All guides

48 guides organised by real-world situation — when to use each tool and how to avoid the common mistakes.

48Guides

Image

HEIC to JPG: A Practical Guide to Converting iPhone PhotosConvert iPhone HEIC photos to JPG in your browser. When to convert, how to batch safely, and when to keep HEIC.
Image Compression: How to Shrink Files Without Ruining QualityCompress images without wrecking quality: pick JPG or WebP, set size and quality in order, and avoid common mistakes.
PNG to JPG: What Happens to Transparent BackgroundsPNG to JPG: why transparency becomes a solid colour, how to choose the fill, and when to keep PNG or use WebP.
WebP Conversion: Choosing Between WebP, JPG, and PNGWebP, JPG, or PNG? How to convert between them, handle transparency, and know when another conversion is not worth it.
Image Resizing: Width, Percentage, and Longest-Edge ScalingResize by width, percentage, or longest edge. Understand the difference, use size presets, and avoid blurry upscaling.
Image Stitching: How to Combine Screenshots into One Long ImageStitch screenshots into a long image. Learn vertical and horizontal layouts, width handling, and why long canvases fail.
SVG to PNG: Exporting Crisp Bitmaps at the Right ScaleExport SVG to PNG at 1x–4x. Choose the right scale, fix missing fonts or images, and know when to keep the SVG.
Image Cropping: Fixed Ratios for Avatars, Covers, and PrintsCrop images to 1:1, 4:3, or 16:9 with a drag box. Learn how export resolution works and when padding beats cropping.
How to Watermark a Batch of Photos Without Ruining ThemWatermark many images at once with six preset positions, width-based sizing, and adjustable opacity and colour. Runs in your browser; photos are never uploaded.

PDF

How to Merge PDFs in the Right OrderCombine contracts, attachments and scans into one PDF in the order you choose. Reorder by drag or arrows; encrypted files are skipped; everything runs locally.
How to Split a PDF by Page RangeSplit a PDF into separate files by ranges like 1-3,5, or one file per page as a ZIP. Pages count from 1; pages past the end are trimmed; everything runs locally.
How to Rotate a PDF That's Sideways or Upside DownRotate every page of a PDF by 90, 180 or 270 degrees in your browser. Orientation changes without re-rendering, so scans keep their quality; nothing is uploaded.
PDF to JPG: Choosing 1x, 2x or 3x ResolutionRender every page of a PDF as a JPG at 1x, 2x or 3x, adjust quality and download the set as a ZIP. Rendering happens in your browser; the PDF is never uploaded.
How to Turn Multiple JPGs into One PDFCombine JPGs and PNGs into one PDF in the order you add them, on A4 or at image size, with adjustable margins. Images are embedded locally; nothing is uploaded.
How to Keep or Delete Pages from a PDFKeep pages 2 and 5-8, or remove ad pages and appendices, and get one new PDF. Ranges like 1-3,5 work; pages come out in ascending order; everything runs locally.
How to Add Page Numbers to a PDFAdd continuous numbers to every page in the 1, 1/N or - 1 - format, with five positions, a starting number and adjustable size. Drawn locally; never uploaded.
How to check a PDF's page count and paper size without AcrobatSee page count, per-page size distribution, A4 or Letter paper size and metadata, and spot landscape pages mixed into a document. Parsed locally in your browser.
How to add a diagonal text watermark to a PDFStamp every page of a PDF with a diagonal text watermark such as DRAFT or CONFIDENTIAL, with adjustable size, opacity and angle. Runs locally in your browser.

Text

Counting words in mixed Chinese and English textCount Chinese characters, English words, characters with and without spaces, paragraphs and reading time separately. Mixed-language text no longer gives a misleading total.
Convert English case: title case, camelCase, snake_case and kebab-caseConvert text to UPPER, lower, Title Case, sentence case, camelCase, snake_case or kebab-case. Naming modes split words first and rejoin them. Runs locally in your browser.
Remove duplicate lines and keep the original orderDeduplicate line-based text while preserving the original order. Optionally ignore case and leading/trailing whitespace; blank lines are dropped. Runs locally, lists are never uploaded.
Markdown preview: render as you type and copy the HTMLWrite Markdown on the left, see rendered HTML on the right, then copy the HTML fragment into a static page or email template. Rendering happens locally in your browser.
Compare two texts line by line and see what was added or removedPaste the old and new versions side by side and get a line-by-line diff with additions and deletions marked. LCS-based matching, optional whitespace and case ignoring. Runs locally.
Remove line breaks and extra spaces from text copied out of a PDFClean up hard returns, blank lines and stray spaces from PDF or web text. Merge into one paragraph, merge without spaces, drop blank lines, trim lines or collapse extra spaces.
Sort lines of text: alphabetical, by length, numeric, deduplicatedPaste one item per line and sort alphabetically, by length, or by the number inside each line. Reverse, shuffle, and remove duplicates, all locally in the browser.
Find and replace in bulk: regex, capture groups, and a preview countReplace text across a whole document with plain text or regular expressions, reference groups with $1, and check the match count before committing.
JSON, HTML, and SQL escaping: three contexts, three different jobsEscape text for a JSON string, HTML entities, or a SQL literal, and reverse it. Each mode solves one context, and none of them is a security boundary.

