ArrayKit guidesPlain-English guides to the ideas behind ArrayKit's tools: PDFs and photos, mortgages and interest, BMI, Unix time, regex, hashes, JSON and more.https://arraykit.com/learn2026-10-18T00:00:00ZUdit Agarwalhttps://arraykit.com/aboutBase64 Encoding, Explainedhttps://arraykit.com/learn/base64-encoder-decoder2026-10-18T00:00:00Z2026-10-18T00:00:00ZBase64 explained: how it turns binary into text, the alphabet and padding, the URL-safe variant, size overhead, use cases, and why it isn't encryption.Linux File Permissions and chmod, Explainedhttps://arraykit.com/learn/chmod-calculator2026-10-18T00:00:00Z2026-10-18T00:00:00ZWhat chmod 755, 644 and 777 mean: owner, group and others, read, write and execute, octal and symbolic modes, umask, setuid, setgid and the sticky bit.CIDR Notation and Subnetting, Explainedhttps://arraykit.com/learn/cidr-subnet-calculator2026-10-17T00:00:00Z2026-10-18T00:00:00ZWhat CIDR notation like /24 means, how to find the subnet mask, network and broadcast address and usable hosts, plus private ranges and how to split a network.Compressing a Photo to an Exact File Sizehttps://arraykit.com/learn/compress-image-to-size2026-10-17T00:00:00Z2026-10-18T00:00:00ZHow to compress an image to 50 KB or reduce a photo to 20 KB for job, exam and government forms: what sets file size, KB vs KiB, and when to resize first.DMS and Decimal Degrees, Explainedhttps://arraykit.com/learn/dms-to-decimal-degrees2026-10-18T00:00:00Z2026-10-18T00:00:00ZHow to convert DMS to decimal degrees and back, how DD, DMS and DDM differ, how many decimal places you need, and which formats Google Maps and GPS accept.Cron Expressions, Explainedhttps://arraykit.com/learn/cron-expression-explainer2026-10-17T00:00:00Z2026-10-18T00:00:00ZCrontab syntax explained: the five cron fields, special characters, common schedules, the day-of-month or day-of-week rule, @daily shortcuts and time zones.HEX, RGB and HSL Color Codes, Explainedhttps://arraykit.com/learn/css-color2026-10-17T00:00:00Z2026-10-18T00:00:00ZHow HEX, RGB and HSL color codes work, how to convert between them step by step, what alpha and 8-digit hex mean, and which WCAG contrast ratios text needs.HEIC Files, Explainedhttps://arraykit.com/learn/heic-to-jpg2026-10-17T00:00:00Z2026-10-18T00:00:00ZWhat a HEIC file is, why iPhones save photos as HEIC, how to open HEIC on Windows, and how to convert HEIC to JPG while keeping quality and dropping GPS data.Markdown Syntax, Explainedhttps://arraykit.com/learn/markdown-preview2026-10-17T00:00:00Z2026-10-18T00:00:00ZA Markdown cheat sheet and guide: headings, lists, links, images, code, tables and task lists, plus the line-break and escaping rules that trip people up.How to Combine PDF Files, and What Survives the Mergehttps://arraykit.com/learn/merge-pdf2026-10-18T00:00:00Z2026-10-18T00:00:00ZHow to merge PDF files into one: file order, what happens to bookmarks, links and forms, password-protected PDFs, and merging on Windows, Mac and iPhone.Ohm's Law and Electrical Power, Explainedhttps://arraykit.com/learn/ohms-law-calculator2026-10-18T00:00:00Z2026-10-18T00:00:00ZThe Ohm's law formula V = I × R explained: how to calculate voltage, current, resistance and power, with worked examples, units, series and parallel circuits.Why PDFs Get Big, and How to Shrink Themhttps://arraykit.com/learn/pdf-compress2026-10-17T00:00:00Z2026-10-18T00:00:00ZHow to reduce PDF size: what makes a PDF large, how compression shrinks images and fonts, why some PDFs barely shrink, and how to meet email and upload limits.Splitting