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According the Amazon's guidelines, a the ideal size for an Amazon Kindle cover image is as follows:

For best quality, your image would be 2820 pixels on the shortest side and 4500 pixels on the longest side

This is a makes the hight 1.6 times the width, which is way more long and narrow than I've ever seen any book. It also seems kind of huge. I don't know any device displaying images on that scale.

What is the logic behind this specification, and is it really the best way?

Note that I am speaking of the cover image that is embedded in the .MOBI file, not the cover for the print-on-demand version, nor the promotional image for the web site. So much of what I've read about cover designs mixes the three that it gets confusing. I only want to know what the best dimensions and size are for the cover that is inside the .MOBI file that will be distributed as a Kindle book.

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The reason why they ask for a big size like this is because of the pixel density of some e-readers and they're probably looking for future device requirements. Indeed, it's huge and your print-ready cover file might not even be big enough to fit this requirement! So it's possible you can't even use the suggested requirement from Amazon for now.

But keep in mind that the numbers you quoted would be for the best quality on the biggest display with the highest density; it's a bit overkill since these numbers are even bigger than what e-readers can actually render. Not every e-reader needs it and you can simply lower these numbers down to fit the average e-readers. Of course if you can use the size you mentioned because your original design allows it, go for it.

And yes, the ratio is different from standard printed cover; the best is to create another version of your design for the Kindle cover. I think the ratio is actually to fit their last generation Kindle Fire e-reader, that's a suggested ratio, and it's different from most e-readers; you are still free to provide a wider one that may look better on thumbnail size.

It depends what you decide is the priority; that the thumbnail cover looks good in the catalog or that it fits better when viewed at 100%. As you guess, because of the different resolution of each e-reader display, you will never get a perfect fit on all of them at 100% unfortunately but you can simply decide to go with the average ratio most of them use.

https://en.wikipedia.org/wiki/Comparison_of_e-book_readers#Electronic-paper_displays

https://developer.amazon.com/public/solutions/devices/fire-tablets/app-development/01--screen-layout-and-resolution#Provide%20Multiple%20Layouts%20for%20Different%20Screens

Amazon says:

A minimum of 625 pixels on the shortest side and 1000 pixels on the longest side For best quality, your image should be 2500 pixels on the longest side

So you can make your height at 2500 pixels, and if you want to keep the same ratio as you mentioned (1.6), your width should be something like 1567 pixels. That's more reasonable and it should be easier to adjust if your print-ready version was made for a 5x8" book.

If you want to fit the average e-reader, the minimum size could be around 1851 pixels width and 2500 pixels height (1.33 ratio.)

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First, a Mobi file is just a converted ePub with Amazon's Kindlegen. You can run Kindlegen through their app or you can run it through the terminal.

Unless you intend on having a nightmare on your hands in trying to create content for every platform, you should take the average of the top ebook stores and build your cover image. These stores range in no specific order from Amazon, Apple, Google, B&N and Kobo. These stores have many quality requirements beyond validation.

The same rule applies in regards to future sells or updates of your ebook. If you ever were to make an update and aren't using the highest setting you could likely get a ticket from an ebook store that will not allow you to sell your ebook and will place the title on-hold. That is why many people that published ebooks a few years back (typically as an ePub2) are now experiencing issues with images because requirements where not really set back then.

At the present time the best ideal size that will work on every platform is 1570 x 2500. I use that exact measurement because that will also work currently on Apple's cover image requirement of an image no larger than 4 million pixels which is actually incorrect because if you send them an image with that exact 4 million pixels total size they will tell you it's too large so the size I send successfully is 3.925 million pixels total which is the max at the present time. This image size can also be used on Apple's store and inside the Kindle.

In regards to the suggested 1851 x 2500 I disagree because that produces a 4.6 million size and you cannot send that out to everyone so it will produce more work when you try to get your ebook sent. So unless you intend to target a file for everyone, which isn't logical or cost effective.

You can use other sizes but since trade paperbacks are typically a 2:3 ratio size it would be more appealing to your customers if your print and digital books were similar because that little detail is what people go by.

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