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.
- Installable as an offline app — works fully offline
Guides & deep-dives
Original, in-depth writing — not tool blurbs. The reasoning behind the numbers.
APR vs APY: the compounding gap that quietly costs you
FinanceRent vs buy: when ownership stops winning
FinanceWhen to refinance: the breakeven math
PrivacyEXIF metadata: what your photos quietly leak about you
DeveloperJWT decoding for non-cryptographers
AIRAG beyond chunk size: what actually moves retrieval quality
All Tools
ASCII Table
Full ASCII reference 0–127 with decimal, hex, binary, character, and HTML entity. Filterable.
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.
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.
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.
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.
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.
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.
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.
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.
HTTP Status Codes
Look up any HTTP status code (1xx–5xx). Meaning, common causes, and the RFC reference.
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.
JSON Formatter
Format, validate, and minify JSON instantly. Errors highlighted with line and column.
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.
JWT Decoder
Paste a JWT to decode its header and payload. All decoding runs in your browser — tokens never leave the page.
Loan Calculator
Compute monthly payment, total interest, and full amortization schedule for any loan. Fixed-rate, simple-interest, principal+interest. Browser-only.
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.
Password Generator
Strong random passwords or memorable passphrases. Generated locally — never sent anywhere.
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.
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.
Regex Cheatsheet
Quick reference: anchors, character classes, quantifiers, groups, lookarounds, flags. Click any pattern to copy.
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.
SQL Formatter
Format and beautify SQL with proper indentation, or minify to a single line. Dialect-aware (ANSI / MySQL / Postgres).
System Prompt Linter
Analyze a system prompt for common issues: vague instructions, conflicting rules, missing examples, oversized context. Heuristic, opinionated, fast.
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.
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 — rates hand-curated and stamped with a "rates as of" date shown beneath each result, 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.