A small reference site for the numbers involved in preparing artwork for print, built because the existing answers are mostly repeated rather than derived — and repetition carries errors forward.
How figures are checked
Where something can be computed, it is computed on the page from a stated formula, so the arithmetic is inspectable rather than asserted. The paper size tables are generated from the ISO 216 rule at build time rather than transcribed, which is why A1 is correct here.
Where a figure has to come from a printer or a standards body, it is linked and dated. Where it could not be verified from a primary source, the sources page says so instead of quietly including it.
What it will not tell you
It cannot tell you what your printer will accept. Requirements vary by a factor of three on bleed and eight on safe margin, and the only authority on your job is the company running it. This site is for understanding what the numbers mean and where they came from.
There are no affiliate links, nothing sponsored and no advertising, which is why it can say that a printer's own spec page contains an arithmetic error.
Corrections
[email protected]. A wrong figure here costs someone a print run, so corrections are genuinely welcome.