Simple Interest Calculator
A simple interest calculator that takes a principal, an annual rate and a term in years and returns the interest earned plus the total amount, using the flat I = P × r × t ÷ 100 formula.
Flat interest maths.
This is a simplified estimate for general information only — not financial, tax, or legal advice. Real figures vary by lender, rate, jurisdiction and your own circumstances, so confirm with a qualified professional before acting.
How to use Simple Interest Calculator
- Enter your principal, annual interest rate, and time in years
- View the interest earned from principal times rate times time
- Read your total amount, formatted as clean currency
About Simple Interest Calculator
The Simple Interest Calculator works out interest on a fixed principal using the classic formula — interest equals principal times rate times time, divided by 100. Type in your principal, the annual interest rate, and the number of years, and it instantly shows the interest and the total amount, both formatted as clean currency. Change any input and the results recalculate in real time, so you can see how a higher rate or a longer term moves the numbers.
Because simple interest is charged on the original amount only, the same tool doubles as a simple-interest loan calculator: enter the loan amount as the principal to see the total interest and the payoff figure. That makes it a good fit for interest-only and simple-interest loans, where the balance stays flat instead of shrinking each month. It also gives you a fast way to sanity-check simple against compound growth before you commit — simple interest grows in a straight line, so it will always trail compound over a long enough term.
It is completely free, with no sign-up, no limits, and nothing to install. Every calculation runs entirely in your browser, so your figures are never uploaded, stored, or shared with any server.
Frequently asked questions
- What formula does it use?
- Interest = principal × rate × time ÷ 100, and the total is the principal plus that interest. The default figures show it: $10,000 at 5% for 3 years earns $1,500, for a total of $11,500.
- How do I figure out simple interest myself?
- Multiply the principal by the annual rate as a decimal, then by the number of years — $10,000 × 0.05 × 3 = $1,500. That is the same as the P × r × t ÷ 100 form the calculator uses with the rate typed as a whole number. Either way you get the same interest.
- Can I use this as a simple interest loan calculator?
- Yes. Enter the loan amount as the principal, the annual rate, and the term in years, and the result is the total interest you would pay plus the payoff total. It assumes the balance stays fixed for the whole term, so it suits simple-interest and interest-only loans rather than amortizing ones where the balance shrinks.
- How do I calculate interest on a simple interest-only loan?
- On an interest-only loan you pay just the interest each period and the principal is untouched until the end, so simple interest applies to the full balance the whole time. Enter the balance, rate and term to get the total interest, then divide by the number of payments for each period's charge. For a loan with declining principal, use the Loan / EMI calculator instead.
- How is this different from compound interest?
- Simple interest is charged on the original principal only, so each year earns the same amount and the balance grows in a straight line. Compound interest also earns on interest already added, so it pulls ahead over time — use the Compound Interest calculator on this site for that.
- How do I enter a period shorter than a year?
- The time field is in years and accepts decimals: 0.5 for six months, 0.25 for three months, 1.5 for eighteen months. For a number of days, divide by 365 — 90 days is about 0.2466.
- The rate is annual — what if mine is monthly?
- Multiply it by 12 before entering it, so 1% a month goes in as 12. The calculator never converts rates for you; it takes the rate exactly as typed and treats it as the yearly figure.
- Can it produce a loan repayment or EMI schedule?
- No. It returns a single interest figure and a total, with no month-by-month breakdown and no assumption about repayments. For monthly instalments, use the Loan / EMI calculator on this site.
- Does it work for currencies other than dollars?
- The results are formatted as US dollars, but the maths is currency-neutral — the numbers are correct whatever currency your principal is in. Only the symbol is fixed.
- Which inputs does it refuse?
- Any blank, non-numeric or negative principal, rate or time. When one of the three is invalid the results disappear and the prompt returns; zeros are perfectly valid and simply produce zero interest.
- How are the figures rounded?
- The interest, principal and total are displayed to two decimal places. The calculation itself is not rounded, so an awkward term such as 1.7 years is only rounded on screen, not in the maths.