Guides
What happens to a file you upload, what a document reveals about you, and which format to choose. Written to answer the question, not to sell an upgrade.
Money and business18
- What is the difference between margin and markup?Markup is profit as a share of your cost; margin is profit as a share of the price you charge. A 50% markup is a 33% margin. Because cost is the smaller number, markup always looks bigger.
- How does compound interest actually work?Compound interest pays interest on interest already earned, so the balance grows on a curve rather than a line. At 8% a year, money doubles roughly every nine years — and doubles again every nine years after that.
- Avalanche or snowball — which clears credit card debt faster?The avalanche method targets the highest interest rate first and always costs less. The snowball targets the smallest balance first and costs more, but clears accounts sooner. The gap is usually smaller than people expect.
- Why do you owe more than the car is worth?A car loses roughly a tenth of its value on the drive home and about a fifth in the first year, while a level-payment loan repays very little principal early. Most buyers owe more than the car is worth for two to three years.
- Is renting throwing money away?No. Buying costs roughly three percent to enter and six percent to leave, plus around 2.6 percent of the home value every year in tax, insurance and maintenance. Below about five years, renting is usually cheaper.
- How much do you actually need to retire?Multiply the annual income you want by about 25 — that is the 4% rule, drawn from historical US data suggesting a portfolio lasts thirty years. Later research argues for 3 to 3.5%, which means 28 to 33 times instead.
- Does the 50/30/20 budget actually work?It works as a diagnostic rather than a plan. Half of take-home pay to needs, thirty percent to wants, twenty percent to saving and debt. Where the split fails is usually housing, which alone can consume most of the needs half.
- Is your pay rise actually a pay rise?Only if it beats inflation. A rise below the inflation rate over the same period is a real-terms pay cut, however positive the number looks. Divide by inflation rather than subtracting it: (1 + rise) ÷ (1 + inflation) − 1.
- APR vs APY — what does the difference actually cost?APR is the nominal rate, ignoring compounding within the year. APY is the effective rate, including it. A card at 22% APR compounded monthly has an APY near 24.4% — that higher figure is what you actually pay.
- Why does removing VAT not give you back the original price?Because the percentage is charged on the net price, not the gross. Adding 20% to 100 gives 120, but taking 20% off 120 gives 96, not 100. To remove VAT you divide by 1.20 rather than subtracting.
- Why does total return tell you so little?Because it has no time in it. Turning 10,000 into 15,000 is a 50% return either way, but over three years that is 14.5% a year and over ten it is 4.1% — one beats the market and the other loses to inflation.
- Why is 50% off plus 20% off not 70% off?Because the second discount applies to the already-reduced price, not the original. Fifty percent off then twenty percent off leaves you paying 40 percent of the original — a 60 percent discount, not 70.
- Should you refinance your mortgage?Only if you will hold the loan past the break-even — closing costs divided by the monthly saving. A better rate over a longer term routinely lowers the payment while raising the total interest, so compare lifetime cost, not the monthly figure.
- What is a good debt-to-income ratio?Under 36% of gross monthly income is comfortable and under 28% on housing alone is the classic guideline. Most lenders will go to 43%, and to around 50% with strong compensating factors. Above that, approval depends on the exception rather than the ratio.
- How much should I be saving each month?A common target is twenty percent of take-home pay, but the order matters more than the number: emergency fund first, then any employer pension match, then high-interest debt, then long-term investing.
- How much can I actually borrow for a house?Most lenders cap housing costs near 28 percent of gross monthly income and total debt payments near 36 percent, with 43 percent a common hard ceiling. Those ratios, not the sale price, decide what you can borrow.
- How do I convert a salary to an hourly rate?Divide annual salary by 2,080 hours for a standard forty-hour week, which assumes exactly fifty-two weeks. The figure understates a contractor rate substantially, because it excludes payroll taxes, unpaid leave and benefits an employer otherwise covers.
- How do I split a bill fairly when orders differ?Tip on the pre-tax total unless local custom says otherwise. Split an uneven bill by charging each person their own items and dividing tax and tip in proportion to those amounts rather than by headcount.
Health and fitness12
- How do you set your macros?Set protein first, from bodyweight — roughly 2.2 g per kg when cutting, 1.6 to 1.8 otherwise. Then set fat at about a quarter of calories, never below 0.6 g per kg. Carbohydrate is whatever calories remain.
- What do heart rate training zones actually mean?Zones are percentages of heart rate reserve — your maximum minus your resting rate — added back to resting. With a maximum of 187 and a resting rate of 60, zone 2 at 60 to 70 percent is 136 to 149 beats per minute.
- When is the fertile window?Ovulation happens about 14 days before your next period starts, not 14 days after your last one began. The fertile window is roughly the five days before ovulation plus the day itself, because sperm survive far longer than the egg does.
- How accurate is a one rep max estimate?Accurate enough at low reps, unreliable at high ones. From a set of three, four standard formulas land within about six kilos of each other on a 100 kg lift. From a set of twelve, they spread by nearly sixteen.
