Guides
Where the tools get tricky, explained with sources
49 guides on how the 12 Lumen Lab web tools calculate and how to use them. Numbers come from laws, official documents and runs we did ourselves, and every guide lists its sources and the date it was last checked.
Calculate
Daycount
Days between dates, countdowns, business days excluding holidays, age and anniversaries.
- How to count the days between two dates (and when to add 1)Subtract the earlier date from the later one, and add 1 only if both the first and last day count. Examples for countdowns, trips, leap years and spreadsheets.
- How to count business days and find a deadlineA business day is a weekday that isn't a holiday. How to find the date 5 or 10 business days away, whether today counts, and which US holidays are skipped.
- US federal holidays on a weekend: which day is observedA federal holiday on a Saturday is observed the Friday before; on a Sunday, the Monday after. Observed dates for 2026 to 2028, including New Year's Day 2028 on Dec 31, 2027.
- Adding months to a date: what happens to January 31January 31 plus one month is February 28 (29 in a leap year). How the end-of-month rule works, why adding a month twice differs from adding two, and how JavaScript differs.
- How to calculate your exact age in years, months and daysCount whole years to the last birthday, then whole months, then the days left. A worked example, birthdays at the end of a month, February 29 birthdays and age in days.
- ISO week numbers: why January 1 can be in week 53ISO weeks start on Monday and week 1 holds the year's first Thursday, so January 1, 2027 is in week 53 of 2026. The rule, 53-week years, and why Excel's WEEKNUM differs.
Scientific Calculator
Fractions and roots shown like a textbook, exact answers like √2/2, plus statistics, equations, matrices and base-n.
- 6÷2(1+2) = 1 or 9? Order of operations on a calculatorWhy 6÷2(1+2) gives 1 on some calculators and 9 on others, and the exact order of operations a scientific calculator uses: brackets, functions, powers, implied multiplication, then × ÷ and + −.
- σ vs s: population or sample standard deviation?The difference between population standard deviation σ and sample standard deviation s, when to use each, and a worked example with the data 2, 4, 4, 4, 5, 5, 7, 9. Also how they map to Excel STDEV.P and STDEV.S.
- Degrees vs radians: why sin 30 gives −0.988 on your calculatorWhy a calculator returns sin 30 = −0.988 instead of 0.5, what degrees and radians mean, the conversion formulas, a table of common angles and when to use each unit.
- Solving quadratic and simultaneous equationsThe quadratic formula and how the discriminant tells you whether roots are real, repeated or complex; when a system of linear equations has no solution or infinitely many; and how to use the equation solver.
- Permutations vs combinations: nPr and nCr explainedThe difference between permutations (nPr) and combinations (nCr), their formulas, how to enter them on a scientific calculator, and worked examples from lottery odds to PIN codes and card hands.
Takehome Korea
Korea net salary calculator laid out like a payslip, plus severance pay, hourly wage and weekly holiday allowance, with the rates and sources shown.
- How payroll deductions work in Korea: the four insurances and withholding taxWhat comes out of a Korean payslip each month: National Pension 4.75%, health insurance 3.595%, long-term care, employment insurance 0.9% and withheld income tax, with a worked example.
- Severance pay in Korea: who qualifies and how it is calculatedKorean severance pay is 30 days of average wages per year of service, for workers with at least one year and 15 hours a week. The formula, with the labor ministry’s sample case worked through.
- Korea minimum wage 2026 and 2027, and the weekly holiday allowanceKorea’s minimum wage is ₩10,320 an hour in 2026 and ₩10,700 in 2027. Why the monthly figure uses 209 hours, and how the weekly holiday allowance is worked out for part-time hours.
Text and files
Inplace
Video to MP3, image and HEIC conversion, PDF merge and split, CSV and Excel, all in your browser with no upload.
- HEIC vs JPG: what's the difference, and should you convert?What HEIC is, why iPhones use it, where it doesn't open, and three ways to get JPGs: the Most Compatible camera setting, automatic conversion on transfer, or converting files.
- 128 vs 192 vs 320 kbps: which MP3 bitrate should you use?MP3 size per minute at 128, 192, 256 and 320 kbps with the exact math, why 320 kbps is the ceiling, and why re-encoding a low-bitrate file at 320 adds nothing.
- Is it legal to convert YouTube videos to MP3?What YouTube's Terms of Service actually say about downloading, how copyright law differs from the Terms, and the ways YouTube does let you save audio and video.
- How to compress a PDF without uploading itWhy scanned PDFs get so big, what PDF compression changes (image resolution and JPEG quality) and leaves alone (text), how to fit Gmail's 25 MB limit, and when compressing won't help.
- How to remove a password from a PDFTake the open password or the printing and copying restrictions off a PDF you're allowed to open, why a forgotten open password can't be removed, and how to protect the file again.
- How to make a video small enough to emailWhat decides a video’s file size (resolution, bitrate, length), the attachment limits in Gmail and Outlook, how to trim and compress a video to fit, and when sending a link works better.
- WebP vs JPG vs PNG: which image format to useHow JPG, PNG and WebP differ in compression, transparency and file size, which suits photos, screenshots and logos, where WebP can still cause trouble, and how to convert between them.
- Turn a PDF into Markdown before giving it to an AIMany AI services read each PDF page as text plus an image. Converting to Markdown drops the page images. What OpenAI, Anthropic and Google document, what we measured, and when to keep the PDF.
Staycode
Free QR code generator with logos, frames and print files. No sign-up, never expires, and it checks that your code scans.
