Guides

Plain-language explainers that go with the calculators on this site. Each guide walks through what the numbers actually mean, the formulas behind them, and the most common ways people get tripped up.

Money

Understanding Mortgages

How a mortgage payment is split between principal and interest, why early payments matter most, and what to actually compare when shopping a loan.

APR vs APY

Two rates that look identical but mean opposite things. The math that explains why a 24% credit card actually costs about 27% a year.

Roth vs Traditional

When pay-tax-now beats pay-tax-later, when it doesn't, and why most savers should hold some of both.

Debt Payoff Strategies

Avalanche vs snowball — when the math wins, and when human behavior wins.

Rent vs Buy

The 5% rule, price-to-rent ratios, and why your time horizon usually matters more than the interest rate.

Online Seller Fees

How marketplace and processor fees stack across eBay, Etsy, PayPal, Stripe, Square, and more — and the hidden line items that get missed.

Health and fitness

Calorie Needs and TDEE

BMR, activity multipliers, the Mifflin–St Jeor formula, and why most online calorie targets are off by a few hundred a day.

BMI Explained

Where BMI is useful, where it falls apart, and what to look at instead.

More guides go up as we find calculators worth a deeper explanation. Spot a topic you'd like covered? Drop a note via the Contact.