How to find the cheapest toilet paper per sheet

Divide the pack's price by its total sheets (rolls × sheets per roll), then multiply by 100 to get the price per 100 sheets. The pack with the lower number is cheaper per sheet.

The method

Find the sheets per roll on the pack. It's usually in small print near the roll count.
Multiply by the number of rolls to get the total sheets.
Divide the price by the total sheets and multiply by 100.
Do the same for every pack and compare.

A worked example

With made-up prices (they're examples, not real store prices):

Pack A: Charmin Ultra Soft Mega XL, 6 rolls × 312 sheets = 1,872 sheets. $9.99 ÷ 1,872 × 100 = 53.4¢ per 100 sheets.
Pack B: Quilted Northern Ultra Soft & Strong Mega, 12 rolls × 255 sheets = 3,060 sheets. $13.49 ÷ 3,060 × 100 = 44.1¢ per 100 sheets.

Pack B (Quilted Northern Ultra Soft & Strong Mega, 12 rolls, $13.49) is the better deal: 17% cheaper per sheet than Pack A (Charmin Ultra Soft Mega XL, 6 rolls, $9.99).

Why shelf tags mislead

Store unit prices use different units: per roll, per 100 sheets or per square foot, sometimes on the same shelf.
"Per roll" hides the roll size. A Mega roll from one brand isn't a Mega roll from another.
"= N regular rolls" uses each brand's own regular roll, and they differ (52 sheets at Charmin, 63 sheets at Quilted Northern and 80 sheets at Angel Soft). Kirkland Signature doesn't make that claim at all and just prints the sheet count.

Per sheet or per square foot?

Per sheet is the simplest. When sheet sizes differ, price per square foot is fairer. The calculator shows both, and Area × ply if you care about thickness.