Tools for the details that break
107 free online tools and 20 guides about the parts of the job that are easy to get wrong: character encodings, hash choices, token debugging, spreadsheet quirks.
Most tools run entirely in your browser — files are never uploaded. No account required.
Guides
All 20 guides →- Age Calculation: Completed Years, Leap Days and Legal AgeHow many years old someone is depends on which convention you use. Completed years versus the Korean age system, what happens to a 29 February birthday, and why two calculators disagree by one.
- Number Bases: Hex, Two's Complement, and Float ErrorsWhy hex always comes in pairs, how computers store negative numbers, why 0.1 + 0.2 is not 0.3, and where JavaScript silently loses integer precision.
- Base64 Is Not Encryption: When to Use ItBase64 is a transport encoding with zero secrecy. Here is what it actually guarantees, why it inflates data by 33%, where it is genuinely the right tool, and how it differs from hashing and encryption.
- Password Hashing: Why SHA-256 Is the Wrong ChoiceA modern GPU tries billions of SHA-256 hashes per second. Passwords need a deliberately slow hash, not a fast one. Here is how bcrypt, scrypt and Argon2 differ, what the cost factor controls, and why salts are not enough.
- BMI: When It Stops Being UsefulBMI is body mass divided by height squared, and it knows nothing about what that mass is. Why athletes read as obese, why the waist-to-height ratio predicts risk better, and the age groups where the thresholds do not apply.
- CSV Opens as Garbage: BOM, Delimiters, and EncodingWhy a CSV that looks fine in the terminal turns into mojibake in Excel, whether a UTF-8 BOM helps or hurts, how delimiters and quoting break naive parsers, and a five-step diagnostic order.
- Favicon Sizes: The Complete ChecklistA 16-pixel icon and a 512-pixel icon are not the same image scaled. The file set that still works in 2026, the maskable safe zone, why browsers ignore your new icon, and what breaks when you ship one big PNG.
- HTML Escaping and XSS: The Five CharactersEscaping angle brackets is not enough. The five characters that must be escaped, why the right rule depends on where the value lands, and why you escape on output rather than on input.
- Image Compression: How Small Before It Looks BadFile size is driven by pixel count, not by the quality slider. Resize first, then pick a format, then tune quality — with the numbers that actually hold up for web, print and screenshots.
- JSON to Table: Flattening Nested FieldsTurning nested JSON into a table is not a copy-paste problem. It is a decision about what one row means. Here is how to flatten objects, what to do with arrays of arrays, and which output format fits which audience.
- Equal Principal vs Equal Instalment: The Real DifferenceBoth repayments clear the same loan. One front-loads the pain and saves interest, the other keeps the payment flat. Where the difference comes from, how to compute it, and whether overpaying is worth it.
- Percentage Traps: Why a 50% Loss Needs a 100% GainPercentages are the easiest arithmetic and the easiest thing to misread. Percentage points versus percent, why a 20% rise then a 20% fall is not zero, and why you cannot average growth rates with an average.
- Regex Patterns Developers Actually UseEmail, URL, IP, date, phone numbers — the patterns you need weekly, why most of them are wrong, and the two features (lookbehind, lazy quantifiers) that trip people up when porting between languages.
- Symmetric vs Asymmetric Encryption: Which to UseAES and RSA are not old and new versions of the same thing. Symmetric encrypts the data, asymmetric hands over the key. Which AES mode to pick, why ECB must go, and what "hybrid encryption" actually means.
- Unix Timestamps: Seconds, Milliseconds, and Time ZonesWhy a 13-digit timestamp breaks your date conversion, why Unix time has no time zone but your display does, and the two places where 32-bit time still runs out.
- URL Encoding: %20, Plus Signs, and Double EncodingWhy a space is sometimes %20 and sometimes +, what encodeURI and encodeURIComponent actually differ on, why Chinese text becomes three percent signs, and why your signature never matches.
- UUID v4 vs v7: Choosing a Primary KeyBoth are 128 bits and both are unique. The difference is what the first bits contain, which decides whether your database inserts append or scatter across the index.
- WCAG Contrast: How to Calculate and Fix ItContrast is a ratio between two relative luminances, which is why "darken the text a bit" sometimes lowers it. The formulas, the AA and AAA thresholds, and the three fixes that work without wrecking the palette.
- What Is a JWT? Structure, Signing, and DebuggingA JWT is three Base64URL segments joined by dots. Here is what each part contains, why the payload is readable by anyone, what the signature actually proves, and the three mistakes that cause most auth bugs.
- YAML, JSON or TOML: Picking a Config FormatYAML is pleasant until indentation or the Norway problem bites. JSON has no comments. TOML is boring on purpose. What each format is actually good at, with the failure modes that decide the choice.