Simple Interest Calculator: What Will You Actually Earn or Owe?

Simple interest is the most straightforward form of interest calculation — and understanding it is essential whether you're evaluating a short-term loan, a CD, a bond, or a savings account. Unlike compound interest, simple interest is calculated only on the original principal, not on accumulated interest. That distinction matters enormously over time. This calculator gives you instant clarity: enter your principal amount, annual interest rate, and time period, and see exactly how much interest you'll earn or pay. You'll also see a comparison to compound interest over the same period, so you understand the real cost difference between lending structures. Simple interest is commonly used for auto loans, short-term personal loans, and some bonds. If you're on the lending side — as with a CD or Treasury bill — the same formula applies. The math is transparent: Interest = Principal × Rate × Time. For short terms (under a year), simple and compound interest diverge only slightly. Over many years, the gap becomes dramatic. A $10,000 investment at 5% simple interest for 20 years returns $10,000 in interest. At 5% compounded annually, it returns $16,533. That's the power of compounding, and why this calculator shows both. Whether you're a student learning the basics, a borrower evaluating loan offers, or an investor comparing short-term instruments, knowing your simple interest calculation is foundational financial literacy.