詳細ガイド
各ツールの詳細ガイド:使い方、利用シーン、内部の仕組み。
Image Optimization for the Web: Formats, Compression, and Best Practices
Why images are the #1 cause of slow websites, which format to use (JPEG, PNG, WebP, AVIF), and how to compress without visible quality loss.
Markdown Cheatsheet: Every Syntax You Need to Know
A complete, practical reference for Markdown — headings, links, code blocks, tables, and the extensions that power GitHub, Notion, and Obsidian.
How Loan Amortization Works: A Complete Guide to Mortgage and Loan Math
Why most of your early mortgage payments go to interest, how to calculate your monthly payment, and what happens when you pay extra.
Percentage Math: Every Formula You Need in One Place
Percentage of a number, percentage change, reverse percentages, compound percentages — every formula explained with clear examples.
How Age Is Calculated: Why Birthdays Are More Complex Than You Think
Exact age calculation across leap years, time zones, and cultural differences — plus why your age changes at different times depending on where you live.
Working Across Time Zones: A Practical Guide to Global Scheduling
How time zones work, why UTC matters, the DST trap, and how to schedule meetings without accidentally waking someone up at 3 AM.
Word Count, Reading Time, and Readability: A Writer's Guide
How reading time is calculated, what Flesch-Kincaid scores mean, and how to use word count effectively when writing for the web.
URL Encoding Explained: Percent-Encoding and Why It Matters
Why URLs can't contain spaces or special characters, what percent-encoding does, and when you need to encode or decode a URL.
Color Spaces Explained: HEX, RGB, HSL, and When to Use Each
Why do designers use HSL while developers reach for HEX? A clear breakdown of every color format and how to convert between them.
Metric vs. Imperial: The Complete Unit Conversion Cheatsheet
Quick-reference formulas and context for every common unit conversion — length, weight, temperature, volume, and more.
How to Interpret Your BMI: A Complete, Balanced Guide
BMI is a useful screening tool, but it has real limitations. Learn what your number actually means, what it misses, and what health professionals look at instead.
Base64 Encoding Explained: What It Is, How It Works, and When to Use It
Base64 is everywhere in web development — in JWTs, data URLs, email attachments, and API keys. Here's how it actually works.
JSON Formatting and Validation: A Practical Guide for Developers
How to read, write, and debug JSON effectively — plus common mistakes, validation tips, and best practices for APIs.
How to Create Strong Passwords: A 2026 Security Guide
What makes a password uncrackable, how modern attacks work, and how to generate and manage strong passwords without going insane.
The Complete Guide to QR Codes: How They Work and How to Use Them
Everything you need to know about QR codes — how they encode data, what the different types mean, and how to generate one for free.