Most of the effort in reporting a disposal is not the arithmetic — it is assembling the acquisition history in one place, in a consistent form, with nothing missing. That assembly is what the calculator here helps with.
What it does
You enter acquisition lots: a date, a quantity, a price per unit and, if you paid one, a fee. It sums the lots, computes the total quantity and the total consideration, divides one by the other for an average per unit, and — if you state a disposed quantity — matches that quantity against the earliest lots first and reports the resulting acquisition cost and the remaining position.
It shows every intermediate figure. The point is that you can check it, not that you should believe it.
What it does not do
It does not know your jurisdiction. Which fees may be included in the basis, whether the earliest-lot convention is required, permitted or forbidden, how corporate actions adjust the figures, how transfers between accounts carry basis with them, how identical securities in different accounts interact — all of that is jurisdiction-specific law, and none of it is encoded here.
It does not know your circumstances, your account history or your prior filings. It does not compute tax, it does not tell you what to declare, and it is not advice.
Blank fields stay blank
If you leave a fee empty, no fee is assumed. The calculator reports how many of your lots carried a fee and totals only those, rather than quietly treating a blank as zero and understating your costs.
A lot missing a quantity or a price is not counted at all, and the number of ignored rows is stated. A silently dropped row is the worst outcome available in a calculation like this.
Where the numbers should come from
Your broker's confirmations and annual statements are the record. Where you were an insider and the acquisition was reported publicly, the filing states the date, the quantity and the price where a price applied, and that filing is checkable against the original.
Note the distinction that catches people out: securities received as compensation are not purchases, and their basis is determined by rules about the value at vesting rather than by anything you paid.