Free Online Developer Tools

The small jobs that interrupt real work: checking a regex, working out what a cron line actually does, formatting a minified file, decoding a token, or generating the structured data a page needs before it ships. Each one is a single page here, grouped by the kind of job, and none of them sends your data anywhere.

Dev Utilities 23

Regex, cron, chmod, git, semver, epoch, subnets, DNS and other daily helpers.

Base Converter

Convert numbers between binary, octal, decimal and hex.

CHMOD Calculator

Convert Unix permissions between rwx, octal and symbolic.

CSS Selector Tester

Test a CSS selector against pasted HTML and list every matching element.

Cookie Parser

Break Set-Cookie and Cookie headers into readable attributes with warnings.

Cron Expression Builder

Build a cron schedule and see when it actually runs.

DNS Lookup

Query any DNS record for a domain with TTLs and explanations.

Email DNS Record Checker

Check an SPF, DKIM or DMARC record before you publish it.

Epoch Converter

Convert Unix timestamps to dates and back again.

Git Cheat Sheet

Find the right git command and whether it is safe to run.

HTTP Status Codes

Look up what any HTTP status code actually means.

IP Address Info

Classify any IPv4 or IPv6 address and see its every form.

IP Subnet Calculator

Network, broadcast, host range and counts for any subnet.

JSONPath Tester

Run JSONPath queries against pasted JSON and see every match.

Keycode Tester

Press a key and see exactly what your browser reports.

MIME Type Lookup

Look up the MIME type for an extension, or the reverse.

Regex Generator

Build a regex from a preset or from sample strings, ready to paste.

Regex Tester

Test a regex and see every match and capture group.

SemVer Tool

Sort, compare and bump semantic version numbers.

URL Query Parser

Split a URL into a readable table of parameters.

User Agent Parser

See what browser and OS a User-Agent string claims to be.

Webhook Payload Viewer

Flatten any JSON webhook payload into searchable paths and values.

XPath Tester

Test XPath expressions against pasted XML or HTML instantly.

cURL Converter

Turn a curl command into code in 24 languages.

Formatters & Validators 10

Beautify and minify JS, CSS and HTML; validate markup, SVG, URLs and emails.

CSS Formatter

Expand minified CSS to one declaration per line, or minify it back.

CSS Validator

Find typos, missing semicolons and unknown properties in pasted CSS.

Email Validator

Check a list of email addresses for syntax errors and likely typos.

HTML Formatter

Indent messy HTML or collapse it safely, keeping pre and script intact.

HTML Validator

Find unclosed tags, duplicate ids and bad nesting in pasted HTML.

Indent Code

Shift every line right by a set number of spaces or tabs.

JavaScript Formatter

Expand minified JavaScript into indented code, or squash it back.

SVG Validator

Find unclosed tags, bad path data and broken references in SVG.

URL Validator

Check a list of URLs for syntax errors, one per line.

Unindent Code

Remove common or fixed indentation from a block of code.

Data & Encoding 17

CSV, JSON, YAML, XML and SQL formatters; Base64, URL, hex and string escaping.

Base64 Encoder / Decoder

Encode and decode Base64, standard or URL-safe.

CSV / JSON Converter

Convert CSV to JSON and back, quoting handled properly.

CSV Column Extractor

Keep, drop and reorder columns in CSV data.

CSV Formatter

Align CSV into readable columns or swap delimiters and fix quoting.

HTML Entity Encoder

Escape and unescape HTML entities in any text.

JSON Formatter

Format, minify and validate JSON, with located errors.

JSON to YAML Converter

Convert JSON to YAML and YAML back to JSON.

Line Ending Converter

Convert text between LF, CRLF and CR line endings instantly.

SQL Formatter

Turn a one-line SQL query into readable indented SQL.

SRT / VTT Converter

Convert SRT subtitles to WebVTT and back, with an optional time shift.

String Escaper

Turn raw text into a safe string literal in any language.

