Free Developer Tools — Browser-Only
- Your data never leaves your tab — no servers, no upload
- No sign-up. No accounts. Your data stays in your browser.
- 15 languages, installable as an offline app
For Schools & Teachers
Free, ad-free, privacy-first. Self-hostable. 15 languages.
All Tools
ASCII Table
Full ASCII reference 0–127 with decimal, hex, binary, character, and HTML entity. Filterable.
Base64 Encoder / Decoder
Encode text to Base64 or decode Base64 back to text. UTF-8 safe and base64url variant supported.
Base64 to Image
Decode a Base64 image or data URI back into a file. Paste the string, preview, download. The inverse of our image-to-Base64 tool.
BMI Calculator
Body Mass Index from height and weight. Metric or imperial. Shows the WHO category — not medical advice.
Cap Rate Calculator
Calculate capitalization rate for a rental property. Enter NOI and property price — get cap rate %. Reverse-solve: enter NOI + target cap rate to derive the offer price. Browser-only.
Case Converter
Convert text between UPPER, lower, Title, Sentence, camelCase, PascalCase, snake_case, kebab-case, CONSTANT_CASE, and dot.case.
Chat Thread Viewer
Paste an OpenAI or Anthropic messages array (JSON) and see it rendered as a readable chat thread. Color-coded by role. Tool-call params expanded. Pure browser, no upload.
CIDR Subnet Calculator
Compute IPv4 subnets from CIDR notation. Network, broadcast, mask, host range, and binary view.
Citation Formatter
Format citations in APA 7, MLA 9, Chicago, and Harvard styles. Enter source fields manually — no DOI lookups, no tracking. Browser-only.
Color-Blind Simulator
Simulate how your game's screenshots / UI / palette appear to color-blind players. Protanopia, deuteranopia, tritanopia, achromatopsia. Upload image, see all four variants side-by-side. Files never leave your browser.
Color Converter
Convert any color between hex, RGB, HSL, and OKLCH. Live preview, copy in one click.
Color Palette Extractor
Extract the dominant colors from any image in your browser. 5–8 swatches, hex codes, copy-paste CSS variables. Files never leave your device.
Color Picker
Pick a color and instantly see it in HEX, RGB, RGBA, HSL, HSLA, HSV, and CMYK. Adjust opacity and copy any value.
Compound Interest Calculator
Calculate compound interest growth for savings or investments. Set principal, rate, period, contribution frequency. See final balance, total interest, year-by-year breakdown, and growth chart.
Credit Card Validator
Validate a card number with the Luhn check and detect the card brand. Runs locally — your number is never transmitted.
Cron Expression Builder
Visually build a cron expression — minute, hour, day, month, weekday — and preview the next 5 fire times.
Cron Expression Parser
Parse cron expressions and see the next 10 fire times. Standard 5-field crontab.
CSS Box Shadow Generator
Build single or multi-layer CSS box shadows visually. Adjust offset, blur, spread, color, and copy CSS.
CSS Gradient Generator
Build linear and radial CSS gradients visually. Edit color stops, copy ready-to-paste CSS.
CSS Minifier
Strip comments, whitespace, and redundancy from CSS. See size before/after and the saving percentage.
CSV to JSON
Convert CSV data to JSON arrays. Header detection, custom delimiters, quoted fields, and embedded newlines handled.
Currency Converter
Convert between 50+ currencies with reference rates as of a known date. No live API, no tracking — useful for quick freelance estimates and quotes. Always verify before invoicing.
Damage Calculator
Combat damage formula tester for RPGs. Attack vs defense, crit chance, resistances, multipliers, armor pen. See expected damage range + crit-adjusted average + damage-per-second. For game balance tuning.
Date Calculator
Days between two dates · add or subtract days/weeks/months/years from a date · age in years, months and days.
