Guides

Short, practical answers to the questions people actually search for, each linked to a free tool that does the job in your browser.

Data and developers

Data and developersHow to create a strong password (according to NIST)How long a password needs to be, why length beats symbols, and what NIST recommends: long, unique passphrases kept in a password manager.4 min read Data and developersHTTP status codes for SEO: 301, 302, 404, 410, 503What each HTTP status code means and which to use: 301 vs 302, 404 vs 410, 401 vs 403, and what Google does with 5xx errors and 429, per its documentation.5 min read Data and developersWhy is my JSON invalid? The most common errorsTrailing commas, single quotes, comments, NaN: the mistakes that trigger 'Unexpected token' in JSON.parse, why each one fails, and how to fix them.5 min read Data and developersUnix timestamps explained: seconds, milliseconds and the year 2038What a Unix timestamp is, how to tell seconds from milliseconds, why a 10-digit number gives a date in 1970, and what the 2038 problem really is.4 min read Data and developersCron expressions explained, with examples and gotchasHow the five cron fields work, ready-to-use examples (every 15 minutes, weekdays at 9), and the traps: day-of-month vs weekday, step values, time zones.5 min read Data and developersHow to read a JWT, and why decoding is not verifyingWhat the three parts of a JSON Web Token contain, how to decode one by hand, what exp and iat mean, and why a decoded token proves nothing until verified.4 min read Data and developersWhat is Base64? Encoding, not encryptionBase64 encrypts nothing: what it does, why the output is a third bigger, what the = padding and the URL-safe variant are, and why btoa() breaks on accents.4 min read Data and developersUUID v4 vs v7: what they are and when to use eachHow a UUID is built, the difference between random v4 and time-ordered v7, how unlikely a collision really is, and why a UUID is not a secret.4 min read Data and developersChecksums and hashes: how to verify a download with SHA-256What a hash is, how to check a file's SHA-256 against the published one, why MD5 and SHA-1 are retired for security, and why SHA-256 is wrong for passwords.4 min read Data and developersOpen a CSV in Excel without garbled text or lost zerosWhy a CSV shows odd characters, lands in one column or loses leading zeros in Excel, and how to import it properly: UTF-8, delimiter, text columns, limits.5 min read

Image and photo

Image and photoPassport photo size and requirements: US, UK and CanadaOfficial passport photo sizes and rules: US 2×2 in, UK 35×45 mm, Canada 50×70 mm, with head size, background, digital file limits and how to make one at home.5 min read Image and photoHow to remove EXIF data and location from a photoWhat your photos hide (GPS, date, device model), when to remove it, and how to do it on iPhone, Android and desktop without recompressing the image.4 min read Image and photoHow to convert HEIC photos to JPGWhat a HEIC file is, why many sites reject it, and four ways to turn iPhone photos into JPG: a camera setting, Mac export, the browser and Windows.4 min read Image and photoJPG vs PNG vs WebP vs AVIF: differences and which to useWhich image format to choose for photos, screenshots, logos with transparency, websites or printing: compression, transparency, support and common mistakes.5 min read Image and photoHow to combine photos and images into one PDFHow to put photos in a single PDF with one page per image: which page size to choose, how to order them, why the file ends up so large and how to shrink it.4 min read Image and photoFavicon for Google: current requirements, sizes and HTML tagsWhat Google requires today to show your favicon (formats, minimum size, one per host), which sizes iPhones and installable apps need, and the HTML tags.4 min read Image and photoDPI for printing: how many pixels does your photo needHow many pixels an image needs to print well: the sum (pixels = inches × dpi), a table for A4, A3 and photo sizes, and why raising the dpi tag improves nothing.4 min read

