On the statement
What Is Interest? covers the concepts. On your own paper, pick one row: a subtotal, a box on a disclosure, a line on a pay stub. Ask which dollar amount that interest is calculated from on that row only.
Read the line
Tax, fees, and promos often reuse one % sign for different underlying amounts. This lesson and this one show two common layouts so you do not assume your card works like your mortgage form.
One real line
Pick a line on a bill that confused you. Write next to it: “percent of ___.” If the blank is empty, you and the issuer may be using different dollar amounts—not different arithmetic.
Check the math
Core lesson
Go deeper: What Is Interest?. Use the calculators below with your own loan or bill numbers, not only the examples on this page.
Use the calculator
FAQ
- Where is the main lesson?
What Is Interest? pulls the topic together in one place, with links to related lessons.
- Which calculator should I open first?
Use the first tool in the list for most questions. If you are reconciling payment rows on a schedule, pick amortization when it appears in the list.