a PDF and Extracting Pages, Explainedhttps://arraykit.com/learn/split-pdf2026-10-18T00:00:00Z2026-10-18T00:00:00ZHow to split a PDF or extract pages: writing page ranges like 1-3, 5, splitting every N pages or by file size, and what happens to bookmarks, links and size.Unix Timestamps and Epoch Time, Explainedhttps://arraykit.com/learn/unix-timestamp-converter2026-10-17T00:00:00Z2026-10-18T00:00:00ZWhat a Unix timestamp is, why epoch time ignores leap seconds, how to tell seconds from milliseconds, converting in JS, Python and SQL, and the 2038 problem.How Exact Age Is Calculatedhttps://arraykit.com/learn/age-calculator2026-10-17T00:00:00Z2026-10-17T00:00:00ZHow to calculate exact age in years, months and days: the borrow method, 29 February birthdays, total days and weeks, and why calculators differ by a day.Amortization Schedules, Explainedhttps://arraykit.com/learn/amortization-calculator2026-10-17T00:00:00Z2026-10-17T00:00:00ZWhat an amortization schedule is, how each row's interest and principal are calculated, why early payments are mostly interest, and what extra payments save.Body Mass Index (BMI), Explainedhttps://arraykit.com/learn/bmi-calculator2026-10-17T00:00:00Z2026-10-17T00:00:00ZThe BMI formula in metric and imperial units, what a healthy BMI is for adults, the WHO categories, and why BMI is only a screening number with real limits.Daily Calorie Needs, Explainedhttps://arraykit.com/learn/calorie-calculator2026-10-17T00:00:00Z2026-10-17T00:00:00ZHow many calories you need a day: BMR vs TDEE, the Mifflin–St Jeor formula, activity multipliers, and what a 250 or 500 calorie deficit means in practice.METs and Calories Burned, Explainedhttps://arraykit.com/learn/calories-burned-calculator2026-10-17T00:00:00Z2026-10-17T00:00:00ZWhat a MET is, the formula for calories burned (MET × 3.5 × kg ÷ 200 per minute), the Compendium of Physical Activities, and why fitness trackers disagree.Colour Vision Deficiency, Explainedhttps://arraykit.com/learn/color-blind-test2026-10-17T00:00:00Z2026-10-17T00:00:00ZWhat colour blindness is: protan, deutan and tritan types, how common it is, how an Ishihara-style color blind test works, and why a screen test only screens.Compound Interest, Explainedhttps://arraykit.com/learn/compound-interest-calculator2026-10-17T00:00:00Z2026-10-17T00:00:00ZThe compound interest formula A = P(1 + r/n)^(nt) and how compound interest works, with real examples, compounding frequency, deposits, APY and the rule of 72.How Much Concrete Do You Need?https://arraykit.com/learn/concrete-calculator2026-10-17T00:00:00Z2026-10-17T00:00:00ZHow much concrete you need for a slab, footing or post hole: volume in cubic yards and m³, 60 lb and 80 lb bag counts, waste allowance, and ready-mix vs bags.Date Calculations, Explained: Counting and Adding Dayshttps://arraykit.com/learn/date-calculator2026-10-18T00:00:00Z2026-10-17T00:00:00ZHow to count the days between two dates, add days, weeks or months to a date, and why calculators disagree: inclusive counts, month ends and leap years.Pregnancy Due Dates, Explainedhttps://arraykit.com/learn/due-date-calculator2026-10-17T00:00:00Z2026-10-17T00:00:00ZHow a due date is calculated: Naegele's rule (LMP plus 280 days), cycle length, conception and IVF dates, early ultrasound, and why few babies arrive on it.How to Add, Subtract, Multiply and Divide Fractionshttps://arraykit.com/learn/fraction-calculator2026-10-18T00:00:00Z2026-10-17T00:00:00ZHow to add fractions with a common denominator, subtract, multiply, divide and simplify them, and