Exact numbers for people who sell printed and digital products
Cover geometry, paper dimensions and marketplace fee maths. Every figure is computed from a stored specification, traced to the source that sets it, and stamped with the date it was checked.
One thing worth knowing up front. The most widely copied book-spine formula
online adds 0.06″ to the spine. That constant is not in the printer's published
specification, and it makes covers about 1.5 mm too wide — enough to push spine text
visibly off-centre. Nothing here uses it.
See the correct formula →
What is covered
Book covers
A perfect-bound cover is one flat piece of artwork — back, spine, front, plus bleed. The spine is the part that moves, and it moves with both page count and paper stock.
Paper sizes
Dimensions in millimetres, centimetres, inches and points, plus the pixel canvas at every resolution worth printing at.
Size comparisons
If you sell printables, the number you actually need is the shared safe area — the rectangle that prints without cropping on both sizes, so one file serves every buyer.
How this site works
Every number comes from one stored value. Paper is stored once in millimetres; cover geometry is stored once as the printer's published constants. Inches, centimetres, points and pixels are all computed from those at build time. Nothing is typed twice, so no two pages can drift apart — and when a specification changes, one edit updates every page that depends on it.
Figures that change — marketplace fees especially — carry the date they were last verified, and the build fails if any of them go stale. Read the methodology →
Common questions
What is SellerSpec?
A reference for the measurements and fee maths that people selling printed and digital products need to get right — cover geometry, paper dimensions and marketplace fees. Every number is computed from a stored specification rather than written by hand, and every source is dated.
Is it free?
Yes. Every page and calculator is free, with no account and no limit.
How do you know the numbers are right?
Constants are traced to the body that publishes them and carry the date they were last checked. Everything derived from them is computed at build time from one stored value, so two pages can never disagree. The methodology page explains the whole process.