Calculator formulas
A single reference page for the payment, payoff, fuel, percentage, tip, and conversion formulas used across the site.
Guides
Plain-English calculator guides.
Use these guides to understand the formulas, assumptions, examples, and limits behind the calculators on this site.
Start with a topic
Guides for everyday estimates
Mortgage affordability
Learn how down payment, rate, loan term, taxes, insurance, HOA, and cash cushion affect a house payment estimate.
Auto loan budgeting
Compare payment, total interest, fees, trade-in value, insurance, fuel, and maintenance before choosing a car loan term.
Debt payoff strategy
Use payoff estimates to compare extra payments, interest savings, avalanche payoff, and snowball payoff methods.
Conversion chart
Reference common metric and standard conversion factors for distance, length, weight, volume, area, and temperature.
Percentage examples
Walk through discounts, tax, raises, price changes, grade percentages, and percent-of calculations with simple examples.
Fuel cost planning
Plan a road trip fuel budget with miles, MPG, local prices, round trips, passenger splits, and real-world efficiency changes.
Mortgage payment basics
Understand principal, interest, taxes, insurance, HOA, and why a calculator estimate can differ from a lender quote.
Car payment basics
Learn how vehicle price, down payment, trade-in, taxes, fees, rate, and term combine into a monthly auto payment.
Why guides matter
Good calculators need explanation, not just outputs
A calculator can return a number in a second, but the number is only useful when the visitor understands what went into it. A mortgage result should not be mistaken for a lender disclosure. A car payment result should not hide taxes, dealer fees, or optional add-ons. A fuel estimate should remind people that real MPG changes with traffic, speed, weather, and cargo.
The guide pages are written to make those assumptions visible. They explain what each calculator includes, what it leaves out, and how to use the result as a planning number before checking final figures from a lender, dealer, tax office, or other official source.