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Base64 Encoder and Decoder — How It Works, With a Worked Example Encode and decode Base64 in your browser, and understand the 3-byte to 4-character algorithm, the padding rule, the 33% size overhead and why btoa fails on emoji. Age Calculator — Exact Years, Months and Days, and Why Tools Disagree Work out your age in years, months, days and total days from a date of birth, with the borrow algorithm shown, plus the month-end and leap-year edge cases that make calculators differ. Password Generator — Entropy in Bits, and What NIST Now Recommends Generate random passwords in your browser, with the entropy formula, a cracking-time table, why a 16-character password beats a complex 8-character one, and why forced complexity rules were dropped. Word and Character Counter — What Counts as a Word, and Why Tools Disagree Count words, characters, sentences and reading time, and understand hyphen and emoji edge cases, weighted character limits on X, and why Google truncates titles by pixel width not character count. BMI Calculator — The Formula, the 1832 Origin, and What It Cannot Tell You Calculate BMI in metric or imperial units, with the formula worked through, healthy weight ranges by height, the different thresholds used for Asian populations, and honest limits. Bill Splitter — Even Splits, Itemised Shares and the Missing-Cent Problem Split a restaurant bill fairly, with worked examples for even and itemised splits, proportional tax and tip, the one-multiplier shortcut, and how to handle leftover cents. Break-Even Calculator — Contribution Margin, Sensitivity and Operating Leverage Find the units and revenue you need to break even, with a fully worked example, a sensitivity table, margin of safety, operating leverage, and the assumptions that make the number misleading. Colour Converter — HEX, RGB, HSL and CMYK Worked Through One Colour Convert between HEX, RGB, HSL and CMYK with every formula shown on a single worked example, plus contrast ratios, why HSL lightness is not perceptual, and why printed CMYK never matches. Compound Interest Calculator — Frequency, APY, Fees and the Inflation Correction Project growth with the compound interest formula worked step by step, compare compounding frequencies and APY, test the rule of 72, and see what fees and inflation remove from the result. CSS Gradient Generator — Angles, the Grey Dead Zone and Banding Fixes Build linear, radial and conic CSS gradients, with the angle convention explained, the gradient-line length formula, why sRGB interpolation muddies colours, and how to kill visible banding. JSON Formatter and Validator — Every Way Valid JSON Goes Wrong Format and validate JSON in your browser, with the six value types, the syntax JSON does not allow, the large-integer precision trap, duplicate keys and prototype pollution explained. Lorem Ipsum Generator — Its Real Source in Cicero, and When Not to Use It Generate placeholder text by words, sentences or paragraphs, plus the actual origin of Lorem Ipsum in Cicero, why the 1500s story is unverified, and the design problems greeking hides. Pomodoro Timer — The Rules Most Apps Leave Out, and What the 25 Minutes Really Is A Pomodoro timer with the full method: the indivisibility and void rules, estimating work in pomodoros, the daily arithmetic, and an honest account of what the evidence does and does not support. Text Reverser — Characters, Words or Lines, and Why Reversing Breaks Unicode Reverse characters, word order or line order, and see the four levels at which naive string reversal fails — surrogate pairs, combining accents, flag sequences and joined emoji. URL Encoder and Decoder — Percent-Encoding, %20 versus Plus, and Double Encoding Encode and decode URLs correctly, with the unreserved and reserved character sets, multi-byte UTF-8 worked out, when to use whole-URL versus component encoding, and why %2520 appears. UUID Generator — 122 Random Bits, Version 7, and the Primary-Key Problem Generate version 4 UUIDs, with the bit layout explained, honest collision figures, why bad entropy causes real duplicates, and why random UUIDs make poor clustered primary keys. Case Converter — Title Case Rules, Programmer Cases, and the Turkish i Problem Convert text between upper, lower, sentence, title, camel, snake and kebab case, with the style-guide disagreements, the lossy conversions, and the locale bugs that break naive case changes. Roman Numeral Converter — The Rules, the Valid Forms, and What the System Cannot Do Convert numbers to Roman numerals and back, with the thirteen-pair algorithm, worked conversions, which forms are invalid, and why clock faces still show IIII. Percentage Calculator — Four Formulas, and the Mistakes That Cost Real Money Work out a percentage of a number, percentage change, and reverse percentages, with worked figures and the traps: percentage points, stacked discounts, margin versus markup and asymmetric recovery. Time Zone Converter — Why Meetings Drift, and What a Converter Cannot Know Convert times between zones, with the overlap arithmetic for real working hours, the three weeks each year when offsets change, and an honest account of the limits of any time zone data. Slug Generator — The Nine-Step Pipeline, and Why Transliteration Has No Right Answer Turn titles into URL slugs, with the exact processing order, why accented characters have no single correct transliteration, how slugs collide, and why changing one later is expensive. Regex Tester — Greedy Matching, Catastrophic Backtracking, and What Regex Cannot Do Test regular expressions against sample