SimpleTax

SimpleTax is a small calculator for tax arithmetic. Its page describes it as free to download for iPhone, iPad, and Android, and states that it calculates before-tax, tax rate, tax amount, or total, entirely on the device. The site emphasises that the calculation works from whichever number you start with, uses currency-aware rounding, and keeps data local rather than sending it to a server. It is a single-purpose utility rather than a filing or accounting product: the page describes arithmetic, not returns or bookkeeping.