Guides: Calculators Explained with Examples and Charts
Plain-English explanations of the maths behind our calculators, with worked examples and charts.
Mortgages and home buying
How a Mortgage Payment Is Calculated
The formula behind your monthly payment, where each dollar goes, and how the loan term and extra payments change the cost.
Mortgages and home buying
How Much House Can You Afford?
How lenders use the 28/36 rule to turn your income and debts into a monthly limit, and how rates and debts move the price.
Saving and investing
How Compound Interest Works
Simple against compound interest, the formula, the effect of time, deposits and returns, and the rule of 72.
Health and fitness
BMI vs Body Fat Percentage
What each number measures, how to work them out, where BMI falls short and how to use both.
Health and fitness
How Many Calories Do You Need?
Estimate BMR and daily calories step by step, and see how deficits really play out over time.
Retirement
Roth IRA vs Traditional IRA vs 401(k)
2026 limits, income phase-outs and a worked example of the tax trade-off, plus why the employer match comes first.
Debt and credit
How Credit Card Interest Works
How APR becomes a daily charge, why minimum payments keep you in debt, and the fastest ways to pay a balance off.
Taxes
Marginal vs Effective Tax Rate
How 2026 tax brackets really work, and why a raise never lowers your take-home pay.
Retirement
How Much Do You Need to Retire?
The 4% rule and the 25 times shortcut, what they assume, and how Social Security and returns change your target.
Mortgages and home buying
Rent vs Buy: How to Compare the True Costs
The full cost of owning, a simple model, and the break-even year that decides which is cheaper.
Saving and investing
How Inflation Erodes Your Money
How the CPI works, what a dollar lost since 1996, and how to judge a return after inflation.
Health and fitness
Macros Explained: Protein, Carbs and Fat
Turn calories into grams, compare common splits and see the official ranges and protein targets.
Guides are for general education and are not financial, medical or legal advice.