Simple Interest Calculator

Calculate simple interest and the total amount from a principal, an annual rate and a time.

Result

Enter any two values and Numerila will work out the third.

Worked example

A principal of 1,000 at 3.3% a year for 3 years gives interest of 1,000 × 0.033 × 3 = 99, so the total amount is 1,099. Entering the 99 of interest instead of the time gives the 3 years.

How we calculated this

Simple interest is calculated on the original amount only: interest = principal × rate ÷ 100 × time in years, and the total is principal + interest. When you enter months, the time is months ÷ 12. Any three of the four boxes give the fourth. Interest is not added to the amount that later interest is worked out on, which is the difference from compound interest.

Finding a principal, rate or time needs the other two factors to be more than zero; when a zero leaves the answer open, the calculator says so. This is plain arithmetic on the numbers you enter. It does not describe any particular loan, account or product, and it is not advice.

A result written as a plain number is exact. A result starting with “≈” has been rounded to 10 significant figures, because its exact decimal never ends or is too long to read comfortably. When an exact value is long, its full digits are shown underneath. The calculation itself is never rounded: only the display is.

What this result means

The box marked Calculated holds whichever of the principal, the annual rate, the time or the interest the other three determine; the interest is earned on the original amount alone, and the result also shows the total of that amount plus the interest.

Common mistakes

Entering the rate as a decimal (0.05) instead of a percentage (5).
Forgetting that months are converted to years first: 6 months is half a year.

Assumptions and limitations

Interest is not added to the amount that later interest is calculated on. For that, use compound interest.
Time is entered in years or months only; there is no day-count option.
This is plain arithmetic on the numbers you enter. It does not describe any particular loan, account or product, and it is not advice.