A borrow is not a sale, and it is not a payment either. In Kindle Unlimited nothing is owed to you at the moment a reader downloads your book; you are paid for pages they actually turn, which means a title with hundreds of borrows and a weak middle section can earn less than a shorter book nobody abandons.
That single fact makes two numbers matter more than the borrow count: the normalised page length of your book, and the proportion of it an average reader finishes. This calculator holds both, multiplies them by the per-page rate you supply, and turns the result into a per-borrow figure, a monthly projection and a reverse calculation showing how many reads a given income target would need.
Read-through is the lever most authors ignore. Take a 300-page book and 100 borrows in a month. At full completion you are paid for 30,000 pages; at 70 per cent completion, 21,000; at 40 per cent, 12,000. Nothing about the book's length, price or borrow count changed between those three lines, and the earnings differ by more than half.
It means the page count is derived from the book rather than from any one reader's screen. Because font size, device and margins all change how many screens a book fills, a raw screen count would pay two authors differently for identical text.
Because the rate is set after the fact, from a fund whose size and total pages read both change every month, and any figure written into this page would be stale within weeks. The calculator ships with defaults so the arithmetic is visible, and a custom field so you can replace them.
Compare pages read against the pages that were available to read. If a hundred borrows of a 300-page book produced 21,000 pages, readers got through 70 per cent on average. It is the one figure here you can actually change by editing.