About Plainwren

Plainwren is a small collection of free calculators and converters for everyday jobs: loan payments, savings growth, VAT, invoices, pay, tips, calorie needs, percentages, dates, time zones, units, random picks, images, PDFs, QR codes, text and JSON. Each page does one job, shows how the answer is worked out, and gives a worked example you can check by hand.

How the tools work

Every tool runs entirely in your browser. When you press a button, the calculation happens on your own device; the numbers and text you type are not sent to us or anyone else. There is no sign-up and no account. Once a page has loaded, it keeps working offline.

How we check the maths

The formulas are standard, published ones (for example the reducing-balance loan formula and the official definitions of units). Each tool's calculation code has automated tests with hand-checked answers, including awkward cases such as leap years, months of different lengths, daylight saving changes and zero interest rates. Each page lists its assumptions and limits.

The tools give estimates and quick answers. They are not financial, tax, legal or medical advice. For decisions that matter, check the figures with the relevant professional or official source.

Credits

The passphrase option of the password generator uses the EFF Large Wordlist created by the Electronic Frontier Foundation, used unchanged under the Creative Commons Attribution 4.0 International licence. A copy of the list is stored on this site.

The QR code generator uses the QR Code generator library by Project Nayuki, under the MIT licence. The library and its licence notice are stored on this site. QR Code is a registered trademark of DENSO WAVE INCORPORATED.

The PDF merge and split tool and the JPG to PDF converter use pdf-lib by Andrew Dillon, under the MIT licence (it bundles small parts of other permissively licensed projects, listed in its licence notice). The library and its licence notice are stored on this site.

The PDF to JPG converter uses pdf.js by Mozilla (version 6.5.438, Apache-2.0). The library, its worker, fonts, character maps and licence notices are stored on this site.

The image to text (OCR) tool uses Tesseract.js and tesseract.js-core (Apache-2.0), which include the Tesseract OCR engine (Apache-2.0), Leptonica and other permissively licensed libraries, with language data from tessdata_fast (Apache-2.0). These files and their licence notices are stored on this site.

Time zone conversions use the IANA time zone database built into your browser.

Found a mistake?

If a result looks wrong, compare it with the worked example on the page and check which assumptions apply.