In the simplest case of a purchase there are two things which are knownthe Quantity (how many/much)
and the Cost per unit (or item)Then multiplying these two together givesthe Total cost
and this can be done in the top part of this calculator.It can also be used to work out what the 'Cost per unit' was if the 'Quantity' and 'Total cost' are known. Or the 'Quantity' if the other two are known. In other words, given any two values out of the three, the third can be worked out. And so, values can be entered in any two (out of the three) boxes at the top (and in any order). To do any of that it is not necessary to know what units are being used to measure whatever it is.
However, if required to make comparisons between different prices which are given for different units (like 1.45 per pound and 3.20 per kilogram) then it is necessary to know just what units are being used for the purchase.
This information is given under 'Comparative Costs per Unit'. Just make sure that, at the top, the entry under 'Cost per unit' is the correct one for the selected 'Units are'. If it is only needed to make comparisons, the 'Quantity' could be entered as '1'. Remember TWO entries are needed.
Values are given to 2 decimal places. This should cover the normal needs of any currency. For instance
et.ceteraThis does mean that a value of less than 0.01 will not be shown. This could be a drawback if it was necessary to get a more accurate 'Comparative cost'. It can be overcome by putting in a 'Cost per unit' that is 100 or 1000 times bigger than it actually is, and then dividing by that (same) amount when considering the values under the heading 'Comparative Costs per Unit'. Remember that the values at the top would also be affected by this change, but it requires a little more thought to make the necessary correction.
There is also an upper limit of 1 billion on the displayed values. This should not be a problem for most people!