text, with the concepts that cause most bugs: greedy versus lazy quantifiers, flag behaviour, exponential backtracking, and the validation jobs regex should not be given. Reading Time Estimator — The Model, Its Assumptions, and What the Label Hides Estimate how long an article takes to read, with the measured speeds by reading mode, adjustments for images and code, and an honest look at whether the read-time label helps at all. Loan EMI Calculator — See the Real Interest You Will Pay Work out the monthly EMI on any loan, then see the full amortisation: how much of each payment is interest, how much kills the principal, and what the loan costs in total. Tip Calculator — Split a Bill and Tip Correctly Calculate a tip at any percentage, split the bill between any number of people, and see the difference between tipping on the pre-tax and post-tax total. Unit Converter — Length, Mass, Volume and Area, Done Exactly Convert between metric and imperial units using the exact defined factors, and understand where conversions go wrong: US versus imperial gallons, mass versus weight, and significant figures. Temperature Converter — Celsius, Fahrenheit and Kelvin Convert between Celsius, Fahrenheit and Kelvin, with the formulas, the point where the two scales meet, and the difference between converting a temperature and converting a temperature change. Water Intake Calculator — How Much You Actually Need Estimate a daily water target from your body weight, activity and climate, and learn why the eight-glasses rule is a misquotation and why urine colour beats any calculator. Macro Calculator — Protein, Fat and Carbs From Your Actual Numbers Work out daily calories with the Mifflin-St Jeor equation, then split them into protein, fat and carbohydrate using evidence-based ranges, with a full worked example. Calorie Burn Calculator — MET Values and What They Really Tell You Estimate calories burned in any activity using published MET values, understand the gross versus net distinction, and see why every burn figure carries a wide error bar. Ideal Weight Calculator — Four Formulas, and Why They Disagree Compare the Devine, Robinson, Miller and Hamwi ideal-weight formulas against the healthy BMI range, and see why a single ideal number is a clinical fiction. Investment Return Calculator — CAGR, Total Return and Real Return Turn a start value and an end value into an annualised return with the CAGR formula, then adjust for inflation to see what the gain was actually worth. Credit Card Payoff Calculator — What the Minimum Payment Really Costs See how long a credit card balance takes to clear at any monthly payment, how much of it is interest, and why the minimum payment can leave a debt outstanding almost indefinitely. Heart Rate Zone Calculator — Karvonen Method, Done Properly Calculate your five training zones from maximum and resting heart rate using the Karvonen reserve method, and see why the 220-minus-age rule is off by ten beats for most people. Unix Timestamp Converter — Epoch Time, Milliseconds and the 2038 Problem Convert Unix epoch timestamps to readable dates and back, tell seconds from milliseconds at a glance, and understand leap seconds, ISO 8601 and the 2038 overflow. Number Base Converter — Binary, Octal, Decimal and Hexadecimal Convert between binary, octal, decimal and hex, with the repeated-division method worked through, the nibble shortcut, and why hex exists at all. Prime Number Checker — Test Any Number and See the Factors Check whether a number is prime using trial division up to its square root, with worked examples, quick divisibility tests, and why 1 is deliberately excluded. GCD and LCM Calculator — Euclid Algorithm With Worked Examples Find the greatest common divisor and lowest common multiple of any numbers, using Euclid algorithm and prime factorisation, plus the identity that links the two. Ratio Calculator — Solve, Simplify and Scale Any Ratio Solve for a missing term in a proportion, simplify ratios to lowest form, scale recipes and mixes, and avoid the part-to-part versus part-to-whole mistake. Life in Weeks — See Your Whole Life as a Grid Convert your age into weeks lived and weeks remaining, and see why weeks are the unit that actually changes how people think about time. Year Progress — How Far Through the Year You Actually Are See the exact percentage of the year completed, with the day-of-year calculation worked through, leap year rules, and quarter and fiscal-year boundaries. Days Until — Countdown to Any Date, Including Working Days Count the days to any future date, see how many are working days, and avoid the inclusive-counting mistake that makes every countdown off by one. Time Since — Elapsed Time From Any Past Date Measure exactly how long ago something happened in years, months, days and hours, and understand why calendar months make elapsed-time arithmetic genuinely tricky. Sleep Debt Calculator — How Much You Owe and How to Repay It Track accumulated sleep debt against your actual need, and understand why one long weekend lie-in does not clear a week of short nights. Work Hours Calculator — Timesheets, Breaks and Overtime Add up worked hours across a week, convert between HH:MM and decimal correctly, subtract unpaid breaks, and calculate overtime without the classic decimal mistake. Retirement Countdown — Time Left and What It Costs Per Month Count the years and months to your retirement date, then see the monthly saving that funds it, why delaying ten years nearly doubles the cost, and where the 4% rule breaks. Life Percentage Calculator — How Much of Your Life Has Passed See what share of an expected