Enter a principal, annual interest rate, and time period to calculate simple interest and the total amount owed or earned.
How to Use the Simple Interest Calculator
Principal, rate, and time — three fields, one formula, no compounding to configure.
No compounding, no reinvestment — simple interest accrues on the original principal only, at the same flat rate, for as long as the money's outstanding. It's the version taught first in every finance class because it's the easiest to verify by hand.
I = P × r × tP is principal, r the annual rate, t the time in years. Some short-term loans and bonds genuinely use simple interest, but plenty of everyday debt — credit cards especially — compounds instead, so it's worth checking which convention actually applies before assuming this is the right formula for a given account.