or click to choose. PRC, AZW and AZW3 Kindle books work too.
Add a Kindle book to compress it. You’ll see how much smaller each picture gets and what takes up space in the file.
Compressed entirely in your browser. Your books are never uploaded.
This MOBI compressor makes Kindle books smaller by shrinking their pictures and removing the leftover source files that KindleGen hides inside every book it builds. The text, chapters, table of contents and bookmarks aren’t touched, so the book reads exactly the same. You can see every picture before and after, check what takes up space in the file, and compress several books at once. Everything runs in your browser and your books are never uploaded.
How to compress a MOBI file
- Drop one or more
.mobior.prcfiles onto the left box, or click it to choose them. - Each book is compressed right away. Results shows the savings and where they came from.
- Open Pictures to see each picture’s old and new size, or Inside to see what the book is made of.
- Change the quality or resize setting if you want more savings or sharper pictures. The books are redone automatically.
- Click Download. With several books, you get one ZIP with all of them.
What makes a MOBI file big
The text in a MOBI file is already compressed, so it’s rarely the problem. The size usually comes from two places:
| Part of the file | What this tool does | Typical savings |
|---|---|---|
| Pictures (covers, photos, comic pages, charts) | Resizes them to fit your Kindle’s screen and re-saves them at the quality you pick | 40–80% on picture-heavy books |
| KindleGen source (SRCS) and build log | Removes them. KindleGen stores a full copy of the book you converted inside the MOBI, and Kindles never read it. | Often a quarter to half of a book made with KindleGen |
| Text, chapters and indexes | Left exactly as they are | None, already compressed |
| Embedded fonts | Left as they are | None, already compressed |
A novel with no pictures that wasn’t made with KindleGen may not shrink at all. The Inside tab shows how much of your book is text, pictures, fonts and leftover source, so you can see why.
Picking the settings
Resize to fit sets the longest side of each picture. A picture bigger than your screen is wasted space, because the Kindle shrinks it anyway when it shows it.
| Setting | Good for |
|---|---|
| 2480 px | Kindle Scribe, and comics or manga you’ll zoom into |
| 1680 px | Kindle Paperwhite, Oasis and Colorsoft. The default, and right for most books. |
| 1448 px | Basic Kindle and older Paperwhites |
| 1024 px | Reading in the Kindle app on a phone, or the smallest possible file |
Picture quality is the JPEG quality used when a picture is re-saved. Balanced is hard to tell apart from the original on an e-ink screen. Use High for photo books you’ll read on a tablet.
Convert PNG photos to JPEG turns photos that were saved as PNG into JPEG, which is often 5–10 times smaller. Screenshots, diagrams and line art stay PNG so text and edges stay crisp, and PNGs with transparency are never converted.
Will the book still open?
Yes. A MOBI file is a list of numbered records, and pictures are found by their record number, not their position in the file. The tool only replaces picture records with smaller versions and updates the record table, so every chapter, link and bookmark still points to the right place. Pictures are only replaced when the new version is actually smaller.
After rebuilding, the tool reads the new file from scratch, checks that every record is where it should be and opens every changed picture. You’ll see Checked in the results when it passes. Keep your original until you’ve opened the compressed book on your Kindle.
Frequently asked questions
Can I still send a MOBI file to my Kindle?
Amazon’s Send to Kindle switched to EPUB in 2022, so it’s no longer the way to load MOBI files. You can still copy a MOBI to a Kindle over USB, and Kindles keep reading the MOBI books they already have. Smaller files copy faster and take less of the Kindle’s storage.
Does it work with AZW3 and KF8 books?
Yes. Most MOBI files made by KindleGen contain both an old-style MOBI 6 book and a newer KF8 book that share the same pictures, and both are handled. You can also compress standalone .azw3 files. In KF8 books, PNG photos are resized but stay PNG, because KF8 text names each picture’s format.
Why did my book barely shrink?
If a book is mostly text, there’s little to gain, because the text is already compressed and it’s never changed. Books with small or already optimized pictures won’t shrink much either. Check the Inside tab to see what the file is made of.
Is this lossless?
Removing the KindleGen source is lossless, since your Kindle never uses it. Re-saving pictures is lossy, like any JPEG. Pick High quality and Don’t resize if you want the pictures as close to the original as possible.
What happens to GIF pictures?
GIFs are left exactly as they are, including animated ones. They’re usually small line art, and older Kindles handle them best as GIF.
Can this remove DRM?
No. If a book with DRM has pictures the tool can read, they’re still shrunk, but the book stays locked to your account. Only your Kindle can confirm a DRM book still opens, so keep the original.