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Step-by-step guides for every tool on Cat Toolbox — how each one works, when to use it, and the details that are easy to get wrong.

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📄PDF Guides4 guides

📄 How to Merge and Split PDF Files Online Combine up to 25 PDFs into one file, or split a document into single pages and custom ranges. Everything runs in your browser, so your documents are never uploaded to a server. → 📄 How to Password-Protect a PDF (and Open a Locked One) Add an open password to a PDF in your browser, understand the difference between user and owner passwords, and what your real options are when a protected PDF will not open. → 📄 How to Reduce PDF File Size Without Ruining It Why PDFs get large, which compression level to pick for email, print, or a size-capped upload portal, and how much smaller a file can realistically get. → 📄 How to Rotate PDF Pages and Fix Wrong Orientation Fix sideways and upside-down pages in a PDF — rotate every page or just the few that are wrong, and understand why scans and phone photos come out rotated in the first place. →

🖼️Image Guides4 guides

🖼️ Batch Resize and Convert Images: What to Know Before You Run It Which files a batch tool will actually accept, how aspect-ratio locking computes the result, why the output count can be lower than you uploaded, and how the ZIP is put together. → 🖼️ How to Compress an Image Without Making It Worse What the quality slider really controls, why max width matters more than quality, when compression makes a file bigger, and how to read the before and after numbers. → 🖼️ How to Convert an Image to PNG, JPEG, or WebP in Your Browser What actually changes when you convert an image file, which formats keep transparency, why a "BMP" conversion can hand you a PNG, and what to check afterwards. → 🖼️ How to Stitch Screenshots Into One Long Image Vertical versus horizontal joining, what the reference width actually controls, why one long screenshot can fail the canvas limits, and how transparency behaves in the output. →

🎵Audio Guides2 guides

🎵 Convert Audio Files in Your Browser: WAV or OGG, and Why OGG Takes Real Time What a browser converter actually does to your file, why the WAV output is always 16-bit, why OGG encoding takes as long as the track plays, and how to pull sound out of a video instead. → 🎵 How to Trim an Audio File Online, and How to Read a BPM Result Sample-accurate cut points, why the exported file is always 16-bit WAV, what the trimmer cannot do — and why an autocorrelation BPM detector keeps handing you double the tempo you expected. →

🎬Video Guides1 guide

🎬 Capturing Video Frames and Making a GIF in Your Browser Why a captured frame can differ from what you saw on screen, what the batch interval really controls, why GIF output is always 10 fps, and how to keep the file from being enormous. →

📝Text Guides3 guides

📝 How to Clean Up and Count Text, and Why the Two Counters Disagree The fifteen formatter actions each redraw your text under different rules — sorting, deduplication and width conversion all have surprises — and the site's two word counts deliberately measure different things. → 📝 Keeping Notes in Your Browser: What the Notepad Saves, and What It Can Lose Auto-save here means localStorage on this device — so it survives a reload but not a cleared browsing history, two open tabs will overwrite each other, and the last few seconds before you close the tab are the risky ones. → 📝 Placeholder Text and Morse Code — Two Generators and Their Hidden Rules Lorem ipsum here builds sentences from a 69-word list that keeps its commas and stray capitals, the count box clamps differently than its labels suggest, and the Morse table supports far more punctuation than its own reference grid shows. →

🧑‍💻Developer Guides3 guides

🧑‍💻 UUID, QR Code and Base64: Where Three Generators Stop Telling You They Failed The UUID page draws real entropy from crypto.getRandomValues, but its count box turns 0 and abc into ten and ignores your format checkboxes until you press Generate again. The QR generator has no error path: ask for more than the error-correction level can hold and it leaves you an invisible canvas that the Download button still exports as a 2,786-byte PNG. The Base64 page trusts your browser's MIME guess, never shows gigabytes, and marks its preview box ready for files it cannot preview. All figures measured by driving the pages. → 🧑‍💻 Comparing Two Versions of a File, and Testing a Regex Before You Ship It The diff is line-based and has no word-level view — so a Windows-vs-Mac paste marks every line as changed — while the regex tester runs on every keystroke, which is exactly what turns a backtracking pattern into a frozen tab. → 🧑‍💻 Reading a Unix Timestamp Without Getting the Wrong Day Seconds or milliseconds is decided here by digit count, which is right almost always and quietly wrong for millisecond values from before September 2001; and only two of the four outputs are unambiguous about time zone. →

