Simple interest is the most transparent way to price a loan or a deposit. Unlike compound interest, it is charged only on the original amount you borrow or save, so the cost never snowballs. That makes it common for short-term personal loans, car financing, and many fixed-term savings products. This guide explains the formula, walks through a real example, and shows how the Simple Interest Calculator gives you the answer in seconds.
How simple interest works
Three inputs drive every simple-interest figure: the principal (P), the annual interest rate (r), and the time in years (t). Interest is the principal multiplied by the rate, multiplied by the time. Because rates are usually quoted as percentages, the formula divides by 100 so a rate of 5 means 5 percent, not 500 percent.
I = P * r * t / 100 Total = P + I
Two details matter. First, the rate and the time must use the same period. The formula above assumes an annual rate and time in years, so six months is 0.5, not 6. Second, simple interest is linear: doubling the term doubles the interest, which is exactly why it is cheaper than compounding over long periods.
Calculating simple interest step by step
- Enter the principal, the amount you are lending, borrowing, or depositing.
- Enter the annual interest rate as a percentage, for example 6 for 6 percent.
- Enter the time in years; use decimals for partial years such as 0.5 for six months.
- Read the interest earned or owed and the total amount alongside it.
A worked example
Suppose you deposit a principal of 2,000 at an annual rate of 5 percent for 3 years. Plugging the numbers in gives I = 2000 * 5 * 3 / 100 = 300. So you earn 300 in interest, and the total grows to 2,300. If the term were only 18 months, t would be 1.5, the interest would be 150, and the total would be 2,150. The relationship stays perfectly proportional to time.
When to choose simple interest
Simple interest favors the borrower on longer terms and is easy to verify by hand, which is why it appears in transparent lending and in classroom math. For savers, it is usually the floor: any compounding account will pay more. Use this calculator to sanity-check loan quotes, compare short-term offers, or estimate the cost of financing before you commit.
Every calculation runs privately in your browser. Nothing you type is uploaded or stored, so you can model sensitive figures with confidence and refine the inputs as often as you like.
