Methodology

This site exists because the numbers it publishes are easy to get wrong and widely got wrong. That only means something if you can check how they are produced.

One stored value per fact

Every physical dimension is stored exactly once. Paper is held in millimetres; book trim sizes are held in inches, because that is the unit the printer specifies them in. Every other representation — centimetres, points, pixels at any resolution — is computed from that single value when the site is built.

This is not a style preference. Hand-entered conversion tables drift: someone rounds 210 mm to 8.27 inches on one page and 8.3 inches on another, and both get copied onward. Computing everything from one source makes that class of error impossible rather than merely unlikely.

Constants are traced, not remembered

Formula constants — paper caliper per page, bleed allowances, fee percentages — are taken from the organisation that sets them, stored with a link to that source and the date they were checked, and never retyped from a secondary article.

A concrete example of why: the book spine formula circulating in most tutorials and calculators is (pages × thickness) + 0.06″. The printer's own specification has no such constant. It is page count times thickness, full stop. The extra 0.06″ makes a cover about 1.5 mm too wide, which is enough to shift spine text off-centre. That error has been copied between sites for years because nobody went back to the source.

Staleness is enforced, not hoped for

Physical standards barely move. Marketplace fees move constantly. So the two are treated differently: every fee schedule carries a verified_on date, and the build fails if any is older than 90 days. The site cannot silently serve a stale fee, because a stale fee stops the site publishing at all.

Worth being precise about how that check happens, because it is less automatic than it sounds. Marketplace fee pages resist automated reading — one returns 403 to any bot, one requires a signed-in account, and one renders its fee tables in JavaScript so the figures never appear in the page source. A scheduled job still runs weekly, but its job is to prompt a person before the deadline, not to confirm the rates itself. The rates on this site are re-read by hand, from the official page, in a normal browser.

Nothing here is ever rewritten from a scraped page. A layout change or a careless pattern match would publish a wrong number, which is the single failure this site cannot absorb — so detection is automated and the correction is not.

What is deliberately not published here

• Numbers that cannot be traced to a source. If a figure only exists in secondary blog posts, it is left out rather than repeated.
• Every possible page. Fifty-one paper sizes could be cross-multiplied into around 1,275 comparison pages. Roughly twenty of those are ever searched for, and publishing the rest would be padding. Comparisons are curated, not generated.
• Thin per-value pages. A trim size and paper stock get one page covering every page count, rather than one page per page count. The reader wants their number and the ones either side of it, on a page they can scan.
• Advice dressed up as data. Where something is judgement rather than measurement — what margin is healthy, which trim size suits a genre — it is written as such.

Sources

• Amazon KDP - Paperback cover dimensions — cover geometry, paper caliper, bleed
• Etsy fee schedule — fee rates and conditions
• eBay fee schedule — fee rates and conditions
• Amazon fee schedule — fee rates and conditions
• ISO 216 and ANSI/ASME Y14.1 — paper series dimensions

Corrections

If a number here is wrong, it is wrong on every page that derives from it, and fixing the stored value fixes all of them at once. Corrections are logged publicly in the data changelog with what changed and why.

Independence

Not affiliated with, endorsed by or operated by Amazon, Etsy, eBay or any other marketplace. Marketplace names are used only to identify whose published schedule a figure came from.