Pint is a Python package to define, operate and manipulate physical quantities: the product of a numerical value and a unit of measurement. It allows arithmetic operations between them and conversions from and to different units.
It is distributed with a comprehensive list of physical units, prefixes and constants. Due to its modular design, you can extend (or even rewrite!) the complete list without changing the source code. It supports a lot of numpy mathematical operations without monkey patching or wrapping numpy.
A command-line script pint-convert provides a quick way to convert between units or get conversion factors.
A simple app to make your calculations easier. Self-hostable.
Github repos for the three components: https://github.com/keepformula
Online app that converts file sizes into other units (i.e., megabytes into bytes, kilobytes into gigabytes).