🔐Crypto Guides1 guide

🔐 What the Password Generator and the Strength Checker Actually Compute The generator rates your settings, the checker rates the string you type, and those two numbers are not comparable. Exact alphabet sizes (26/26/10/27), the four entropy bands, the 40-word common list, the crack-time model — plus the one unit label on the checker that is off by a factor of ten. →

🧮Calculator Guides3 guides

🧮 BMI, Age and Date Calculators: Which Numbers to Trust, and When The BMI page classifies the rounded number, not the real one. The age page anchors its counters to UTC midnight and keeps ticking after you clear the box. The date page counts days from local midnight, so a clock change can cost you one. Every figure below was measured on the pages themselves. → 🧮 Mortgage, Interest and Percentage Calculators, and the Numbers They Do Not Show You The mortgage page recomputes as you type, the interest page waits for a button, and the percentage page answers a division by zero with 0%. Everything below was read out of each page's own script: the annuity formula, the rounding drift in the repayment table, the growth-curve point that lands a year past your term, and the axis label that is ten times off. → 🧮 Unit and Base Converters: Factors, Display Rounding and the Edges of the Range The unit converter multiplies by one stored factor per unit; the base converter re-encodes an integer exactly. Both are reliable in the middle and surprising at the edges — exactly 1,000,000 switches to scientific notation, 1 PB comes out as 1023.999994 TB, and the box you type in keeps the leading zeros you gave it. Every figure below was measured on the pages themselves. →

⏱️Time Guides1 guide

⏱️ Countdown, Pomodoro and Stopwatch: Which One Still Keeps Time in a Hidden Tab The stopwatch recomputes the elapsed time from the system clock every 20 ms, so a throttled tab cannot make it lose a second; the countdown and the Pomodoro timer count ticks instead and fall behind. Measured on the pages: a finished countdown restarts in one press while the Pomodoro timer needs two, a 200-minute focus block is accepted despite the field saying 120, and a negative entry leaves 0-5:00 on screen. →

🎨Color Guides1 guide

🎨 Color Converter, Palette Extractor and Gradient Builder: What Each One Changes Without Telling You All three colour pages accept what you give them and quietly rewrite it. The converter's HEX box parses a number, so #FF0000ff becomes #0000FF; its HSL box rounds, so 89.4% of colours do not survive a HEX→HSL→HEX round trip. The extractor analyses a 4000×3000 photo through five of its 4,000 columns, pins its first cluster centre to the top-left pixel, and reports a 1% accent as 100% because the white it sits on was filtered out. The gradient builder refuses a fifth stop. Every figure below was read out of the pages' own scripts or measured by driving them. →

🔤Encoding Guides1 guide

🔤 Base64, URL Escapes and Unicode Escapes, and Why None of Them Hide Anything Base64 of a UTF-8 text, percent-escapes that leave a plus sign alone, and a Unicode converter that rewrites even the letter A — the three encode pages on this site are reversible by design, so the useful details are the boundaries: padding, plus signs, surrogates and which box the result lands in. →

🧾Format Guides1 guide

🧾 JSON, Escape and Markdown Tools: What These Four Pages Do to Your Text Without Asking All four pages take your text, rewrite it, and never mention it. The JSON formatter turns 1e309 into null and calls that a success; the code generator types every array by its first element, so [1,\"a\"] becomes number[] and duplicate keys become two interfaces with the same name. The HTML escaper knows exactly five characters and double-escapes anything you already escaped. The Markdown preview renders raw HTML, and a single quote in a link URL lets that HTML carry event handlers. Every figure here came from reading these pages' scripts or driving them in a browser. →