2 years, SI = (100,000 × 8 × 2) / 100 = 16,000, so the total amount is 116,000. The calculator shows each step.
How do I calculate simple interest for months or days?
Convert the period to years first. Divide months by 12, so 9 months is 0.75 years. Divide days by 365, or by 360 if your lender uses the actual/360 convention, so 90 days is 0.2466 years on a 365-day year and 0.25 on a 360-day year. Pick Months or Days and the calculator converts for you.
What is the difference between simple interest and compound interest?
Simple interest is charged on the original principal only, so it adds the same amount every year. Compound interest is charged on the principal plus the interest already added. At 8% on 100,000, simple interest is 16,000 after 2 years and 80,000 after 10, while annual compounding gives 16,640 and 115,892.50.
How do I find the rate, the time or the principal if I know the interest?
Rearrange the formula. Rate R = (SI × 100) / (P × T), time T = (SI × 100) / (P × R), and principal P = (SI × 100) / (R × T). This calculator only works forward, from principal, rate and time to interest and total, so use these formulas when one input is unknown.
Is a car loan or personal loan simple interest?
Many are described as simple interest loans, meaning interest accrues on the balance you still owe and never on unpaid interest. A repayment schedule with monthly instalments changes that balance every month, so this single-period formula does not give the instalment. For monthly payments use a loan or EMI calculator, and check your lender’s day-count method.
Is it safe to type real loan or deposit amounts into this calculator?
Yes. The calculation runs in your browser with JavaScript and nothing you type is uploaded, so the amounts stay on your device. You can confirm it in the Network tab of your browser’s DevTools: changing a value makes no request.