A book’s size is something a reader feels before reading a word. Choose the trim by how it sits in the hand.
The best trim size for a KDP book is the one that feels right in a reader’s hands and matches what the top books in your genre already use. Look at the 5 to 10 most popular, most-reviewed comparable titles on Amazon and size yours to match them.
Most authors are readers first, and they know exactly what a good book feels like. That instinct disappears the moment they put on the publisher hat and start guessing at specs.
I’m Daniel J. Middleton, and Scribe Freelance is the shop I built around one idea: a book’s design should never be the reason it fails to connect.
This piece covers why trim size is a physical decision, what thin books and novellas need, and how to check your genre before you commit.
Why Does Trim Size Feel Like a Spreadsheet Decision Instead of a Physical One?
Because the options arrive as a list of numbers. Every reader already knows what a book should feel like: the balance of thickness, weight, and length. That knowledge gets set aside once trim size turns into an abstract spec.
On KDP, trim size matters less to the print bill than people expect. Printing cost is driven mainly by page count, ink type, and marketplace, though books wider than 6.12 inches or taller than 9 inches count as large trim sizes with their own printing costs. Within the regular sizes, you’re choosing mostly for the reader.
KDP offers a long list of standard trim sizes, from 5 x 8 up through 8.5 x 11, and accepts custom sizes within limits. The list changes over time, so check the current options when you set up your title.
How Do Thin Books and Novellas Change the Trim Size Decision?
Both are short formats, so trim size changes how the book feels in the hand more than it does for a full-length title.
What Trim Size Works for a Thin Book Under 100 Pages?
A book under roughly 100 pages belongs in a smaller format, such as 5 x 8 or even smaller, depending on the content. A larger trim spreads a thin book across the page and makes it feel slight in the hand. KDP’s custom sizes start at 4 inches wide by 6 inches tall, so there’s room to go smaller than 5 x 8.
Low-content formats like planners and coloring books are a separate case. They’re meant to be written in or colored in, so the room matters.
Which Trim Size Suits a Novella?
One a reader can cup comfortably while curling up with it at night. A 7 x 10 trim works actively against that. A novella is built for a particular kind of reading, and its size should serve it.
How Do You Check Your Genre’s Trim Size Against Amazon Bestsellers?
Don’t guess. Use what’s already selling in your genre:
Find your genre’s bestsellers. Look at Amazon’s bestsellers in your actual target genre, not across the whole store.
Choose 5 to 10 comparable titles. Take the most popular, most-reviewed books that are truly like yours.
Record each trim size. It’s listed in the product details on each book’s Amazon page.
Size to the pattern. Where most of them land is what readers in that genre already have in hand.
Matching the norm removes a barrier. It doesn’t sell the book.
What Happens if You Change Trim Size After the Interior Is Done?
Your page count changes, and so does your spine. KDP’s Cover Calculator asks for the page count at your formatted trim size, so a new trim means a new count and a fresh template, as how to calculate spine width for a KDP paperback explains. Choose the trim size before the interior is built and you skip all of that.
The right trim size is the one a reader never notices, because the book simply behaves like the others on their shelf.
What Should You Do Before You Lock Your Trim Size?
Pull your 5 to 10 comparable titles and check them against the size you had in mind. If yours differs from nearly all of them, make sure there’s a reason.
If you’d like a second opinion on trim size before your interior is built, the get-started page is where to begin.
