Guides
Practical articles that explain the concepts behind the tools — with examples and step-by-step walkthroughs.
✅ Validators
How CPF and CNPJ Validation Works (the Check Digit)
The step-by-step check-digit algorithm for CPF and CNPJ (modulo 11), the new alphanumeric CNPJ, and why a valid format does not mean the number exists.
9 min read
How to Validate a Credit Card with the Luhn Algorithm
What the Luhn algorithm checks (and what it does not), the step-by-step calculation, identifying the card brand by BIN, and the ethics of test numbers.
7 min read
Pix Key: The Types, How to Validate It and the Pix QR Code
The five types of Pix key, how each is formatted, how to validate one before saving, and what the BR Code behind copy-and-paste Pix really is.
8 min read
🔄 Converters
Binary, Octal, Decimal and Hex: A Visual Guide
What a number base is, why computers speak binary, why programmers love hexadecimal, and how to convert by hand.
7 min read
Base64: What It Is, What It's For, and When NOT to Use It
The problem Base64 solves, how 3 bytes become 4 characters, legitimate uses like data URLs, and the myth that Base64 "encrypts".
6 min read
Unix Timestamp, ISO 8601 and Time Zones: Taming Dates in Code
What the Unix epoch is, the seconds-vs-milliseconds confusion, the unambiguous ISO 8601 format, and the daylight-saving-time nightmare.
7 min read
Web Colors: HEX, RGB, HSL and CMYK Explained
How screens form colors with additive RGB, HEX as hexadecimal RGB, the logic of HSL, CMYK for print, and contrast for accessibility.
7 min read
🔐 Security
How to Create a Strong Password (and Never Forget It)
What actually makes a password strong, the most common mistakes, and how to generate and remember secure passwords with passphrases and 2FA.
8 min read
MD5 vs SHA-256 vs bcrypt: Which Hash to Use and When
The difference between hashing and encryption, why MD5/SHA-1 were retired, and when to use SHA-256 for integrity or bcrypt for passwords.
8 min read
📈 SEO
Title and Meta Description: The On-Page SEO Guide That Converts
What Google shows in the SERP, the ideal title-tag length, whether the meta description still matters, and how Open Graph appears on social.
7 min read
Robots.txt, Sitemap and Canonical: The Indexing Trio
How Google discovers and indexes pages, what robots.txt does (and does not), the role of the XML sitemap, canonical against duplication, and hreflang.
8 min read
🧮 Calculators
Simple vs Compound Interest: The Difference That Changes Everything
The two formulas side by side, the snowball effect of compounding, the rule of 72, and where it hits your wallet on savings, loans and credit cards.
8 min read
Price Table vs SAC: Which to Choose for Your Loan
How an installment is built, the difference between fixed (Price) and decreasing (SAC) payments, the total cost of each system and the role of the APR.
8 min read
CDB, Savings or Treasury: Comparing the Real Return
The Selic and CDI benchmarks, the savings-account rule, how regressive income tax eats into returns, and what a "percentage of CDI" actually means.
9 min read
From Gross to Net: How INSS and Income Tax Cut Your Salary
The difference between gross and net, the progressive INSS table, how the income-tax base is built, and the other common payroll deductions.
8 min read
Vacation Pay and Termination: What Actually Goes Into the Math
How vacation pay is calculated with the constitutional third, selling vacation days, the types of termination, and what each one pays out.
8 min read
BMI: What the Number Really Means (and Where It Fails)
How BMI is calculated, the WHO ranges, why it misleads for athletes and older adults, and which indicators complement it better.
7 min read
Training Heart Rate: Find Your Zones
The maximum-heart-rate formulas, the five training zones, the Karvonen method, and how to apply it all to your real workouts.
6 min read
How Much Water to Drink per Day, Based on Your Weight
Where the 35 ml-per-kilo recommendation comes from, what raises your needs, common hydration myths, and the signs you are drinking too little.
5 min read
📅 Dates
How to Count Business Days Between Two Dates (Holidays Included)
The difference between calendar, business and banking days, how to exclude weekends, and the tricky moving holidays like Carnival and Easter.
7 min read
How to Calculate Exact Age and Time Lived
Why age is not just "this year minus birth year", how to count years, months and days correctly, and the February 29 edge case.
5 min read
🛠 Dev
JSON in Practice: Format, Validate and Convert Without the Headache
JSON syntax in five minutes, the most common errors, pretty-print vs minify, and how to convert to CSV, YAML and XML.
7 min read
Regular Expressions From Scratch: A Practical Beginner's Guide
What a regex is and where it lives, the essential metacharacters, groups and captures, ready-made patterns, and the mistakes that make regex slow.
9 min read
Inside a JWT: The Anatomy of a Token
The three parts of a JWT, what goes (and never should go) in the payload, how the signature protects it, and why decoding is not trusting.
7 min read