- How much water should you actually drink?Roughly 35 millilitres per kilogram of bodyweight, plus about 500 more per hour of exercise. Around a fifth of that arrives in food, so a 70 kg person needs about 2.5 litres total and roughly 2 litres from drinks.
- Why does every body fat method give a different number?Because none of them measures fat directly. The tape method infers it from circumferences, calipers from pinch thickness, scales from electrical resistance. Expect a spread of three to five points between methods on the same body, the same day.
- Is there really an ideal weight?No. The four clinical formulas disagree by around three kilograms at the same height, and none was designed as a health target — they were built to calculate drug doses. A healthy BMI range spans about twenty kilograms.
- Kilojoules, calories and kilocaloriesOne kilocalorie is exactly 4.184 kilojoules. The "calories" on a food label are kilocalories, so an 8,700 kJ daily reference is about 2,079 kcal. Dividing kilojoules by four is a useful estimate but runs roughly five percent high.
- How much sleep do I actually need?Most adults need seven to nine hours. The proportion of people who genuinely function well on six or less is very small — around one percent — and almost everyone who believes they belong to it does not.
- Is BMI accurate?BMI is a useful population statistic and a crude individual one. It cannot distinguish muscle from fat or say where fat is stored, which is why waist measurement adds information it structurally cannot provide.
- Why does a calorie deficit stop working?A deficit is energy eaten below energy spent. The familiar 3,500 calories per pound is a static estimate from 1958; it holds early on and overstates later loss, because a smaller body spends less and the deficit narrows on its own.
- How is a pregnancy due date worked out?The standard method adds 280 days to the first day of the last menstrual period, so week one precedes conception by about two weeks. It assumes a 28-day cycle with ovulation on day fourteen, which many cycles do not follow.
PDFs and documents6
- Is it safe to upload documents to an online converter?Usually, but not always, and you cannot verify it. Uploading sends a complete copy of your document to a company’s servers, where retention, staff access and breach exposure are governed by their policy rather than by anything you control.
- What your PDF is telling people, and how to checkA PDF usually carries your name, the software you used, creation and modification times, and sometimes GPS coordinates from embedded photos. Black boxes drawn over text almost never delete it — the words remain selectable underneath.
- How do you actually reduce a PDF’s file size?Almost all PDF bulk is embedded images, so shrinking their pixel dimensions is the fix that works. Removing pages, fonts or metadata saves far less than people expect, and re-compressing already-compressed scans usually degrades quality for almost nothing.
- How to reverse PDF page orderReversing rewrites the list of pages a PDF carries, not the pages themselves, so nothing is re-encoded and no quality is lost. A scan usually comes out backwards because a sheet feeder stacks output face-up, leaving the last sheet on top.
- Why does my document print with the wrong margins?A4 is 210 by 297 mm and US Letter is 216 by 279. Letter is wider and shorter, so neither fits inside the other — printing one on the other clips an edge or leaves an uneven margin.
- What should I check before sending a PDF?Fix page order and rotation first, then add numbering and any watermark, then flatten the form fields, and strip metadata last. Doing metadata first is wasted, because every later edit writes new metadata over it.
Images and photos6
- PNG, JPG or WebP — which should you actually use?Use JPG for photographs, PNG for logos, diagrams and anything needing transparency, and WebP when you want either of those roughly a third smaller. The deciding question is whether the image has hard edges or smooth gradients.
- Why did my file get bigger after converting it?Almost always because the new format is a poor fit for that content — a photograph saved as PNG, or an already-compressed image re-saved losslessly. Compression is not a property of the file; it is a bet about what the content looks like.
- What does a photo reveal about you?A phone photo usually records GPS coordinates accurate to a few metres, the exact date and time, the device model and its serial-adjacent identifiers. Anyone who receives the original file can read all of it in seconds, without special tools.
- What favicon sizes do you actually need?Six: 16 and 32 for browser tabs and bookmarks, 48 for Windows, 180 for apple-touch-icon, and 192 and 512 for Android home screens and install prompts. Anything beyond those is legacy.
- What aspect ratio should my image be?An aspect ratio is width divided by height, and it survives any proportional resize. Changing it requires either cropping away content or distorting what remains, so the choice is which loss you prefer rather than whether to accept one.
- How do I get an image under a specific file size?Encoders take a quality setting, not a size, and the resulting bytes depend on image content. Hitting a target requires encoding repeatedly at different qualities and keeping the largest result that fits, which is what a target-size tool automates.
Text and writing2
- What reading level should I write at?Aim for a Flesch Reading Ease of 60 to 70, roughly an eighth to ninth grade level, for general audiences. The formulas count syllables and sentence length only, so they measure surface difficulty rather than whether the writing is clear.
- Why does text copied from a PDF break onto every line?A PDF stores positioned glyphs rather than flowing paragraphs, so a copy reconstructs line breaks from where text sat on the page. Every visual line becomes a real newline, which is why the paste arrives broken.