Debt Payoff Planner
Plan how to pay off multiple debts. Pick snowball (smallest balance first) or avalanche (highest interest first). See timeline, total interest, monthly schedule.
Dice Roller
Roll dice with standard D&D notation — 2d6+3, 1d20, 4d6 keep highest 3. Crypto-secure RNG.
Email Validator
Check whether an email address is syntactically valid (RFC 5322 friendly), with breakdown of local part, domain, and common pitfalls.
Emoji Picker
Searchable emoji palette by name and keyword. Click any emoji to copy. Categories: smileys, animals, food, travel, objects, symbols, flags.
EXIF Stripper
See what your photos reveal about you — GPS coordinates, camera model, exact timestamp — then strip it all. 100% in-browser. The file never leaves your device.
Expense Splitter
Split a group expense evenly or by custom shares (percentages or fixed amounts). Output per-person totals + transfer instructions ('A owes B €X').
File Size Converter
Convert between bytes, KB, MB, GB, TB and the binary KiB, MiB, GiB, TiB. Decimal vs binary clearly distinguished.
Grade Calculator
Calculate weighted grades for a class. Set categories (assignments / tests / projects / final) with weights, enter your scores, see your running grade. What-if mode: 'What do I need on the final to get an A?'
Gross Rent Multiplier (GRM)
Quick property screening with GRM = price ÷ annual gross rent. Lower GRM = better cash yield. Compare against market range for your city. Browser-only.
Hash Generator
Hash text with SHA-1, SHA-256, SHA-384 or SHA-512 using your browser's WebCrypto. Computed locally — input never leaves the page.
HTML Encoder / Decoder
Escape HTML special characters or decode entities back. Useful for safely embedding user input or debugging encoded markup.
HTML Formatter
Format and beautify HTML or minify it. Indent size, comment stripping, and self-closing tag awareness.
HTTP Status Codes
Look up any HTTP status code (1xx–5xx). Meaning, common causes, and the RFC reference.
Image Color Picker
Upload an image and click anywhere to get the hex, RGB, and HSL of that pixel. Build a custom palette by clicking multiple points. Pure browser, no upload.
Image Compressor
Compress PNG, JPG, or WebP images right in your browser. Adjust quality, see before/after sizes, download. Files never leave your device.
Image Contrast Checker
Click any two points in an image to check WCAG contrast — AA, AAA, large-text thresholds. Built for designers verifying real screenshots, not just hex codes.
Image Format Converter
Convert images between PNG, JPG, and WebP in your browser. Drop, pick a target format, download. No upload, no server.
Image Resizer
Resize images to any width or height in your browser. Lock aspect ratio, scale down social-media-friendly sizes, download instantly. No upload.
Image to Base64
Convert any image to a Base64 data URI ready for inline use in HTML, CSS, or Markdown. Files stay in your browser.
Inflation Calculator
See how inflation erodes purchasing power. Enter amount + years + inflation rate → real value today (or future). Useful for raises, retirement targets, savings goals.
Invoice Generator
Generate a clean invoice in markdown + print-ready HTML. Fill form, see live preview, save as PDF via browser print. No upload, no signup.
JavaScript Minifier
Fast structural JavaScript minify — strip comments, collapse whitespace, drop blank lines. See size before/after and the saving percentage.
JSON Diff
Structural diff for two JSON documents — keys added, removed, changed, and value changes shown side by side.
JSON Formatter
Format, validate, and minify JSON instantly. Errors highlighted with line and column.
JSONPath Tester
Run JSONPath queries against any JSON document. See matched nodes and their paths in real time.
JSON Schema → Pydantic
Convert JSON Schema to Pydantic v2 model code, in your browser. Paste schema, get a ready-to-paste Python class. Useful for LLM structured-output workflows.
JSON to CSV
Convert JSON arrays of objects to CSV. Auto-detects fields, supports custom delimiters, escapes properly for Excel and Google Sheets.
