Simple Interest Calculator (I = P·r·t)

How Simple Interest Works

Simple interest is the straightforward way to calculate interest on a loan or investment. It adds a fixed amount each period based only on the original principal, so the growth is linear rather than compounding. This calculator helps you estimate total interest and final amount quickly, which is useful for short-term loans, personal notes, or basic savings plans that use simple interest.

The idea is easy: the lender or borrower agrees on a principal amount, an annual interest rate, and a time period. The interest is calculated on the original principal only, so it does not “snowball” over time. That makes simple interest predictable and easy to compare across offers.

How to Use the Calculator

1. Enter the principal amount (the starting balance).
2. Type the interest rate as a percentage, such as 5 for 5%.
3. Choose the time period and enter years, months, or days.
4. Click calculate to see total interest and the final amount.
5. Use “Copy results” if you want to save the output.

Example: If you borrow $1,000 at 6% simple interest for 2 years, the interest is $120 and the total repayment is $1,120. If you extend the same loan to 3 years, the interest becomes $180 because the interest increases in a straight line with time.

Simple Interest Formula

• Interest: \(I = P\cdot r \cdot t\)
• Total amount: \(A = P(1+rt)\)
• \(P\) = principal, \(r\) = annual rate as a decimal (5% → 0.05), \(t\) = time in years

Tip: 18 months = 1.5 years; 90 days ≈ 90/365 years (actual day count can vary by contract).

Simple interest shows up in places like auto loans, short-term business financing, promissory notes, and some government or corporate bonds. It is also common in basic classroom problems where the goal is to practice percent and time calculations. If your loan or investment compounds interest, this calculator will understate the total, so use a compound interest calculator in that case.

Simple Interest: FAQ

What is the simple interest formula?

Simple interest uses \(I = P\cdot r \cdot t\) where \(P\) = principal, \(r\) = annual rate (decimal), \(t\) = years. The total is \(A = P(1+rt)\).

Does this calculator compound interest?

No—this tool uses simple interest only. For compounding, try our Compound Interest Calculator.

Are my inputs private?

Yes. Everything runs in your browser—no uploads, no accounts.

What units should I use for time?

Enter years directly, or choose months/days and we’ll convert to years automatically (months ÷ 12; days ÷ 365).