convert mixed numbers and decimals, with worked examples.Gas Mileage and Fuel Economy, Explainedhttps://arraykit.com/learn/gas-mileage-calculator2026-10-17T00:00:00Z2026-10-17T00:00:00ZHow to calculate gas mileage with the full-tank method, convert MPG to L/100 km and km/L, US vs UK gallons, and work out the fuel cost per mile or km.Grade Point Average (GPA), Explainedhttps://arraykit.com/learn/gpa-calculator2026-10-17T00:00:00Z2026-10-17T00:00:00ZHow to calculate GPA from letter grades and credit hours, the 4.0 GPA scale with plus and minus values, weighted vs unweighted GPA, and cumulative GPA.GST Tax Invoices in India, Explainedhttps://arraykit.com/learn/gst-invoice-generator2026-10-17T00:00:00Z2026-10-17T00:00:00ZWhat a GST tax invoice in India must show: GSTIN, HSN/SAC, place of supply, CGST + SGST vs IGST, a worked example with rounding, and e-invoicing rules.Cryptographic Hash Functions, Explainedhttps://arraykit.com/learn/hash-generator2026-10-17T00:00:00Z2026-10-17T00:00:00ZWhat a cryptographic hash is, how MD5, SHA-1, SHA-256 and SHA-512 compare, how checksums verify downloads, and why fast hashes are wrong for passwords.HTTP Status Codes and MIME Types, Explainedhttps://arraykit.com/learn/http-reference2026-10-18T00:00:00Z2026-10-17T00:00:00ZWhat HTTP status codes mean, from 200 to 504: 301 vs 302, 401 vs 403, 404 vs 410, soft 404s and redirects for SEO, plus MIME types and Content-Type.PNG vs JPG vs WebP, Explainedhttps://arraykit.com/learn/image-converter2026-10-17T00:00:00Z2026-10-17T00:00:00ZPNG vs JPG vs WebP: lossy and lossless compression, transparency, file sizes for the same photo, and the best image format for the web, photos and logos.Turning Images into a PDF, Explainedhttps://arraykit.com/learn/image-to-pdf2026-10-18T00:00:00Z2026-10-17T00:00:00ZHow to convert JPG and other images to PDF: page size and orientation, margins, image order, why photo PDFs get large, and how to make a scanned PDF searchable.Image to Text: How OCR Workshttps://arraykit.com/learn/image-to-text2026-10-17T00:00:00Z2026-10-17T00:00:00ZHow OCR turns an image to text: what affects accuracy (resolution, contrast, skew, fonts, language), why handwriting is hard, and how to make PDFs searchable.Lorem Ipsum, Explainedhttps://arraykit.com/learn/lorem-ipsum2026-10-18T00:00:00Z2026-10-17T00:00:00ZWhat lorem ipsum means, where it comes from (Cicero, 45 BC), how it became the standard placeholder text, its downsides, and why it should never go live.How a Mortgage Payment Is Calculatedhttps://arraykit.com/learn/mortgage-calculator2026-10-17T00:00:00Z2026-10-17T00:00:00ZHow is a mortgage payment calculated? The formula, what tax, insurance, PMI and HOA dues add, 15- vs 30-year costs, and checking it with a mortgage calculator.Strong Passwords, Explainedhttps://arraykit.com/learn/password-generator2026-10-17T00:00:00Z2026-10-17T00:00:00ZWhat makes a password strong: length vs complexity, password entropy in bits with worked numbers, passphrases, password managers and current NIST advice.PDF to JPG and PNG, Explainedhttps://arraykit.com/learn/pdf-to-images2026-10-18T00:00:00Z2026-10-17T00:00:00ZHow to convert a PDF to JPG or PNG: which DPI to pick, pixel sizes for A4 and Letter pages, JPG vs PNG for text and photos, and fixing blurry or huge images.Converting PDF to Word, Explainedhttps://arraykit.com/learn/pdf-to-word2026-10-17T00:00:00Z2026-10-17T00:00:00ZWhy converting PDF to Word is hard, what survives (text, headings, lists) and what breaks, why scanned PDFs need OCR first, and what a DOCX file really