The steps
Step one: describe part, whole, or change in plain language. Step two: make sure months match months and years match years. Step three: calculate. What Is Interest? explains why skipping the first two steps costs people real money.
Catching mistakes
Round and estimate first if you can. If your rough answer and the exact answer are far apart, you likely switched which balance or time period you were using mid-way.
Mental shortcut
10% of 80 equals 20% of 40 (same math, different way of saying it). For a bill or a loan, use your real numbers in a calculator before you rely on mental math.
Two approaches
Fast: round first, then verify Slow: write it in words, then calculate
What moves the result
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.