String Unescaper

Decode backslash escapes and quoted literals back into plain text.

Text to Hex Converter

Convert text to hexadecimal bytes and back again.

URL Encoder / Decoder

Percent-encode and decode URLs, values and form data.

XML Formatter

Indent messy XML or squash it to one line, with tag checking.

XML to JSON Converter

Convert XML to JSON and JSON back to XML.

YAML Formatter

Tidy inconsistent YAML and pinpoint syntax errors by line.

Security & Identifiers 17

Hashes, HMAC, bcrypt, JWT decoding, password and UUID generation and validation.

Bcrypt Generator

Generate and verify bcrypt password hashes.

CORS Tester

Test a URL for CORS headers or analyse pasted ones.

CSP Builder

Build a Content-Security-Policy header from per-directive source lists.

CSP Tester

Explain a CSP, flag weaknesses, and test whether a URL would load.

HMAC-SHA256 Generator

Sign a message with HMAC-SHA256 and verify signatures.

HTTP Header Inspector

Parse pasted HTTP headers, explain each one and audit security.

Hash Generator

Generate MD5, SHA-1, SHA-256 and SHA-512 digests from text.

JWT Decoder

Decode a JWT and read its claims and expiry.

Memorable Password Generator

Random words, a symbol and digits — strong, and you can remember it.

Passphrase Generator

Diceware passphrases from the EFF wordlist, generated locally.

Password Generator

Strong random passwords generated in your own browser.

Random PIN Generator

Random 4, 6 or 8 digit PINs, avoiding 1234 and 0000.

Random String Generator

Cryptographically random strings in any length and character set.

SSL Certificate Inspector

Decode a PEM certificate and check its dates, names and fingerprints.

UUID Generator

Random v4 or time-ordered v7 UUIDs, generated in your browser.

UUID Validator

Check a list of UUIDs and decode their version, variant and timestamp.

WiFi Password Generator

WPA2/WPA3 keys with no look-alike characters, easy to read aloud.

SEO 21

Structured data, meta tags, previews, robots, sitemaps, redirects and page audits.

Article Schema Generator

Build Article, NewsArticle or BlogPosting JSON-LD with the properties Google requires.

Breadcrumb Schema Generator

Build BreadcrumbList JSON-LD and an accessible nav from a Name | URL trail.

Canonical URL Generator

Normalise URLs to their canonical form and generate rel=canonical tags.

FAQ Schema Generator

Turn a list of questions and answers into FAQPage JSON-LD structured data.

Heading Structure Checker

List a page's h1–h6 in order and flag skipped levels, missing h1, empties.

Internal Link Analyzer

List a page's links with anchor text, internal/external split, nofollow and repeats.

Local Business Schema Generator

Build LocalBusiness JSON-LD with address, geo, opening hours and social profiles.

Meta Description Generator

Draft meta descriptions from your own page text, trimmed to length.

Meta Tag & Open Graph Checker

Check title, description, canonical, Open Graph and Twitter card tags in pasted HTML.

Product Schema Generator

Build Product and Offer JSON-LD with price, availability, identifiers and rating.

Redirect Generator

Turn an old-to-new URL list into 301 rules for Apache, Nginx, Netlify and Vercel.

Robots.txt Checker

Check a robots.txt file line by line before you deploy it.

SERP Preview

Preview your page title and description as a Google search result.

Schema Markup Validator

Validate JSON-LD schema.org markup for required properties and format errors.

Sitemap Validator

Validate sitemap.xml and sitemap index files against the sitemaps.org protocol.

Title Length Checker

Check page titles and meta descriptions by pixel width, not just characters.

Twitter Card Preview

Preview your link as an X (Twitter) card and copy the meta tags.

UTM Campaign URL Builder

Build correctly encoded UTM campaign links with consistent naming.

WCAG Checker

Find WCAG accessibility failures in pasted HTML, with the criterion each breaks.

