Simple Interest Calculator

What Is a Simple Interest Calculator?

A simple interest calculator works out interest using just the starting amount, not any interest that's already piled up. That's the "simple" part; the rate applies to the same principal every single period, so the interest you pay (or earn) grows in a straight line instead of speeding up over time.

You'll run into simple interest more often than you'd expect: short-term personal loans, a lot of car dealership financing, certain fixed-income bonds, and some student loan terms all use it. Comparing that against a compound interest calculator for the same amount and term is the fastest way to see whether a lender's offer is actually the better deal or just marketed that way.

The Formula

I = P × R × T

• I — interest earned or owed
• P — principal, the amount you start with
• R — annual interest rate, written as a decimal (6% becomes 0.06)
• T — time, in years

That's the whole calculation. No exponents, no period-by-period recalculation multiply the three numbers, and you're done.

How to Use This Calculator

1. Enter the principal the amount you're borrowing or investing.
2. Enter the annual rate as a percentage.
3. Set the time period. You can enter years, months, or days, the calculator converts it for you, so you don't have to do the (months ÷ 12) math by hand.
4. Hit Calculate. You'll get the interest amount and the total payable or receivable.

If your loan or deposit term isn't a clean number of years, say, 18 months or 45 days, use the months or days input instead of converting it yourself first. Manual rounding is where most people's numbers drift off from what the lender quotes.

A Real Example

Lisa takes a short-term personal loan of $3,500 at 9% simple interest for 18 months to cover a move.

• Principal: $3,500
• Rate: 9% per year
• Time: 18 months (1.5 years)

Interest: $3,500 × 0.09 × 1.5 = $472.50 Total repayment: $3,972.50

Lisa knows the exact cost before she signs anything, no guessing what the balance will look like by the final payment.

Here's the same formula from the saver's side: if Lisa instead put $3,500 into a fixed deposit paying 6% simple interest for 2 years, she'd earn $3,500 × 0.06 × 2 = $420, for a maturity value of $3,920. Same formula, opposite direction, it's why this one calculator covers both loan costs and fixed deposit returns.

Simple Interest vs. Compound Interest

Simple interest charges (or pays) the rate on the original principal only, every period. Compound interest charges the rate on the principal plus whatever interest has already accrued, so the balance grows faster the longer it runs.

For short terms and small rates, the difference is minor. Stretch the term out, a 5-year car loan, a 20-year investment, and compounding pulls noticeably ahead or behind, depending on which side of the transaction you're on. If you're borrowing, simple interest is generally the cheaper structure over a long term. If you're saving or investing, compounding works in your favor. Run the same principal, rate, and term through our compound interest calculator to see the gap for your own numbers rather than guessing at it.

Comparing a loan or deposit? Try the compound interest calculator, loan calculator, or interest rate calculator to see how the same numbers play out under a different structure.

Frequently Asked Questions