Guides
A calculator gives you a number. These guides explain what the number means, where the formula comes from, and how to use the result in real life. Every guide is written and fact-checked by the same people who build the tools.
How Your Pregnancy Due Date Is Calculated (and Why It's an Estimate)
Your due date is a single day, but only about 1 in 20 babies arrive on it. Here's how the 40-week estimate is built and what the milestones mean.
Read guideTipping in the United States: A Practical Guide for Every Situation
Restaurants, bars, delivery, rideshare, salons, hotels — a situation-by-situation guide to what to tip in the US, and how to handle automatic service charges.
Read guideWhat's a Good Running Pace? A Realistic Guide by Age and Goal
A 'good' pace depends on your age, experience, and what you're training for. Here's how to set a realistic target and predict your race times.
Read guideHow Loan Amortization Works: Where Your Monthly Payment Really Goes
Your payment is fixed, but the split between interest and principal shifts every month. Understand amortization and you'll see exactly how extra payments save you years.
Read guideUnderstanding BMI: What the Number Tells You (and What It Doesn't)
BMI is a fast screening number, not a diagnosis. Here's how it's calculated, why the categories exist, and the cases where the number misleads.
Read guideHow to Read Your Paycheck Stub: Every Line Explained
Gross pay, FICA, federal and state withholding, pre-tax deductions — here's what every line on your pay stub actually means and why your take-home is lower than your salary.
Read guide