PDF

PDFAre online PDF and photo converters safe?What happens to your file in an online converter or compressor, what the FBI warned about, and how to check in a minute whether a site uploads your files.5 min read PDFPDF too large to email? How to reduce its sizeAttachment limits for Gmail (25 MB) and Outlook (20 MB), and how to shrink a heavy PDF without making it unreadable: compress, rescan, split or share a link.4 min read PDFHow to redact a PDF properlyA black box drawn over a PDF does not delete the text underneath. How to test it in thirty seconds and how to redact names, IDs or account numbers for good.5 min read PDFMerge, split, reorder and rotate a PDF without softwareMerge PDFs, delete or extract pages, reorder them, rotate sideways scans and split a PDF in your browser with nothing uploaded, plus what to check first.4 min read PDFHow to convert a scanned PDF to text (OCR)How to tell whether your PDF is an image, how to extract the text with OCR in the browser, and how to get good recognition: resolution, skew, borders, contrast.4 min read

Codes and validators

Codes and validatorsHow to create a QR code (link, Wi-Fi, contact) that scans reliablyHow to make a QR code for a link, your Wi-Fi or a contact card, what size and contrast it needs to scan reliably, and which error correction level to choose.5 min read Codes and validatorsRoman numerals: rules, table and how to convert themRoman numeral rules, a table to convert any number, the current years (2026 is MMXXVI), where they are used and the century trap, checked against the RAE.4 min read Codes and validatorsSubnet mask and CIDR: how to calculate hosts and rangesWhat /24 or 255.255.255.0 means, why a /24 network holds 254 hosts and not 256, how to work out network, broadcast and range by hand, and which IPs are private.4 min read Codes and validatorsEAN, UPC and ISBN check digit: how it is calculatedHow the last digit of an EAN-13, EAN-8, UPC-A barcode or an ISBN is calculated, with worked examples, which errors it catches and which ones slip past it.5 min read

Web and SEO

Web and SEOrobots.txt and sitemap.xml: what to put and what to avoidHow to write a robots.txt and sitemap.xml that work: location, syntax, Google's limits, why robots.txt does not hide pages and how to control AI crawlers.5 min read Web and SEOTitle and meta description length: measured in pixels, not charactersGoogle sets no character limit for a title or meta description: it truncates by width. What its docs say, real pixel measurements and how to check yours.4 min read Web and SEOOpen Graph image size: official Facebook and LinkedIn requirements and how to update itWhat Meta and LinkedIn require for the image shown when a link is shared (size, weight, 1.91:1 ratio), why it is missing the first time and how to refresh it.4 min read

Dates and time

Dates and timeWhen do the clocks change? Dates for the US, UK and Europe through 2030Daylight saving dates in the US, UK and EU through 2030, the exact hour it happens, and why London–New York meetings are off by an hour for a few weeks.4 min read Dates and timeWhat week of the year is it? How ISO week numbers workHow weeks are numbered under ISO 8601, why 2026 has 53 weeks, why 1 January can be week 52 or 53 and why Excel gives a different number. With worked examples.5 min read

Numbers and calculators

Numbers and calculatorsHow to calculate VAT: add it, remove it, and avoid the classic mistakeHow to add VAT to a net price and take it out of a gross one (without the classic mistake), the three UK rates (20%, 5%, 0%) and worked examples.4 min read Numbers and calculatorsHow many MB in a GB: 1000 or 1024Why 1 GB is 1000 MB or 1024 MiB depending on who measures it, why your 500 GB drive shows 465, and the difference between megabits and megabytes.4 min read

Color

ColorAccessible color contrast: the ratio WCAG requiresWhy light gray on white fails: WCAG 2.2's 4.5:1, 3:1 and 7:1 ratios, how contrast is calculated, and which everyday colors pass or fail.4 min read

How these guides are written

Every guide starts from a real task, such as fitting a file under a size limit or checking a document before sending it, and ends with a tool that does it without uploading anything.

When a guide states a rule that can change, such as a file size limit, a tax rate or a photo size, the figure comes from the official source listed at the bottom of the page, together with the date it was last checked. If a figure could not be confirmed, the guide says so instead of guessing.

What a guide says about our own tools, such as their limits, formats and what they do and do not do, we check against how the tool actually behaves, and where a calculation is shown we have worked it through to confirm the result.

If you spot a mistake or an outdated figure, tell us at hola@utilsdock.com and we will correct it.