The phrase “your cover must be 300 DPI” is practically a mantra in self-publishing, yet it’s a common source of confusion. Many authors see “72 DPI” listed in their file’s information, hurriedly change that number to 300 in their editing software, and upload the same exact image, wondering why it still gets flagged. The model behind it is simple.
DPI is a relationship, not a quality
A digital file is just a grid of pixels—for example, 3900 across and 2775 tall. The term DPI, or “dots per inch,” only becomes meaningful when you decide how large you’re going to print that grid of pixels. It’s a simple ratio:
DPI = pixels ÷ printed inches
Take that 3900 x 2775 pixel image. Print it at 13 by 9.25 inches, and the DPI is 3900 ÷ 13, which equals 300 DPI. Print the same file as a poster twice that size, and the DPI drops to 150. The “DPI” number saved inside a JPEG or PNG file is just a note attached to the pixels. Changing that note doesn’t magically add detail or sharpen a blurry image.
The real question is this: does your image have enough raw pixels for the physical size you need?
To find out, you just do the reverse of the formula:
required pixels = print inches × 300
Practical examples
Here are the pixel dimensions you’d need for some common paperback sizes:
| Print target | Required pixels |
|---|---|
| 6 × 9 front cover (with bleed: 6.125″ × 9.25″) | 1838 × 2775 |
| Full 6 × 9 wrap, 300 pp cream (13.0″ × 9.25″) | 3900 × 2775 |
| Full 5 × 8 wrap, 220 pp cream (10.8″ × 8.25″) | 3240 × 2475 |
(You can calculate your own exact needs with this cover size calculator—it’s designed to give you the required pixel dimensions for a 300 DPI print.)
The real reasons files fail the “300 DPI” check
When a platform like KDP rejects your cover for resolution, it’s using that same formula: it divides your image’s pixel dimensions by the size it occupies in the final PDF. You usually fail for one of these reasons:
- Stretching a small image. You can enlarge a 1600-pixel-wide image to 3900 pixels using software, but you’re not creating new detail—you’re just making the existing pixels bigger. The file might technically pass a size check, but the printed result will look fuzzy and soft. True resolution has to be captured or created from the start.
- Using a high-resolution canvas with low-resolution parts. Your overall document might be set to 300 DPI, but if you drop in a photo and stretch it to three times its original size, its effective DPI plummets. Quality checkers often analyze the individual images inside your PDF file.
- Starting with low-quality source art. Images grabbed from a website or taken as a screenshot are typically around 100 DPI at screen size. They will never produce a sharp, crisp print when blown up to cover dimensions.
Ebook covers: It’s all about the pixels
For Kindle covers, the DPI field is completely meaningless—digital screens are measured in pixels, not inches. Amazon’s requirement is a specific pixel grid: 1600 × 2560 (full ebook spec). Whether your file says 72 DPI or 300 DPI in its metadata has zero effect on how it appears on a reader’s device.
In short
The DPI number in the file’s metadata can be ignored. The print size in inches times 300 gives the pixel dimensions, and the image then has to contain that many pixels of real detail, not an upscaled version of a smaller file.
Sources
- KDP: Paperback Submission Guidelines — the 300 DPI requirement
- KDPEasy: cover FAQ — dimensions, DPI, bleed — how effective DPI is evaluated
