Convert tools

Conversions look trivial until the edge cases arrive

Every tool in this category answers a question that seems too simple to need a tool: how many days, what percentage, how many kilograms. The reason they exist is that each one has a definition problem hiding inside it. Two people counting the days between the same pair of dates can get different answers and both be right, depending on which convention they were taught. The tools here pick a convention, state it, and show the alternatives rather than quietly choosing for you.

Dates: the counting convention is the whole question

Date difference and age both come down to whether you count the endpoints. Counting the interval between two dates — the arithmetic difference — is not the same as counting the days involved, which includes both ends and is therefore one larger. Contracts, notice periods, and hospital stays each tend to use one convention or the other, so the number you need depends on what the number is for.

Leap years complicate the second layer. Age in years is a calendar operation, not a division by 365.25: someone born on 29 February has a birthday that most years does not exist, and the sensible answers differ between jurisdictions. Both tools do their arithmetic on calendar dates in UTC rather than on local timestamps, because subtracting local times across a daylight-saving transition produces intervals of 23 or 25 hours and silently shifts the result by a day.

Percentages: two traps that cost real money

Percentage calculations go wrong in two specific ways. The first is confusing a percentage with a percentage point: a rate moving from 3% to 4% has risen by one percentage point but by 33% in relative terms, and financial and medical reporting frequently blurs the two. The second is assuming the operation is reversible. A price cut by 20% and then raised by 20% does not return to where it started — it lands 4% lower, because the second percentage applies to a smaller base. The same asymmetry is why a 50% loss requires a 100% gain to recover.

Units: same name, different quantity

Unit conversion is mostly mechanical, with one persistent hazard: several unit names refer to more than one quantity. A US gallon and an imperial gallon differ by about 20%. A US fluid ounce and an imperial fluid ounce differ too, in the other direction relative to their gallons. Tons come in short, long, and metric. Recipes, fuel economy figures, and shipping weights are where this bites, and the error is invisible because the number looks reasonable either way — so check which system the source figure came from before converting it.

Files: what survives a conversion and what does not

Merging and splitting PDFs preserves page content but not everything attached to it. Bookmarks pointing into the merged documents, and interactive form fields, do not survive the page-copy operation that merging uses; that behaviour is measured by an automated test on this site rather than asserted, so the claim stays true as the underlying library changes. If a document's outline or its form matters, keep the original.

QR codes involve a trade-off that is worth understanding before printing anything. Higher error-correction levels let a damaged or partly obscured code still scan, but they consume capacity, so the same payload produces a denser grid. A long URL at the highest correction level can become dense enough that a phone camera struggles at small print sizes. Shortening the URL helps far more than increasing the printed size.

Everything runs in your browser

The file tools in this category — PDF and QR — do their work locally. Your document is opened, rewritten, and handed back without ever being uploaded, which is both faster than a server round-trip and a straightforwardly better arrangement when the file is a contract, an invoice, or a scan of something personal. This is verified rather than promised: an automated test loads real files into the tools and fails the build if any network request carries data out.