is.How to Calculate Percentageshttps://arraykit.com/learn/percentage-calculator2026-10-17T00:00:00Z2026-10-17T00:00:00ZHow to calculate percentage problems: X% of Y, what percent one number is of another, the percentage increase formula, reverse percentages and common mistakes.Random Number Generators, Explainedhttps://arraykit.com/learn/random-number-generator2026-10-18T00:00:00Z2026-10-17T00:00:00ZHow a random number generator works, how to pick a random number between 1 and 100, random numbers without repeats, modulo bias and real lottery odds.Reaction Time, Explainedhttps://arraykit.com/learn/reaction-time-test2026-10-17T00:00:00Z2026-10-17T00:00:00ZWhat reaction time is, the typical simple visual reaction time for adults, why screens and mice add delay, what slows you down, and how to get a fair score.Regular Expressions (Regex), Explainedhttps://arraykit.com/learn/regex-tester2026-10-17T00:00:00Z2026-10-17T00:00:00ZRegular expressions explained: character classes, greedy vs lazy quantifiers, groups, flags and lookarounds, a regex cheat sheet, and catastrophic backtracking.Converting Pay Between Hourly, Monthly and Annualhttps://arraykit.com/learn/salary-calculator2026-10-17T00:00:00Z2026-10-17T00:00:00ZHow to convert hourly pay to an annual salary and back: the 2,080-hour year, why a month is not 4 weeks, biweekly vs semi-monthly pay, and gross vs net.How to Calculate Square Footagehttps://arraykit.com/learn/square-footage-calculator2026-10-17T00:00:00Z2026-10-17T00:00:00ZHow to calculate square footage for rectangles, L-shaped rooms, triangles and circles, convert sq ft to m², add a flooring waste allowance and price per sq ft.How to Calculate Standard Deviationhttps://arraykit.com/learn/standard-deviation-calculator2026-10-18T00:00:00Z2026-10-17T00:00:00ZHow to calculate standard deviation step by step, with a worked example, variance, sample vs population (n − 1 or n), the 68–95–99.7 rule and STDEV.S.Text Diff, Explainedhttps://arraykit.com/learn/text-diff2026-10-17T00:00:00Z2026-10-17T00:00:00ZHow a text diff works: line, word and character diffs, the longest common subsequence, reading added and removed lines, and the whitespace traps to avoid.Time Zones, UTC and Daylight Saving, Explainedhttps://arraykit.com/learn/time-zone-converter2026-10-17T00:00:00Z2026-10-17T00:00:00ZUTC vs GMT, offsets such as +05:30 and +05:45, IANA zone names, why EST and IST are ambiguous, daylight saving shifts, and using a time zone converter well.AI Tokens and Token Counts, Explainedhttps://arraykit.com/learn/token-counter2026-10-18T00:00:00Z2026-10-17T00:00:00ZWhat a token is in AI, how many words a token is, why GPT, Claude and Gemini count the same text differently, and how tokens set context limits and API costs.ZIP Files, Explainedhttps://arraykit.com/learn/unzip-online2026-10-17T00:00:00Z2026-10-17T00:00:00ZWhat a ZIP file is, how ZIP compression works, why photos and videos barely shrink, how to unzip safely on any device, and how ZIP compares with 7z and RAR.UPI QR Codes, Explainedhttps://arraykit.com/learn/upi-qr-code-generator2026-10-17T00:00:00Z2026-10-17T00:00:00ZHow a UPI QR code works: the upi://pay link and its pa, pn, am, cu and tn fields, static vs fixed-amount codes, checking the payee name, and print sizes.URL Encoding (Percent-Encoding), Explainedhttps://arraykit.com/learn/url-encoder-decoder2026-10-17T00:00:00Z2026-10-17T00:00:00ZHow URL percent-encoding works: reserved and unreserved characters, %20 vs + for spaces, encodeURIComponent vs encodeURI, UTF-8 bytes and double-encoding