Tax arithmetic is where small businesses lose money quietly. Not through fraud or negligence — through a division done in the wrong direction. The classic case is removing VAT from a gross price by subtracting the rate, which takes off too much and leaves your revenue understated and your reclaim overstated. It is one of the most common errors an accountant will find, and it takes seconds to avoid.
The tools here handle the calculations that recur constantly: adding and removing tax at any rate, working out what a payment term means as a date, and understanding what money is actually available versus what is merely invoiced. They are built for the person doing the books on a Friday afternoon, not for a finance department with a system already in place.
As with every calculation on this site, the formula is shown alongside the result. Tax figures end up on documents that other people check, so being able to explain how you got to a number is worth more than getting it fast. If your accountant asks why a line reads the way it does, you should be able to answer.