Guides
The ideas behind the calculators — written by the person who builds them, with every number verified the same way the tools are.
Debt Snowball vs. Avalanche: The Math and the Psychology
The avalanche method always wins on paper — so why do so many people pay off more debt with the snowball? A worked example with real numbers, and how to choose.
Companion tool: Debt Snowball Calculator
What Goes on an Invoice (and How to Follow Up When It Isn't Paid)
The elements every professional invoice needs, the payment terms that actually get you paid, and a calm escalation path for overdue invoices.
Companion tool: Invoice Generator
Password Entropy, Explained: Why Length Beats Complexity
What entropy bits actually measure, how long different passwords survive a modern cracking rig, and why four random words beat P@ssw0rd! every time.
Companion tool: Password Generator
How Compound Interest Actually Works
The formula behind compound growth, what compounding frequency really changes, the Rule of 72 — and a worked 10-year example you can verify yourself.
Companion tool: Compound Interest Calculator
What BMI Does — and Doesn't — Tell You
Where the BMI formula came from, what the standard categories mean, and the well-documented cases where the number misleads.
Companion tool: BMI Calculator