bugs.UUIDs, Explained: v4, v7 and GUIDshttps://arraykit.com/learn/uuid-generator2026-10-17T00:00:00Z2026-10-17T00:00:00ZWhat a UUID is, how the 8-4-4-4-12 format works, UUID v4 vs v7 and the other versions, how unlikely a v4 collision is, and how UUIDs affect database indexes.How Video Compression Workshttps://arraykit.com/learn/video-compressor2026-10-17T00:00:00Z2026-10-17T00:00:00ZHow video compression works: codecs vs containers, bitrate and the file size formula, CRF quality settings, and how to get a video under 25 MB or 10 MB.VLOOKUP, INDEX MATCH and XLOOKUP, Explainedhttps://arraykit.com/learn/vlookup-formula-generator2026-10-17T00:00:00Z2026-10-17T00:00:00ZHow VLOOKUP works, why FALSE matters for an exact match, what causes #N/A, and when INDEX MATCH or XLOOKUP is the better lookup in Excel and Google Sheets.Spin the Wheel Pickers, Explainedhttps://arraykit.com/learn/spin-the-wheel2026-10-18T00:00:00Z2026-10-17T00:00:00ZHow a spin the wheel picker chooses a winner, whether a random name picker wheel is fair, how weights and duplicates change the odds, and tips for giveaways.Word Count, Explainedhttps://arraykit.com/learn/word-counter2026-10-17T00:00:00Z2026-10-17T00:00:00ZWord count explained: how words and characters are counted, words per page, reading and speaking time, and the limits for posts, meta descriptions and essays.Image Size, Resolution and DPI, Explainedhttps://arraykit.com/learn/image-resizer2026-10-13T00:00:00Z2026-10-13T00:00:00ZWhat image dimensions, DPI and file size mean, how to pick the right pixel size for web and print, and why resizing is not the same as compressing or cropping.Internet Speed Test Results, Explainedhttps://arraykit.com/learn/internet-speed-test2026-10-13T00:00:00Z2026-10-13T00:00:00ZWhat download speed, upload speed, ping, jitter and bufferbloat mean, how much speed streaming, calls and gaming need, and why you get less than your plan.JSON Formatting, Explainedhttps://arraykit.com/learn/json-formatter2026-06-25T00:00:00Z2026-10-13T00:00:00ZA complete guide to JSON: what it is, its data types and syntax rules, beautify vs minify, validation and JSON Schema, common errors, and best practices.JSON Web Tokens (JWT), Explainedhttps://arraykit.com/learn/jwt-decoder2026-06-25T00:00:00Z2026-10-13T00:00:00ZA complete guide to JWTs: the three-part structure, header and payload claims, signing algorithms, decoding vs verifying, and the security pitfalls to avoid.BPM, Tempo and the Metronome, Explainedhttps://arraykit.com/learn/online-metronome2026-10-13T00:00:00Z2026-10-13T00:00:00ZWhat BPM means, how tempo markings like Allegro map to beats per minute, how time signatures and subdivisions work, and how to practice with a metronome.The Pomodoro Technique, Explainedhttps://arraykit.com/learn/pomodoro-timer2026-10-13T00:00:00Z2026-10-13T00:00:00ZHow the Pomodoro Technique works: the 25-minute focus session, the 5-minute break, the long break after four, popular variations like 50/10, and tips that help.QR Codes: Error Correction, Size and Logos, Explainedhttps://arraykit.com/learn/qr-code-with-logo2026-10-13T00:00:00Z2026-10-13T00:00:00ZHow QR error correction works, how large a logo can be, what size to print, which colors scan reliably, and when to use PNG or SVG so your code always scans.Typing Speed and WPM, Explainedhttps://arraykit.com/learn/typing-test2026-10-13T00:00:00Z2026-10-13T00:00:00ZHow WPM is calculated, how net and raw speed differ, what counts as a good typing score, and how to type faster with fewer mistakes. Test your speed online.