Points and picas for paper dimensions
Connect layout units with inches and millimeters.

In the digital layout convention used by this site, an inch contains 72 points or 6 picas. A pica contains 12 points.
A Letter example
An 8.5 × 11 inch Letter page is 612 × 792 points. In picas, it is 51 × 66. The paper stays the same size; only the unit changes.
To convert millimeters to points, multiply by 72 ÷ 25.4. To convert millimeters to picas, multiply by 6 ÷ 25.4. Keep enough precision while designing and round only the displayed result.
Choose pt or pc in the site’s unit selector to inspect a format.