Guides
The tools do the arithmetic. These explain what the arithmetic means — why a loan’s early payments are mostly interest, why a scanned PDF is enormous, why length beats symbols in a password.
Calculators
How loan interest actually works
Why your early payments are nearly all interest, what APR includes that a rate does not, and what an extra $50 a month really buys you.
Reading health calculators honestly
BMI is weight over height squared and knows nothing else. Daily calorie figures are population equations. What both can and cannot tell you.
Text & Developer Tools
Choosing a strong password: length beats complexity
Why a long passphrase beats a short tangle of symbols, what password entropy measures, and the advice that has quietly been withdrawn.
Counting words and characters: why the numbers disagree
Graphemes, code units and bytes all claim to be characters. Which one your limit means, and why two tools report different totals.
Image Tools
JPG, PNG or WebP: choosing an image format
Which image format to use and why, what lossy compression actually discards, and the conversions that cost you quality for nothing.
PDF Tools
Why PDF files get so large, and what actually shrinks them
Scanned pages, embedded fonts and images saved at print resolution — where the megabytes really go, and which fixes are worth trying.
Business Tools
What an invoice needs to contain
The fields that make an invoice usable, why a unique number matters more than the layout, and where local tax rules take over.