Developer

JSON formatter: pretty-print, minify, and locate syntax errorsPaste JSON to pretty-print it with indentation, minify it to a single line, or let the parser point at the syntax error. Everything runs locally in your browser.
Base64 encode and decode: UTF-8 text without mojibakeEncode text to Base64 and back, handling UTF-8 internally so Chinese, Japanese, and emoji survive the round trip. Everything is computed locally.
URL encoding and decoding: whole strings, parameter values, and %20Percent-encode non-ASCII characters and spaces, or decode strings like %E4%B8%AD%E6%96%87. Encode a whole string or only the values inside a query string.
Unix timestamp converter: 10-digit seconds, 13-digit millisecondsConvert Unix timestamps to dates and back, with automatic detection of 10-digit seconds and 13-digit milliseconds. Shows local time, UTC, and ISO 8601.
SHA-256 hash generator: HEX and Base64 output, without sending dataCompute SHA-1, SHA-256, SHA-384, or SHA-512 with the browser's WebCrypto API, and get both HEX and Base64. MD5 is not available natively.
UUID Generator Guide: Batch v4 IDs Done RightHow to generate UUID v4 in batches without the usual mistakes: casing, storage, why a random primary key slows writes, and what to use instead.
JWT Decoder Guide: Header, Payload and ExpiryWhat a JWT decoder can and cannot tell you: the three-part structure, why exp is a UTC timestamp in seconds, and why decoding is not verification.
Regex Tester Guide: Live Matches and GroupsHow to debug a regular expression in the browser: pick the right dialect and flags, test strings that must not match, read groups, avoid backtracking blowups.
Cron Expression Guide: Five Fields, Next RunsHow to write a 5-field cron expression that runs when you meant: field ranges, the day-of-month and day-of-week OR rule, what */n means, and time zones.

Calculators

BMI Guide: Chinese 24/28 vs WHO 25/30How BMI is calculated, why the Chinese adult cut-offs of 24 and 28 differ from the WHO 25 and 30, and what the number cannot tell you about muscle and fat.
Loan Guide: Equal Payment vs Equal PrincipalThe real difference between the two repayment methods: a flat instalment with front-loaded interest versus a declining one with less total interest.
Unit Converter Guide: Length, Weight, Area, Volume, TemperatureWhere unit conversion actually goes wrong: jin versus pound, US versus imperial gallons, temperature as an offset not a ratio, and why 1 TB shows 931 GB.
Age Guide: Full Years, Months, Days, CountdownHow full age differs from Chinese nominal age, why you cannot divide days by 30 for months, what a 29 February birthday does, and which time zone counts.
Days Between Two Dates: Natural Days, Weeks, and Calendar MonthsEnter a start and end date to get the gap in calendar days, weeks, and whole months, and see why dividing days by 30 gives you the wrong month count.
Percentage Calculator: Share of a Total, Change Rate, and Percentage PointsWork out A as a percentage of B, the percentage change from an old value to a new one, and the difference between percent and percentage points.
Discount Calculator: Sale Price, Percent Off, and Stacked OffersWork out the final price and the amount saved, and see why stacked discounts multiply instead of adding up.
Salary Converter: Monthly, Daily, Hourly, and Annual Pay (21.75-Day Basis)Convert between monthly, daily, hourly, and annual pay using the 21.75 paid-days and 8-hour conventions, and see how that differs from actual days worked.
Payroll Tax Estimator: Cumulative Withholding and Why Each Month DiffersEstimate Chinese payroll tax with cumulative withholding and see why the same salary deducts differently each month. An estimate, not tax advice.

AI Assist

Token Counter: Estimate Prompt Tokens and API CostEstimate prompt tokens and API cost with rules of thumb: about 4 characters per token in English and 1.5 in Chinese, within roughly ±15%.
How to Write a Structured Prompt: Role, Task, Constraints, OutputAssemble a structured prompt from role, task, input, constraints, and output format to cut rework on GPT, Claude, and other models.
Chinese to English Image Prompt: Structure, Style, and --arTurn a Chinese scene description into a structured English prompt — subject, style, composition, lighting, colour — for Midjourney and Stable Diffusion.