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 already accrued. It is the easier calculation and the rarer product, which is why 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 is 60,000 a year, every year, unchanged. The result panel puts the compound figure alongside it — the same principal, rate and term compounding annually would generate 2,02,464 of interest instead. The 22,464 gap over three years is small; over fifteen it becomes the dominant term.
Interest = P × R × T ÷ 100The quoted rate and the rate you actually pay
Here is where simple interest costs borrowers real money. A consumer loan advertised at a "12% flat rate" over three years computes interest exactly as above: 1,80,000 on the full 5,00,000, divided into 36 instalments of 18,888.89.
But you are not holding 5,00,000 for three years. You are repaying it steadily, so your average outstanding balance is roughly half the original. Solve for the reducing-balance rate that produces the same 18,888.89 instalment and it comes out at about 21.2% a year — nearly double the advertised figure.
Flat and reducing rates are not comparable, and a lender quoting one against a competitor quoting the other is not offering you a like-for-like choice. If you are 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 rather than reinvesting it — non-cumulative products.
- Statutory interest on delayed payments, penalties and refunds in many jurisdictions, which is usually specified as simple by the rule that creates it.
- Car and consumer finance quoted on a flat basis, as above.
For anything longer than a year where interest is retained rather than paid out, compound is the realistic model.
What this calculation leaves out
The formula is deliberately spare, and so are its blind spots. It assumes the principal is constant for the entire term — no part-payments, no drawdowns, no early settlement. It ignores every fee, and on a small consumer loan an arrangement fee can exceed the interest. It applies no tax to interest received, which for a depositor is material.
It also assumes whole years. Day-count conventions differ (actual/365, actual/360, 30/360), and on a short-term instrument the choice between them 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.