Simple Interest Calculator
Calculate straightforward interest yields based strictly on principal values.
Simple Interest Calculator workspace
Parameters Configuration
The currency selector changes the symbol and number formatting only. Amounts you enter are never converted between currencies.
Ready for calculations
Configure parameters on the left and press Calculate.
Using Simple Interest Calculator
Enter Principal: Type the initial principal or loan amount.
Define Interest Rate: Enter the annual flat interest rate.
Set Duration: Choose the duration in days, months, or years.
Inspect Payout: Review the calculated interest earned and the final payout amount.
If you lend $5,000 to a business partner at a fixed 6% annual simple interest rate for 3 years, the interest accrued is calculated linearly as $5,000 × 0.06 × 3, which equals $900, resulting in a final repayment of $5,900. Unlike compound structures where interest is earned on previous interest, simple interest keeps the growth static relative to the principal, making it standard for short-term personal notes or invoice late fees.
Simple interest is charged on the original principal and never on interest that's already accrued. It's the easier calculation and the rarer product, so the most useful thing this page can do is show you what the same rate would cost if it compounded.
Flat interest, worked through
Borrow 5,00,000 for three years at 12% simple:
- Interest: 1,80,000
- Total repayable: 6,80,000
That's 60,000 a year, every year, unchanged. The result panel shows the compound figure next to it: the same principal, rate and term compounding annually would generate 2,02,464 of interest instead. A 22,464 gap over three years is small, but over fifteen years compounding takes over.
Interest = P × R × T ÷ 100The quoted rate and the rate you actually pay
This is where simple interest costs borrowers real money. A consumer loan advertised at a "12% flat rate" over three years works out interest exactly as above: 1,80,000 on the full 5,00,000, split into 36 instalments of 18,888.89.
But you aren't holding 5,00,000 for three years. You're paying it back steadily, so your average outstanding balance is roughly half the original. Solve for the reducing-balance rate that gives the same 18,888.89 instalment and it comes out at about 21.2% a year, nearly double the advertised figure.
You can't compare flat and reducing rates directly. If one lender quotes a flat rate and another quotes a reducing rate, you aren't looking at a like-for-like choice. When you're comparing loans, convert both to a reducing-balance basis first.
Where simple interest genuinely applies
- Short-term bridging and some inter-company loans, where the term is too short for compounding to matter.
- Certain fixed deposits and bonds that pay interest out instead of reinvesting it (non-cumulative products).
- Statutory interest on delayed payments, penalties and refunds in many jurisdictions, which the rule that creates it usually specifies as simple.
- Car and consumer finance quoted on a flat basis, as above.
For anything longer than a year where the interest stays in the account and isn't paid out, compound is the realistic model.
What this calculation leaves out
The formula is deliberately bare, and it has blind spots to match. It assumes the principal stays the same for the entire term, with no part-payments, drawdowns or early settlement. It ignores fees, and on a small consumer loan an arrangement fee can be bigger than the interest. It also doesn't apply any tax to interest received, which matters a lot if you're the depositor.
It assumes whole years too. Day-count conventions differ (actual/365, actual/360, 30/360), and on a short-term instrument the one you pick changes the interest by a noticeable amount. This page uses plain years.
Further reading on this site
To see the compounding version in full, with a choice of frequency, use the compound interest calculator. To work out an instalment on a reducing-balance basis, use the EMI calculator. For a deposit that compounds quarterly by convention, see the FD calculator.
Frequently asked
What is the difference between simple and compound interest?
Simple interest is calculated only on the initial principal. Compound interest is calculated on the principal plus any accumulated interest, which causes your balance to grow faster.
How do I convert days to years for simple interest?
To convert days to years, divide the number of days by 365 (standard year) or 360 (often used in commercial banking calculations).
When would a lender actually use simple interest?
Mostly on short-term instruments: some car loans, bridging finance, treasury bills, and the penalty interest clauses in commercial contracts. Anything running for years — mortgages, savings accounts, credit cards — compounds, and the gap widens every period.