Code and data12
- Why does my JSON say invalid when it looks fine?Almost always a trailing comma, single quotes, an unquoted key or a comment. JSON is far stricter than JavaScript object syntax, and every one of those is legal in a JavaScript file and illegal in a JSON document.
- Is it safe to paste a token or config into an online tool?Only if the tool runs in your browser. A JWT is a bearer token — whoever holds it can use it until it expires — so pasting one into a decoder that posts to a server has handed over a working credential, not a puzzle.
- How should I store dates and times in a database?Store an instant as UTC — a Unix timestamp or a timestamp with time zone. Store a future appointment as local time plus its IANA zone name, because the offset for that zone may change before the date arrives.
- What actually makes a password strong?Length, and being unique to one account. A long passphrase of ordinary words resists guessing better than a short string of symbols, because attacks scale with the number of possibilities, and each character multiplies them.
- What is a hash function, and why can it not be reversed?A hash function turns any input into a fixed-length fingerprint. The same input always gives the same output, and the output reveals nothing about the input — there is no reverse operation, only guessing.
- Why does my text show as é or ’?Text was written in one encoding and read in another. é means UTF-8 was read as Latin-1; a black diamond question mark means a byte could not be decoded at all.
- How do regular expressions actually work?A regular expression describes a pattern rather than a literal string. Most of the language is a handful of character classes, quantifiers and anchors — perhaps a dozen constructs cover the overwhelming majority of real use.
- Which colour format should I use in CSS?Hex and RGB are the same sRGB channel values in different notation. HSL is easier to reason about but its lightness is not perceptual. OKLCH is perceptually uniform, so equal lightness values genuinely look equally bright.
- Why does my CSV break when I convert it?CSV carries no type information, so every value arrives as text and identifiers with leading zeros lose them. JSON carries nesting that CSV has no way to express, so flattening is a choice the converter makes on your behalf.
- Is minifying CSS and HTML still worth it?Minification removes characters; compression encodes repetition. Because gzip and brotli already handle repeated whitespace efficiently, minifying before compressing typically saves far less than the raw character count suggests — often only a few percent.
- Why does my 1 TB drive show as 931 GB?Manufacturers count a terabyte as 1,000,000,000,000 bytes while Windows divides by 1,024 three times and still labels the result GB. The bytes are all present; the two are using different definitions of the same prefix.
- What does /24 mean in an IP address?The number after the slash counts how many leading bits identify the network. A /24 fixes the first 24 bits, leaving 8 for hosts — 256 addresses, of which 254 are usable once the network and broadcast addresses are excluded.
Measuring and converting11
- The unit conversions people get wrong, and whyThe three big ones: applying a linear factor to an area or volume, treating feet-and-inches or stone-and-pounds as decimals, and converting a temperature difference as though it were a temperature. Each is a systematic error, not a rounding one.
- Horsepower, PS and kilowattsMechanical horsepower is 745.7 watts and metric horsepower, written PS or CV, is 735.5 watts. The same engine therefore carries a slightly higher number in PS than in hp. Kilowatts are the unit vehicle registration documents actually use.
- How much paint, flooring or wallpaper do I need?Measure the area, add a waste allowance set by the material and the method, and round up to whole units. The allowance is not padding — it covers offcuts, uneven surfaces and pattern matching.
- Why does my recipe not work when I convert it?Volume measures vary by country and by how you scoop. A cup of flour ranges from 120 to 150 grams depending on technique alone, which is why weighing is the only repeatable method for baking.
- Why is MPG a misleading way to compare cars?MPG measures distance per fuel and L/100km measures fuel per distance, so they are reciprocals rather than a scale. That inversion makes equal MPG gains represent wildly unequal fuel savings, an effect known as the MPG illusion.
- Are inches and centimetres an exact conversion?Most everyday imperial-to-metric factors are exact by definition, not measured. An inch is exactly 25.4 millimetres and a pound exactly 0.45359237 kilograms, both fixed by international agreement in 1959.
- How do I work out what an appliance costs to run?A watt is a rate and a kilowatt-hour is a quantity. Running cost is power in kilowatts times hours used times price per kilowatt-hour, so a high-wattage appliance used briefly can cost less than a modest one left on.
- How is a GPA actually calculated?A GPA is a credit-weighted average: multiply each course grade point by its credit hours, add those products, and divide by total credit hours. A four-credit course therefore moves the average twice as much as a two-credit one.
- What does a property listing actually measure?Floor area depends on where the measurement stops. Gross external, gross internal and net internal areas can differ by twenty percent for the same building, and listings rarely say which standard they used.
- How do I work out my running pace?Pace is time divided by distance, the inverse of speed. Because it is a reciprocal, a fixed time saving per kilometre is a much larger effort at fast paces than at slow ones, which is why pace targets get harder as they improve.
- Why do different countries count age differently?Most of the world counts completed years since birth, so age increases on a birthday. East Asian reckoning traditionally counted a newborn as one and added a year at new year, making a person up to two years older by the same date.