Every developer keeps a short list of utility sites that save time. The problem with most of them is that they want your data: upload your JSON, register an account, or send your passwords to a server. The tools below are different — each one runs entirely in your browser, works offline once loaded, and never sees your private data.
1. JSON Formatter & Validator
Debugging an API response or a config file means reformatting JSON over and over. A good formatter does more than indent — it validates syntax, highlights errors with line numbers, and offers minified output for compact storage. This is the tool you open first thing in the morning and keep open all day. Try the JSON formatter →
2. Unix Timestamp Converter
Database values, API payloads, and log lines all throw epoch timestamps at you. The right converter shows both seconds and milliseconds, converts to UTC and your local time, and works both directions — epoch to date, and date to epoch. No installation, no extension, just an open tab. Open the timestamp converter →
3. QR Code Generator
Sharing a Wi-Fi password, a test URL, or contact details on a screen is still the fastest way to move data to a phone. A browser-based QR generator lets you control size and encoding, produces a clean PNG, and does it all locally — which matters when the content is sensitive. Open the QR generator →
4. Secure Password Generator
The one rule that overrides everything else in passwords: generate randomly, never compose by hand. A good generator runs on your own device, lets you choose length and character sets, and never transmits the result. If a generator has a server, it is already doing it wrong. Open the password generator →
5. Color Code Converter
Design systems live in HEX, RGB, and HSL, and you will switch between them constantly. A converter with a live preview saves the mental math of translating #2563eb into an RGB string for a tooltip or a gradient stop. Open the color converter →
6. Text Character & Word Counter
Meta descriptions, commit messages, tweets, and form labels all have limits. A real-time counter with characters, words, sentences, and paragraphs keeps you inside the constraint without guesswork. Open the word counter →
7. CM to Inches Converter
Front-end work crosses measurement systems constantly: screen sizes, print dimensions, furniture mockups. Having the conversion table memorized for the common values — and a calculator for the rest — saves a search every time. Open the converter →
8. Celsius to Fahrenheit Converter
Device temperatures, server rooms, and international specs mix scales without warning. A converter with a chart of the common reference points (0°C = 32°F, 100°C = 212°F) makes the numbers stick. Open the converter →
9. Speed Distance Time Calculator
Estimating latency-related distances, download times, or transfer windows is a recurring back-of-envelope problem. The SDT calculator finds any of the three when you know the other two, and shows the units so you do not mix hours and minutes. Open the calculator →
10. Percentage Calculator
Discounts, price changes, API rate limits, and test coverage percentages — everyday work is full of "what percent is this of that" questions. Three forms in one tool: percent of a number, percentage change, and reverse percentage. Open the calculator →
The Rule Behind the List
Every tool on this list runs client-side. That is not a marketing detail — it is the feature. When the tool is your browser tab instead of a remote service, there is nothing to sign up for, nothing to trust, and nothing to clean up later. Bookmark the ones you use daily, and the list itself becomes your personal toolbox.
Looking for more? Browse the full set at zzml.com developer tools, unit converters, and everyday calculators.