Simple Interest Calculator
Simple interest grows at a flat rate on the original principal only. Enter the principal P, annual rate r and time t in years; the calculator returns the interest I = P·r·t and the final amount A = P + I.
Frequently asked questions
How is simple interest different from compound interest?
Simple interest applies the rate to the original principal only. Compound interest reapplies the rate to the accumulated balance, so interest itself earns interest.
Can the time be a fraction of a year?
Yes. Enter the time in years — six months is 0.5, ninety days is roughly 0.2466.
Should I enter the rate as a percent or a decimal?
Enter the rate as a percent (5 for 5%). The calculator converts it to a decimal before applying the formula.