It's a fundamental question when you're publishing your book, but it can be a mind-boggling one when it's your first time going through the process.
In theory, your cover size can be anything you want - but which of the many options is going to be best for your particular book?
There's a wide range of standard trim sizes on the market - and then some that aren't so standard. As I'm talking mainly about digital printing for self-publishing authors here - rather than the more expensive, large-scale, offset printing process - I'll stick with talking about standard sizes.
I'd love to tell you that there's an absolute rule - a formula to make it easy to 'calculate' yours - but there isn't.
What I can do is give you some guidelines that will help you choose a trim size that's appropriate, functional and cost-effective for your particular book - and will also look appealing (this is a must if you're going to sell it well).
First things first: what the hell's a 'trim size'?
Essentially, 'trim size' is the publishing term for your book cover dimensions, and is given width x height.
Say you're having a paperback published. When your book is manufactured, all the internal pages are assembled together in a big stack, then the cover is mechanically wrapped around the stack of pages, and glued into place at the spine, holding all the pages in place. At this point in the manufacturing, your book has extra material all around the open edges, which gets trimmed off at the end of the process to give a clean, even finish - hence the finished size of your book is called the 'trim' size.
If you're in Europe this will be in millimetres, if you're the US, it'll be in inches. If you're in the UK, like the many things we can't decide upon as a nation, it can be in either - depending on which publisher, or what size your decide on (just to be extra confusing).
Printers will charge more per-page to trim larger sizes of books - but on the flip-side of this, your costs could be reduced if there are fewer pages in your book (because you've opted for for a bigger trim size).
Secondly: your word count, and how much space you’d like in your layout (which you decide when your book is being typeset/formatted) will both have a bearing on how thick your book will end up being.
If you have a large word count - something approaching War And Peace - then a small trim size would result in a very chunky spine (think pocket dictionaries, or those small bibles you get in hotels!). This could make it easy to damage, clumsy for the reader to use and uncomfortable to hold in the hand, so it's best avoided. Unless you're specifically aiming at this type of format (perhaps with the intention of it being a portable cube of a book).
On the other end of the scale, if you only have a slim volume (imagine a novella or poetry book sort of size), then a large book format isn’t going to be cost-effective, because you’re paying for space (and trimming) you don’t really need.
Each printer has a minimum spine size that they can physically produce. If your page count is so small that the spine falls below this minimum, there's not enough bulk to glue it properly. If this is the case, the printer will likely recommend a stitched (usually called saddle-stitching) or stapled spine (and look more like a pamphlet). This is something else you want to avoid - publishing a pamphlet has way less kudos than publishing a book (however small it's printed).
Also think about whether you'd like to include images and diagrams in your book.
Your book needs to be big enough to accommodate these at a size where the reader can view them properly. There's no point in going to the trouble of including a useful diagram or a stunning photo if your reader needs a magnifying glass to see them because your book is so tiny.
Thirdly: study other books within the same genre as yours.
Have look through your own bookshelves - or visit a bookshop armed with a tape measure - to get a good idea of what feels 'right' in your hand, and what works for your own genre.
Aim for a similar size for your own book so that - along with a fitting cover design - it'll look at home on the shelf with these other titles, and be believable as a contender for an authority within its genre.
Your book needs to answer readers pre-conceived notions about the genre and meet their expectations*.
As an example, a standard contemporary fiction paperback is around 5”x 8”. Most fiction books are printed in this size, and it's a size that is widely available to self-publishers, so and easy option to go with for your own book.
Many business books tend to have a trim size around 6” x 9.25”. This gives more impact (literally, in size, and also from a marketing aspect), and from a practical point of view, the larger size allows more space for the text to breathe in the layout.
These books usually contain a mass of heavyweight information, so spacing it out allows the reader to digest everything more fully and find reference points more easily. The larger trim size also gives space for the author to include clear, readable diagrams.
There are many different books sizes, from different countries and publishing standards. In the good old British publishing industry (we love our traditions, don't we?), some sizes have an assortment of names - Demy, Royal, Crown etc. These hark back to the folded paper sizes (in inches) used for books at the dawn of machine-press publishing. Different self-publishing companies will have different options available, so if you see these terms, don't panic, they usually give the dimensions as well, and you can choose the most fitting.
If you’re using Amazon's Print On Demand service (KDP Print) or other POD publishers like IngramSpark, all the trim size dimensions will be in inches. Blurb is another good self-publisher to look at—more so if you're interested in producing art, photography or other creative books with large format or predominantly image focused rather than text heavy. They have both US and UK sites depending on where you're ordering from.
All these services have more information on their websites. They'll be able to give you comparison tables for the different trim sizes, costs and different volumes and distribution options they offer, so you can do your homework and get some costing estimates before you make solid enquiries about your own book.
Although this all sounds complicated, once you have some basic information to hand, you should be able to quickly narrow down the many options available to just two or three trim sizes suitable for your book—then the whole thing becomes less daunting.
After that, the decision comes down to making sure your book size has a good balance between personal preference, aesthetics, marketing attributes and practicalities.
Summing up, here's a checklist of points to consider when choosing your book's trim size:
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Julia is a motorcycle-riding, cat-herding, food loving artist, illustrator and book designer. She loves food and cooking but then has to run it off at some point later.
She takes great pleasure in making paintings and artwork to sell, and with her design hat on, she helps self-publishing authors get high-end book design and illustration to boost their marketing and help them sell lots of books — all without having to approach big scary design firms.