JSONL Viewer
View JSONL (newline-delimited JSON) files in a paginated table. Paste up to thousands of lines, search, filter, expand individual records. Files never leave your browser.
JWT Decoder
Paste a JWT to decode its header and payload. All decoding runs in your browser — tokens never leave the page.
Lesson Plan Template
Generate a structured lesson plan from a fillable form. Standard sections (objectives, materials, steps, assessment, reflection). Markdown + printable HTML output.
Loan Calculator
Compute monthly payment, total interest, and full amortization schedule for any loan. Fixed-rate, simple-interest, principal+interest. Browser-only.
Lorem Ipsum Generator
Generate paragraphs, sentences, words, or list items of placeholder Latin text. Configurable length and starting phrase.
Markdown Table Editor
Visually edit a table — rows, columns, alignment per column — and copy the GitHub-flavoured Markdown.
Markdown to HTML
Convert Markdown to clean HTML with a live preview. Supports headings, lists, code, tables, images, and links.
Markdown to PDF
Render markdown to a print-ready PDF in your browser. Paste, preview, save as PDF via browser print dialog. No upload, no server.
Markup & Margin Calculator
Set retail prices from cost. Enter any two of cost / markup % / margin % / selling price — get the other two plus profit. Browser-only.
MIME Type Lookup
Search MIME types by extension or by type. ~120 common types — image, video, audio, application, text, font.
Mortgage Affordability Calculator
How much house can I afford? Income + monthly debts + down payment + interest rate → max purchase price under the 28/36 DTI rule.
Mortgage Refi Comparison
Compare your current mortgage to a refinance offer. See break-even point, monthly payment difference, total lifetime savings. Account for closing costs.
NOI Calculator (Net Operating Income)
Calculate Net Operating Income for a rental property. Enter gross rental income, vacancy %, and operating expenses. Get NOI, effective gross income, and expense ratio. Browser-only.
Number Base Converter
Convert between binary, octal, decimal, hexadecimal, and any base from 2 to 36.
OG Tag Preview
Paste OG meta tags or fill title/description/image — preview share cards as they look on Twitter/X, Facebook, LinkedIn, and Discord.
OpenAI ⇄ Anthropic Converter
Convert message arrays between OpenAI Chat Completions and Anthropic Messages API formats. Handles system messages, tool calls, image content blocks, multi-turn. In-browser, no API keys.
Password Generator
Strong random passwords or memorable passphrases. Generated locally — never sent anywhere.
Percentage Calculator
Five percentage calculators in one: of, what %, increase/decrease, change, and tip/tax.
Placeholder Image Generator
Generate inline-SVG placeholder images at any size with custom text and colours. Output as data URI or downloadable SVG.
Prompt Diff
Compare two prompts (or two versions of a prompt) side by side. Line-level diff with highlighted changes. Designed for iterating on LLM prompts.
Property IRR Calculator
Compute the levered IRR + cash-on-cash + equity multiple over a property hold period. Year-0 down payment, year-by-year cash flows, exit sale proceeds. Browser-only.
QR Code Generator
Generate QR codes for any text, URL, Wi-Fi, or vCard. Custom size, colors, and error correction. Export PNG or SVG.
Query String Builder
Add key/value rows; get a correctly URL-encoded query string out. Supports array (a[]=1) and bracket-less repeated keys.
RAG Text Chunker
Split text into token-sized chunks for RAG / embeddings prep. Multiple strategies: recursive char, sentence-aware, semantic boundaries. Configurable overlap. All in-browser.
Readability Checker
Compute Flesch Reading Ease, Flesch-Kincaid Grade Level, Gunning Fog, SMOG, and Coleman-Liau scores on your text. Spot dense paragraphs, target grade-level appropriately.
Regex Cheatsheet
Quick reference: anchors, character classes, quantifiers, groups, lookarounds, flags. Click any pattern to copy.