lifespan you have used, and why the naive subtraction understates how long you are likely to live. Meeting Cost Calculator — What That Hour Actually Costs Price a meeting in salary terms using fully loaded cost, see what a recurring weekly meeting costs over a year, and count the hidden cost that salary alone misses. Salary Negotiation Calculator — What One Conversation Is Worth See the lifetime value of a starting-salary difference once annual raises compound, and what to weigh besides the base number. Freelance Rate Calculator — What You Must Charge to Match a Salary Work out an hourly freelance rate from a target income using billable utilisation, and see why matching a 60,000 salary needs roughly two and a half times the employee hourly rate. Savings Goal Calculator — Monthly Amount to Hit a Target by a Date Calculate the monthly contribution needed to reach a savings target by a deadline, with and without interest, and see how much the interest actually contributes. Cost Per Use Calculator — The Only Price That Matters Divide price by realistic lifetime uses to compare purchases honestly, including resale value and running costs, and see where the metric misleads. Email Tone Checker — Find the Hedging Before You Send Spot the hedges, apologies, intensifiers and passive constructions that weaken an email, with before-and-after rewrites and the limits of automated tone detection. Palindrome Checker — With the Unicode Problems Nobody Mentions Check whether text reads the same backwards, understand which characters get normalised away, and see why naive string reversal breaks on emoji and accents. Sort Text Lines — Alphabetical, Numeric, Natural and by Length Sort lines of text, deduplicate, reverse and shuffle, and understand why item10 comes before item9 and why Zebra sorts before apple. Steps to Calories Calculator — What 10,000 Steps Actually Burns Convert step count to calories using weight, stride and walking speed, see the gross and net figures, and where the 10,000-step target came from. Fasting Timer — Windows, Protocols and What the Evidence Says Track an intermittent fasting window across 16:8, 18:6, 20:4 and OMAD, see what happens at each stage, and read an honest summary of what trials actually found. Decision Matrix Calculator — Weighted Scoring Done Honestly Score options against weighted criteria to compare them systematically, with a worked job-offer example and a clear account of where weighted scoring misleads. Random Decision Maker — Fair Picks, and When to Actually Use One Pick randomly from a list, flip a coin or roll dice with genuinely unbiased randomness, and understand which decisions randomness is the correct tool for. Priority Matrix — Urgent Against Important, Used Properly Sort tasks into the four urgency and importance quadrants, see where time actually goes against where it should, and understand what the matrix cannot resolve. Habit Streak Tracker — What the Research Says About Chains Track consecutive days and consistency percentage, and learn why 21 days is a myth, why one missed day does not matter, and where streaks turn against you. Goal Calculator — Break a Target Into a Daily Rate Convert a large goal into the daily or weekly rate that reaches it, add realistic slack, and see where linear planning breaks down. Meeting Agenda Builder — Timeboxed Items With Named Outcomes Build an agenda where every item has a type, an owner, a timebox and a defined outcome, and understand why an agenda alone does not fix a bad meeting. Focus Timer — Pomodoro and the Intervals That Suit the Task Run focused work intervals with breaks, compare 25:5 against longer blocks, and understand why the standard interval is arbitrary and when it works against you. Cognitive Load Estimator — How Much You Are Actually Holding Estimate the mental load of your current open work against real working-memory limits, and learn why the famous seven-item figure was revised down to about four. Is This Meeting Worth It — A Test Before You Send the Invite Score a proposed meeting against its cost, whether a decision exists, and whether async would be slower, with an honest case for when meetings beat writing. Procrastination Cost Calculator — What Delay Actually Charges You Put figures on the cost of putting something off, including compounding financial loss, the rushed-work penalty, and why procrastination is an emotional problem rather than a scheduling one. Email Subject Line Scorer — Length, Spam Triggers and Truncation Check a subject line against mobile truncation limits, spam trigger patterns and specificity, and understand why open-rate benchmarks stopped being reliable in 2021. Negotiation Power Score — Where Leverage Actually Comes From Assess your leverage across alternatives, time pressure, information and replaceability, and see why improving your alternatives beats every tactic at the table. Life ROI Calculator — Purchases Priced in Hours of Your Life Convert a price into hours of your life using your real hourly wage after tax, commute and work costs, and see which spending genuinely returns time. Decision Flip — Using a Coin to Reveal What You Already Prefer Assign two options to a coin, then use your reaction to the result as the data, plus the reversibility, 10-10-10 and pre-mortem tests that come first. Name Value Calculator — The Arithmetic, and What It Does Not Predict Compute a name value under Pythagorean, Chaldean and ordinal systems, see the neat mathematical reason two of them always agree, and read an honest account of what numerology can tell you.

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