XML Sitemap Generator

Turn a list of URLs into a valid sitemap.xml in seconds.

hreflang Generator

Generate hreflang link tags, headers or sitemap XML for multilingual pages.

Accessibility 9

Simulate, generate and check the things that make a page usable by everyone.

ARIA Attribute Builder

Generate the right role and aria attributes for any common widget.

ARIA Label Generator

Write clear aria-label text for icon buttons, links and unlabelled inputs.

Alt Text Helper

Compose the right alt attribute for decorative, informative and functional images.

Colour Blindness Simulator

View any image through protanopia, deuteranopia, tritanopia or achromatopsia.

Focus Indicator Generator

Build a visible, WCAG-friendly focus ring and copy the :focus-visible CSS.

Keyboard Navigation Tester

Audit HTML for controls that keyboard users cannot reach, operate or see.

Skip Link Generator

Generate a skip-to-content link that appears on focus, with HTML and CSS.

Tab Order Tester

List every Tab stop in your HTML in order and flag focus order problems.

Touch Target Checker

Check a control's size and spacing against WCAG, Apple and Android minimums.

How the developer tools are organised

Dev Utilities is the everyday shelf. Regex testing and generation, cron and chmod, common git commands, semantic version comparison, epoch and number-base conversion, HTTP status codes, keyboard key codes, user-agent parsing, cURL to code, URL query parsing, IP subnetting and DNS record lookup.

Formatters & Validators takes a file and either tidies it or tells you what is wrong with it: JavaScript, CSS and HTML formatting; CSS, HTML and SVG validation; URL and email address validation against the actual specifications rather than a rough pattern.

Data & Encoding is for structured text and the representations it passes through: CSV to JSON and back, CSV column selection, JSON, YAML, XML and SQL formatting, JSON to YAML and XML to JSON conversion, Base64, URL encoding, hex, string escaping for source code and HTML entities.

Security & Identifiers covers hashing (MD5 through SHA-512), HMAC signing, bcrypt hashing and verification, JWT decoding, strong password generation, and UUID generation and validation.

SEO is the largest group: schema generators for articles, breadcrumbs, FAQs, local businesses and products; a schema validator; canonical and hreflang generators; meta description and title length tools; a SERP preview and a Twitter card preview; robots.txt and sitemap checking; redirect rules; heading structure, internal link and WCAG checks.

Why client-side matters here more than anywhere

Developer tools are where the most sensitive strings end up being pasted: bearer tokens, connection strings, a config file with a secret in it, a production log line. A hosted tool that sends that to its own server has just leaked it, whatever its privacy policy says. Every page in this section runs in your browser and makes no network request with your input. Hashing uses the Web Crypto API, formatting and parsing are pure JavaScript, and the SEO tools generate their output locally — the only thing that leaves your machine is what you choose to copy.

Accuracy over convenience

Where a tool implements a specification — cron syntax, semver precedence, RFC 5322 addresses, JSON-LD vocabulary — it follows the specification rather than a simplified approximation, and says so on the page. Where a convention varies between systems (which fields a UUID version carries, how a shell escapes a string), the tool states the convention it is using so the output can be trusted in the place you are about to paste it.

FAQ

Is my data sent to a server by any of these tools?

No. Encoding, formatting, hashing and subnet maths all run in JavaScript on your device. Nothing you paste leaves your machine, so decoding a real token or formatting a real config file is safe.

Can I decode a hash back to the original value?

No. Hashing is a one-way function — the input cannot be recovered from the digest. Services that appear to reverse a hash are looking the value up in a precomputed table of common inputs, which is exactly why passwords should be salted.

Is Base64 a form of encryption?

No. Base64 is a reversible encoding designed to move binary data through text channels. Anyone can decode it in seconds, so it provides no confidentiality at all.

Do these tools work offline?

Once a page has loaded, yes. The logic ships with the page, so the tool keeps working if your connection drops. Reloading the page requires a connection unless your browser has cached it.