Simple Interest Calculator

How the Simple Interest Calculator Works

Our simple interest calculator computes the interest earned or paid on a principal amount over a given time period at a fixed annual rate. Unlike compound interest, simple interest is calculated only on the original principal — it does not earn interest on previously accrued interest. This makes it straightforward to compute and easy to verify by hand.

Simple Interest Formula

I = P × R × T    A = P + I

Where:

• I = Interest earned
• P = Principal (initial amount)
• R = Annual interest rate (as a decimal)
• T = Time in years
• A = Total amount (principal + interest)

For example, if you invest $10,000 at 5% simple interest for 3 years, the interest is $10,000 × 0.05 × 3 = $1,500, and the total amount after 3 years is $11,500. Each year earns a flat $500 in interest, which never changes because the principal stays the same.

Simple vs. Compound Interest

The key difference is whether interest earns interest. With simple interest, only the original principal generates interest each period, so the growth is linear. With compound interest, each period's interest is added to the principal, and the next period's interest is calculated on that larger balance — so growth accelerates over time. Over short periods the difference is small, but over decades it becomes enormous. A $10,000 investment at 5% for 30 years earns $15,000 in simple interest but about $33,200 under annual compounding.

Where Simple Interest Is Used

Simple interest is commonly applied to short-term loans, some auto loans, bonds that pay a fixed coupon, and certain savings products. Most mortgages, credit cards, and long-term investments use compound interest instead. When you take out a short-term personal loan, the lender may quote a simple interest rate — but always confirm whether the interest is precomputed (added to the balance upfront) or accrues on the declining balance, since that affects the total you pay.

Calculating Time in Partial Years

If your loan or investment spans a fraction of a year, you can enter the time as a decimal. For example, 18 months is 1.5 years, and 6 months is 0.5 years. The formula works the same way — a 6-month, $10,000 investment at 5% simple interest earns $10,000 × 0.05 × 0.5 = $250. For periods shorter than a year, you can also convert months to years by dividing by 12, or days by dividing by 365.

Tips for Using This Calculator

• Confirm whether your loan or investment uses simple or compound interest before relying on the result.
• For auto loans, check whether interest is precomputed or accrues on the declining balance.
• Use partial years (e.g., 1.5) for periods shorter than a full year.
• Compare the simple interest result with a compound interest calculator to see the long-term difference.
• Remember that simple interest always grows linearly — the yearly interest never changes.