Most calculation mistakes are not arithmetic, they are picking the wrong question. "What is 19 % of 250" and "250 is what percent of 19" are different sums that people reach for interchangeably. These calculators make you choose the question first, then show the formula they used.
That also makes them checkable. If the formula under the answer is not the one you meant, you picked the wrong mode, and you can see it immediately. The three money trackers take the same principle to paper: type your target, your debts or your bills, and the sheet that prints is your arithmetic, dated and totalled.
Questions
Why show the formula?
So you can tell whether the tool answered the question you meant. A bare number gives you nothing to check.
Is anything rounded?
Results display a few decimals. Nothing is rounded internally, so chained calculations stay accurate.
Do these handle VAT?
Yes, in both directions, which is the part people usually get wrong. Getting the net from a gross price means dividing, not subtracting the rate.
Do the money trackers need my bank details?
No. They work from the numbers you type: an amount, balances, due days. Each tool page states its data handling.