Regex LLM Output Extractor
Extract structured data (JSON, code blocks, key-value pairs) from messy LLM responses. Pick from common templates or write your own regex. Live preview.
Regex Tester
Test JavaScript regular expressions live. See matches, capture groups, and apply replacements as you type.
Rent vs Buy Calculator
Compare total cost of renting vs buying a home over your holding period. Models mortgage, taxes, maintenance, appreciation, rent inflation, and opportunity cost. Finds the break-even year. Browser-only.
Rental Yield Calculator (Gross + Net)
Compute gross rental yield (annual rent ÷ price) and net rental yield (NOI ÷ price). Most popular metric in UK / EU buy-to-let. Includes vacancy + expense ratio sliders. Browser-only.
Retirement Projection
Project your retirement savings: current balance, annual contributions, expected return, inflation, target year. See real (inflation-adjusted) and nominal balances. Stress-test with multiple return scenarios.
ROI Calculator
Calculate return on investment + annualized CAGR for any holding. Compare up to 3 investments side-by-side: stocks, real estate, crypto, anything with a buy + sell value.
Rubric Generator
Generate printable rubrics (markdown + HTML preview + print view) from a list of criteria and performance levels. Designed for teachers planning assessments.
Slug Generator
Turn any title into a clean URL slug — transliterates accents, strips punctuation, joins with hyphens.
SQL Formatter
Format and beautify SQL with proper indentation, or minify to a single line. Dialect-aware (ANSI / MySQL / Postgres).
SVG Optimizer
Minify SVG files in your browser. Strips comments, useless whitespace, default attributes, and rounds long decimals. Files never leave your device.
System Prompt Linter
Analyze a system prompt for common issues: vague instructions, conflicting rules, missing examples, oversized context. Heuristic, opinionated, fast.
Text Diff
Compare two blocks of text and see line-by-line additions, removals, and unchanged context. Side-by-side or unified view.
Unix Timestamp Converter
Convert between Unix timestamps and human-readable dates. Seconds and milliseconds, UTC and local.
Time Zone Converter
Convert a date and time between IANA time zones. See offsets, DST status, and weekday for both ends.
Tip & Split Calculator
Bill total + tip % + people = per-person amount. Round-up option, tip and grand total shown.
Token Counter
Estimate token counts across GPT-4, Claude, Llama, Gemini, and other LLMs. Paste text, see counts per model side-by-side. Files never leave your browser.
Unicode Inspector
Paste text → table of every code point. Hex, decimal, UTF-8 bytes, category. Spot invisible characters.
Unit Converter
Convert between metric and imperial units of length, weight, temperature, volume, and area.
URL Encoder / Decoder
Percent-encode strings for URLs or decode percent-encoded ones back to plain text.
URL Parser
Paste any URL — see the protocol, host, port, path, query parameters (decoded), hash, and origin laid out.
UUID Generator
Generate RFC 4122 UUIDs (v4 random or v7 time-ordered). Batch up to 100. Cryptographically secure.
VAT Calculator
Calculate VAT for any EU country, UK, Switzerland, and more — with current 2025 rates including post-reform updates many tools miss. Standard, reduced, and zero rates supported.
WCAG Contrast Checker
Check the contrast ratio between two colors. Pass/fail verdict for WCAG AA and AAA at every text size.
Word Counter
Count words, characters, sentences, paragraphs, and estimate reading + speaking time as you type.
XML Formatter
Format and minify XML. Validate well-formedness with line and column on errors.
XML ↔ JSON Converter
Convert XML to JSON or JSON back to XML. Handles attributes, text nodes, and arrays sensibly.
XP Curve Calculator
Generate XP-per-level tables for RPG progression. Linear, polynomial, exponential, or custom curves. See the full table + visual graph + copy as JSON/CSV.
YAML ↔ JSON Converter
Convert between YAML and JSON in either direction. Useful for Kubernetes manifests, CI configs, and OpenAPI specs.