How to use it
- Choose what you want to find: interest, principal, rate or time.
- Enter the other values and pick the units for the rate and the time.
- Read the result, the total amount A = P + I and the worked formula.
Questions and answers
What is the simple interest formula?
I = P × r × t, where P is the principal, r the rate as a decimal (5% = 0.05) and t the time in the same unit as the rate. The total amount is A = P + I.
How do I use days with a yearly rate?
Divide the days by the days in a year: 90 days at 6% a year is 90/365 of a year (or 90/360 under the banker’s rule). Pick 365 or 360 in the calculator.
What is the difference between simple and compound interest?
Simple interest is always charged on the original principal; compound interest is charged on the principal plus the interest already earned. The calculator shows what compounding at the same rate would give.