Eight reference pages and five calculators for preparing artwork for print. Every figure is either computed in front of you or carries the source and the date it was checked.
Why this exists
Most prepress answers online are repeated rather than derived, and the repetition carries errors. The top result for "gsm to lb" gives three mutually contradictory answers on one page. The consensus KDP spine formula includes an additive term that belongs to Lulu, making every answer about thirteen per cent too wide. A widely-cited resolution page attributes "150 lpi" to ISO 12647-2, which specifies a range and never names a single value.
So the working is shown here. Where a number is computed, the formula and its constants are on the page. Where it comes from a printer or a standard, it is linked and dated on the sources page. Where it could not be verified, that is said rather than glossed.
What is deliberately absent
There is no creep or shingling calculator, even though nothing on the web offers one and the gap is obvious. No printer publishes a numeric creep allowance — I checked seven — so any figure here would be a derivation presented as a specification, which is exactly the failure this site is a reaction to.
Named colour systems are not covered either. Those libraries are licensed data rather than published specifications, which puts them outside what this site can source and stand behind.
Corrections
Wrong numbers here cost someone a print run. If you find one: bigolronald@gmail.com.