Guides
Original articles on the decisions behind the calculators — not just the formulas.
Every tool on this site explains the formula it uses. These guides go a step further: they walk through a real decision — how much house to buy, whether a BMI number means what you think it means, whether a longer car loan term is actually a good idea — in more depth than a single calculator page can.
How Much House Can You Actually Afford?
A lender will approve you for more than you should spend. Here is how to find your own number.
Why Your BMI Number Might Be Misleading You
BMI measures a ratio, not your health. The difference matters more than most BMI calculators let on.
The Real Cost of a 72-Month Car Loan
The lower payment is real. So is the extra interest and the longer stretch of owing more than the car is worth.
Why Does the US Still Use Fahrenheit and Imperial Units?
It is not that the US never tried to switch. It tried once, made it voluntary, and the momentum ran out.
How Much Should You Actually Tip in 2026?
The suggested percentages on a payment screen and what is actually customary are not always the same number.
How Many Working Days Are Really in a Year?
Most PTO and project-timeline math starts from the wrong base number. Here is the one that is actually correct.
Why a Regex That Works in Testing Can Fail (or Hang) in Production
Passing every example you tried is not the same as being correct. Here is the gap between the two.
How Much Emergency Fund Do You Actually Need?
Three to six months of what, exactly? The answer changes the target by tens of thousands of dollars.
APR vs APY vs Interest Rate
A 22.99% card APR costs you 25.84% a year. A mortgage APR understates the cost of a loan you sell in five years. Neither is a mistake in the disclosure.
The Percentage Mistakes That Cost Real Money
These are not random errors that cancel out. Every one of them is biased against the person doing the arithmetic.
Time vs Rate: What Actually Drives Compound Growth
A worse investor with a longer runway beats a better investor who started late. Here is by how much.
Why Two Calorie Calculators Give You Different Numbers
The equations barely disagree. The dropdown you fill in yourself is where the error lives.
Two Expensive Unit-Conversion Failures, and What They Actually Teach
Nobody in either incident thought pounds were kilograms. That is what makes them worth studying.
ISO Week Numbers, and Why Your Spreadsheet Disagrees With Your Colleague
A week number is not a number. It is half of a pair, and the missing half is where the bugs live.
The Invisible Characters Breaking Your Data
Two strings that look identical, compare unequal, and diff clean. Here is where they differ and how to see it.