Interest Calculator
Calculate simple and compound interest on any amount for any period.
Choose what to solve for
Interest Earned
$1,500.00
Principal
$10,000
The amount invested or borrowed
Total Interest
$1,500
5.00% for 3.00 years
Total Amount
$11,500
Principal + interest
The calculation
Interest = Principal × Rate × Time
$1,500 = $10,000 × 5.00% × 3.00 yr
| Year | Interest Earned | Balance |
|---|---|---|
| 1 | $500 | $10,500 |
| 2 | $1,000 | $11,000 |
| 3 | $1,500 | $11,500 |
Simple interest is calculated on the principal only — it does not compound. For interest that earns interest, use the Compound Interest Calculator.
How to use this calculator
- Choose what to solve for — Interest, Principal, Rate or Time. The variable you pick becomes the result; the other three become inputs.
- Enter the three known values — for example, to find interest, enter principal, rate and time.
- Read the result — the solved value appears as the headline figure, with the full picture (principal, interest, total amount) below it.
The year-by-year table shows how simple interest accumulates in a straight line.
How it works
Simple interest uses one formula:
Interest = Principal × Rate × Time
Interest is calculated only on the original principal. It is never added back to the balance to earn further interest — that is what separates simple interest from compound interest. As a result, the same amount of interest is earned every year, and the total grows linearly.
Because the formula has four variables, knowing any three gives the fourth:
- Interest = P · r · t
- Principal = I ÷ (r · t)
- Rate = I ÷ (P · t)
- Time = I ÷ (P · r)
If your money actually compounds — as in a savings account — simple interest will understate the result; the Compound Interest Calculator is the right tool there.
Frequently Asked Questions
What is simple interest? ▾
Simple interest is interest calculated only on the original principal — it does not earn interest on previously earned interest. The formula is I = P · r · t, where P is the principal, r is the annual rate, and t is the time in years. Because it never compounds, simple interest grows in a straight line.
How is simple interest different from compound interest? ▾
Simple interest is always charged on the starting principal alone. Compound interest is charged on the principal plus all interest accumulated so far, so it grows faster and faster. Over short periods the difference is small; over long ones it is large. For compounding, use the Compound Interest Calculator instead.
Can this calculator solve for the rate or time? ▾
Yes. The formula I = P · r · t rearranges, so you can solve for any of the four variables. Pick what you want to find — Interest, Principal, Rate or Time — enter the other three, and the calculator returns the unknown.
Where is simple interest actually used? ▾
Simple interest appears in some short-term personal loans, certain car loans, many bonds (coupon interest), and short promissory notes. Savings accounts and most long-term loans use compound interest, not simple interest.
What does 'total amount' mean? ▾
The total amount, sometimes called the maturity value, is the principal plus all the simple interest earned over the period — what you would have at the end, or owe at the end, depending on whether you are saving or borrowing.