Money and math guides
Plain-English explainers with real numbers worked out: what you can afford, how interest grows, what to tip, and more. Each guide links to the calculator that does the math for you.
Money & loans
6 guides
How Much House Can I Afford?
A common guideline keeps your full monthly housing payment near 28% of gross income, with all debts under 36%. Here is how that turns into a home price, using clearly stated example numbers.
5 min read15-Year vs 30-Year Mortgage: Which Costs Less?
A 15-year mortgage has a higher payment and much lower total interest than a 30-year loan. Here is how big the gap is on a $300,000 loan at example rates, plus a middle path.
4 min readHow to Pay Off a Car Loan Early
Extra principal payments shorten a car loan and cut the interest you pay. Here is how much $50, $100 or $200 a month saves on a $30,000 loan, and when the money may do more elsewhere.
5 min readAPR vs APY vs Interest Rate
Interest rate, APR and APY all describe a yearly percentage, but each counts different things. Here is what goes into each one and which to compare when you borrow or save.
5 min readThe Rule of 72: How Long Does It Take to Double Your Money?
Divide 72 by an annual interest rate and you get a fast estimate of how many years it takes to double your money. Here is how accurate that shortcut is and when to use something else.
5 min read$15 to $60 an Hour Is How Much a Year? Hourly to Salary Chart
A full-time hourly job at 40 hours a week works out to 2,080 paid hours a year. This chart turns common hourly wages into yearly, monthly, biweekly and weekly gross pay.
5 min readShopping & percentages
4 guides
How Much to Tip in the U.S.: Restaurants, Delivery, Hotels and More
Most U.S. tipping lands in a 15% to 20% range for personal service, with smaller or optional tips at the counter. Here are the typical ranges by service and a fast way to do the math.
5 min readSales Tax by State: 2026 State and Combined Rates
Your sales tax rate is the state rate plus any local taxes where you shop. Here are the state and average combined rates for every state and DC as of July 1, 2026.
5 min readHow to Tell If a Sale Is Really a Good Deal
A big red sign tells you how a store wants you to feel about a price. A few minutes of math tells you what you actually save.
5 min readHow to Calculate Percentages in Your Head
Most percentage problems you meet in daily life can be solved without a phone. A few building blocks and two shortcuts cover tips, discounts, raises and test scores.
4 min readDates, school & health
2 guides
How to Raise Your GPA
How much you can raise your GPA in one semester depends mostly on how many credits you already have. The earlier you start, the more each grade counts.
5 min readBMI vs Body Fat: What Your BMI Can and Cannot Tell You
BMI is a quick screening number based on your height and weight. It says nothing directly about how much of your weight is fat, which is why doctors often look at other measures too.
4 min read