- Do QR codes expire? Static vs dynamic codesThe QR image never expires; dynamic codes that route through a provider can stop. How trials switch codes off, how to spot one, and what to do if printed.
- WiFi QR codes: format, special characters and hidden networksA WiFi QR code holds one line like WIFI:T:WPA;S:name;P:pass;; How password symbols are escaped, hidden and open networks, iPhone and Android, and sign tips.
- QR code print size: how big should it be?Close-up QR codes: at least 2 cm (0.8 in) wide, scanning from about 10 times their width. A size table, dense codes, quiet zone and sticker sheets.
- QR code with a logo: error correction, size and contrastA logo works thanks to error correction and uses up part of it. Error correction levels, logo size, and the 2:1 and 4:1 contrast rules of our scan check.
- vCard QR codes: what goes inside and how it savesA vCard QR code stores contact details as text; phones offer to add the contact when scanned. The lines stored, how fields add density, and print tips.
- How to scan a QR code safely: check the link before you open itQR scams send people to fake login or payment pages. Read the real domain before opening a scanned link: where to look and the warning signs.
Onesheet
Printable year-on-a-page and monthly calendars with holidays, as PDF, image or print.
- How to Print a Calendar on One Page (Letter vs A4)Why a calendar spills onto a second page or gets cut off, and the print settings that fix it: paper size, scale, margins, headers and footers.
- 2027 Long Weekends from US Federal HolidaysThe ten three-day weekends that US federal holidays create in 2027, with dates, plus the two Thursdays where one day off makes four.
- Week Numbers: ISO 8601 vs US Week NumberingTwo common ways to number weeks give different answers around New Year. How each works, with worked examples for 2026 and 2027.
- South Korea Public Holidays in 2027 (in English)All 24 public holiday dates in South Korea for 2027 with weekdays, including the substitute holidays and the two holidays added by the 2026 rule change.
Kankan
Word and character counter that shows every counting rule side by side, plus X, Instagram, YouTube and SMS limits.
- How X counts characters: emoji, links and the 280 limitX counts a weighted length, not characters: Latin letters count 1, Korean, Chinese and Japanese characters 2, every link 23 and each emoji 2. A table of examples counted with our tool and checked against twitter-text.
- Why a 161-character text costs two messagesOne SMS holds 160 GSM-7 characters, but split messages hold only 153 each, and a single emoji or curly quote drops the limit to 70. Tables of where the breaks fall, counted with our SMS tool.
- Characters, code points, bytes: why 👨👩👧 is 1, 5 or 18One emoji can be 1 character, 5 code points, 8 UTF-16 units or 18 bytes. What each number measures, a table of common cases counted four ways, and which one a length limit is likely to use.
Groups, practice, time and devices
Pickboard
Wheel spinner, ladder game, name picker and team generator from one list, with a replay link for every result.
- Is an online wheel spinner really random?Three things decide whether a wheel spinner is fair: where its random numbers come from, how they are turned into a slot, and whether the animation decides anything. Here is how Pickboard handles each.
- How to pick winners from a list without repeatsA short, checkable way to draw several winners from a list: clean the list, draw once, keep alternates, and share a link that replays the result.
- Ladder lottery (Ghost Leg): how it works and how fair it isA ladder lottery always gives everyone one result, but with a normal number of rungs people tend to land near where they started. We measured how much, and what fixes it.
Todok
Typing test and practice in English and Korean that shows which keys you missed and where you slowed down.
- How WPM is calculatedWPM is characters divided by five, divided by minutes. See worked examples of net and gross WPM and accuracy, the rounding we use, and the choices that make typing sites report different numbers.
- Touch typing basics: the home row and finger chartTouch typing starts with eight fingers resting on the home row. Here is the finger chart for every letter key, the order to learn the rows in, and how to tell which keys are holding you back.
- Why accuracy first makes you fasterA fixed typo costs three key presses for one character. See the arithmetic for 30 seconds of typing with and without errors, and how to use a missed-key map to practice the keys that slow you down.
Oneturn
A dial timer that rings on time in a background tab, a lap stopwatch, an alarm and a world clock with each city's next daylight saving change.
- Why online timers run late in a background tabWhy a web timer can ring up to a minute late in another tab or stop when a phone screen turns off, explained from Chrome's documentation, and how to avoid it.
- Daylight saving time 2026–2027: when clocks change and how time differences shiftWhen clocks change in 2026 and 2027 in the US, UK, EU, Australia and New Zealand, how the gaps between cities shift for a few weeks, and who stopped changing.
- Lap time vs split time on a stopwatchLap time is how long one lap took; split time is the total from the start to that point. How to read the same laps both ways and spot your fastest lap.
Checkbench
Test your keyboard, mouse, mic, webcam, speakers, screen and gamepad in one place. Nothing is uploaded.
- Keyboard typing double letters: how to confirm chatteringOne press, two letters? Confirm double-typing (chattering) with the keyboard tester, see how the 30 ms rule decides and rule out other causes.
- Key rollover and ghosting: test how many keys register at onceHold keys together and see how many your keyboard really reports. A practical way to test key rollover and spot ghosting or blocked combinations.
- Microphone not working in the browser: a permission checklistMic silent in a browser call? Run the mic test, read its message and follow the matching fix: site permission, system setting, device or another app.
- How to check a new monitor for dead pixels and the right refresh rateA ten-minute check for a new monitor or laptop screen: full-screen colors to find bad pixels, what to note down and the refresh rate it really runs at.
Fewmoves
See the moves and win rates at your rating from 17.4 million Lichess games, with 81 opening guides and a spaced-repetition trainer.